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ReStructuredText
579 lines
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ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==============================
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How To Write Linux PCI Drivers
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==============================
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:Authors: - Martin Mares <mj@ucw.cz>
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- Grant Grundler <grundler@parisc-linux.org>
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The world of PCI is vast and full of (mostly unpleasant) surprises.
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Since each CPU architecture implements different chip-sets and PCI devices
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have different requirements (erm, "features"), the result is the PCI support
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in the Linux kernel is not as trivial as one would wish. This short paper
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tries to introduce all potential driver authors to Linux APIs for
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PCI device drivers.
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A more complete resource is the third edition of "Linux Device Drivers"
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by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.
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LDD3 is available for free (under Creative Commons License) from:
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https://lwn.net/Kernel/LDD3/.
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However, keep in mind that all documents are subject to "bit rot".
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Refer to the source code if things are not working as described here.
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Please send questions/comments/patches about Linux PCI API to the
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"Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.
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Structure of PCI drivers
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========================
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PCI drivers "discover" PCI devices in a system via pci_register_driver().
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Actually, it's the other way around. When the PCI generic code discovers
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a new device, the driver with a matching "description" will be notified.
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Details on this below.
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pci_register_driver() leaves most of the probing for devices to
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the PCI layer and supports online insertion/removal of devices [thus
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supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver].
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pci_register_driver() call requires passing in a table of function
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pointers and thus dictates the high level structure of a driver.
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Once the driver knows about a PCI device and takes ownership, the
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driver generally needs to perform the following initialization:
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- Enable the device
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- Request MMIO/IOP resources
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- Set the DMA mask size (for both coherent and streaming DMA)
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- Allocate and initialize shared control data (pci_allocate_coherent())
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- Access device configuration space (if needed)
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- Register IRQ handler (request_irq())
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- Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
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- Enable DMA/processing engines
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When done using the device, and perhaps the module needs to be unloaded,
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the driver needs to take the follow steps:
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- Disable the device from generating IRQs
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- Release the IRQ (free_irq())
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- Stop all DMA activity
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- Release DMA buffers (both streaming and coherent)
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- Unregister from other subsystems (e.g. scsi or netdev)
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- Release MMIO/IOP resources
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- Disable the device
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Most of these topics are covered in the following sections.
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For the rest look at LDD3 or <linux/pci.h> .
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If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
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the PCI functions described below are defined as inline functions either
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completely empty or just returning an appropriate error codes to avoid
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lots of ifdefs in the drivers.
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pci_register_driver() call
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==========================
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PCI device drivers call ``pci_register_driver()`` during their
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initialization with a pointer to a structure describing the driver
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(``struct pci_driver``):
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.. kernel-doc:: include/linux/pci.h
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:functions: pci_driver
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The ID table is an array of ``struct pci_device_id`` entries ending with an
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all-zero entry. Definitions with static const are generally preferred.
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.. kernel-doc:: include/linux/mod_devicetable.h
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:functions: pci_device_id
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Most drivers only need ``PCI_DEVICE()`` or ``PCI_DEVICE_CLASS()`` to set up
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a pci_device_id table.
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New PCI IDs may be added to a device driver pci_ids table at runtime
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as shown below::
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echo "vendor device subvendor subdevice class class_mask driver_data" > \
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/sys/bus/pci/drivers/{driver}/new_id
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All fields are passed in as hexadecimal values (no leading 0x).
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The vendor and device fields are mandatory, the others are optional. Users
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need pass only as many optional fields as necessary:
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- subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)
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- class and classmask fields default to 0
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- driver_data defaults to 0UL.
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- override_only field defaults to 0.
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Note that driver_data must match the value used by any of the pci_device_id
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entries defined in the driver. This makes the driver_data field mandatory
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if all the pci_device_id entries have a non-zero driver_data value.
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Once added, the driver probe routine will be invoked for any unclaimed
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PCI devices listed in its (newly updated) pci_ids list.
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When the driver exits, it just calls pci_unregister_driver() and the PCI layer
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automatically calls the remove hook for all devices handled by the driver.
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"Attributes" for driver functions/data
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--------------------------------------
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Please mark the initialization and cleanup functions where appropriate
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(the corresponding macros are defined in <linux/init.h>):
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====== =================================================
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__init Initialization code. Thrown away after the driver
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initializes.
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__exit Exit code. Ignored for non-modular drivers.
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====== =================================================
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Tips on when/where to use the above attributes:
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- The module_init()/module_exit() functions (and all
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initialization functions called _only_ from these)
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should be marked __init/__exit.
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- Do not mark the struct pci_driver.
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- Do NOT mark a function if you are not sure which mark to use.
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Better to not mark the function than mark the function wrong.
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How to find PCI devices manually
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================================
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PCI drivers should have a really good reason for not using the
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pci_register_driver() interface to search for PCI devices.
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The main reason PCI devices are controlled by multiple drivers
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is because one PCI device implements several different HW services.
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E.g. combined serial/parallel port/floppy controller.
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A manual search may be performed using the following constructs:
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Searching by vendor and device ID::
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struct pci_dev *dev = NULL;
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while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
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configure_device(dev);
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Searching by class ID (iterate in a similar way)::
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pci_get_class(CLASS_ID, dev)
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Searching by both vendor/device and subsystem vendor/device ID::
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pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
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You can use the constant PCI_ANY_ID as a wildcard replacement for
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VENDOR_ID or DEVICE_ID. This allows searching for any device from a
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specific vendor, for example.
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These functions are hotplug-safe. They increment the reference count on
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the pci_dev that they return. You must eventually (possibly at module unload)
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decrement the reference count on these devices by calling pci_dev_put().
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Device Initialization Steps
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===========================
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As noted in the introduction, most PCI drivers need the following steps
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for device initialization:
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- Enable the device
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- Request MMIO/IOP resources
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- Set the DMA mask size (for both coherent and streaming DMA)
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- Allocate and initialize shared control data (pci_allocate_coherent())
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- Access device configuration space (if needed)
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- Register IRQ handler (request_irq())
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- Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
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- Enable DMA/processing engines.
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The driver can access PCI config space registers at any time.
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(Well, almost. When running BIST, config space can go away...but
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that will just result in a PCI Bus Master Abort and config reads
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will return garbage).
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Enable the PCI device
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---------------------
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Before touching any device registers, the driver needs to enable
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the PCI device by calling pci_enable_device(). This will:
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- wake up the device if it was in suspended state,
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- allocate I/O and memory regions of the device (if BIOS did not),
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- allocate an IRQ (if BIOS did not).
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.. note::
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pci_enable_device() can fail! Check the return value.
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.. warning::
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OS BUG: we don't check resource allocations before enabling those
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resources. The sequence would make more sense if we called
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pci_request_resources() before calling pci_enable_device().
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Currently, the device drivers can't detect the bug when two
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devices have been allocated the same range. This is not a common
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problem and unlikely to get fixed soon.
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This has been discussed before but not changed as of 2.6.19:
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https://lore.kernel.org/r/20060302180025.GC28895@flint.arm.linux.org.uk/
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pci_set_master() will enable DMA by setting the bus master bit
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in the PCI_COMMAND register. It also fixes the latency timer value if
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it's set to something bogus by the BIOS. pci_clear_master() will
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disable DMA by clearing the bus master bit.
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If the PCI device can use the PCI Memory-Write-Invalidate transaction,
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call pci_set_mwi(). This enables the PCI_COMMAND bit for Mem-Wr-Inval
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and also ensures that the cache line size register is set correctly.
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Check the return value of pci_set_mwi() as not all architectures
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or chip-sets may support Memory-Write-Invalidate. Alternatively,
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if Mem-Wr-Inval would be nice to have but is not required, call
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pci_try_set_mwi() to have the system do its best effort at enabling
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Mem-Wr-Inval.
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Request MMIO/IOP resources
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--------------------------
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Memory (MMIO), and I/O port addresses should NOT be read directly
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from the PCI device config space. Use the values in the pci_dev structure
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as the PCI "bus address" might have been remapped to a "host physical"
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address by the arch/chip-set specific kernel support.
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See Documentation/driver-api/io-mapping.rst for how to access device registers
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or device memory.
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The device driver needs to call pci_request_region() to verify
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no other device is already using the same address resource.
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Conversely, drivers should call pci_release_region() AFTER
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calling pci_disable_device().
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The idea is to prevent two devices colliding on the same address range.
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.. tip::
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See OS BUG comment above. Currently (2.6.19), The driver can only
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determine MMIO and IO Port resource availability _after_ calling
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pci_enable_device().
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Generic flavors of pci_request_region() are request_mem_region()
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(for MMIO ranges) and request_region() (for IO Port ranges).
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Use these for address resources that are not described by "normal" PCI
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BARs.
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Also see pci_request_selected_regions() below.
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Set the DMA mask size
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---------------------
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.. note::
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If anything below doesn't make sense, please refer to
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Documentation/core-api/dma-api.rst. This section is just a reminder that
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drivers need to indicate DMA capabilities of the device and is not
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an authoritative source for DMA interfaces.
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While all drivers should explicitly indicate the DMA capability
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(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
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32-bit bus master capability for streaming data need the driver
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to "register" this capability by calling dma_set_mask() with
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appropriate parameters. In general this allows more efficient DMA
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on systems where System RAM exists above 4G _physical_ address.
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Drivers for all PCI-X and PCIe compliant devices must call
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dma_set_mask() as they are 64-bit DMA devices.
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Similarly, drivers must also "register" this capability if the device
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can directly address "coherent memory" in System RAM above 4G physical
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address by calling dma_set_coherent_mask().
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Again, this includes drivers for all PCI-X and PCIe compliant devices.
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Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
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64-bit DMA capable for payload ("streaming") data but not control
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("coherent") data.
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Setup shared control data
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-------------------------
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Once the DMA masks are set, the driver can allocate "coherent" (a.k.a. shared)
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memory. See Documentation/core-api/dma-api.rst for a full description of
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the DMA APIs. This section is just a reminder that it needs to be done
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before enabling DMA on the device.
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Initialize device registers
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---------------------------
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Some drivers will need specific "capability" fields programmed
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or other "vendor specific" register initialized or reset.
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E.g. clearing pending interrupts.
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Register IRQ handler
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--------------------
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While calling request_irq() is the last step described here,
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this is often just another intermediate step to initialize a device.
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This step can often be deferred until the device is opened for use.
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All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
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and use the devid to map IRQs to devices (remember that all PCI IRQ lines
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can be shared).
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request_irq() will associate an interrupt handler and device handle
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with an interrupt number. Historically interrupt numbers represent
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IRQ lines which run from the PCI device to the Interrupt controller.
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With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
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request_irq() also enables the interrupt. Make sure the device is
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quiesced and does not have any interrupts pending before registering
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the interrupt handler.
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MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
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which deliver interrupts to the CPU via a DMA write to a Local APIC.
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The fundamental difference between MSI and MSI-X is how multiple
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"vectors" get allocated. MSI requires contiguous blocks of vectors
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while MSI-X can allocate several individual ones.
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MSI capability can be enabled by calling pci_alloc_irq_vectors() with the
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PCI_IRQ_MSI and/or PCI_IRQ_MSIX flags before calling request_irq(). This
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causes the PCI support to program CPU vector data into the PCI device
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capability registers. Many architectures, chip-sets, or BIOSes do NOT
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support MSI or MSI-X and a call to pci_alloc_irq_vectors with just
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the PCI_IRQ_MSI and PCI_IRQ_MSIX flags will fail, so try to always
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specify PCI_IRQ_LEGACY as well.
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Drivers that have different interrupt handlers for MSI/MSI-X and
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legacy INTx should chose the right one based on the msi_enabled
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and msix_enabled flags in the pci_dev structure after calling
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pci_alloc_irq_vectors.
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There are (at least) two really good reasons for using MSI:
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1) MSI is an exclusive interrupt vector by definition.
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This means the interrupt handler doesn't have to verify
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its device caused the interrupt.
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2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
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to be visible to the host CPU(s) when the MSI is delivered. This
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is important for both data coherency and avoiding stale control data.
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This guarantee allows the driver to omit MMIO reads to flush
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the DMA stream.
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See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
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of MSI/MSI-X usage.
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PCI device shutdown
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===================
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When a PCI device driver is being unloaded, most of the following
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steps need to be performed:
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- Disable the device from generating IRQs
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- Release the IRQ (free_irq())
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- Stop all DMA activity
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- Release DMA buffers (both streaming and coherent)
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- Unregister from other subsystems (e.g. scsi or netdev)
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- Disable device from responding to MMIO/IO Port addresses
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- Release MMIO/IO Port resource(s)
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Stop IRQs on the device
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-----------------------
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How to do this is chip/device specific. If it's not done, it opens
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the possibility of a "screaming interrupt" if (and only if)
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the IRQ is shared with another device.
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When the shared IRQ handler is "unhooked", the remaining devices
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using the same IRQ line will still need the IRQ enabled. Thus if the
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"unhooked" device asserts IRQ line, the system will respond assuming
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it was one of the remaining devices asserted the IRQ line. Since none
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of the other devices will handle the IRQ, the system will "hang" until
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it decides the IRQ isn't going to get handled and masks the IRQ (100,000
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iterations later). Once the shared IRQ is masked, the remaining devices
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will stop functioning properly. Not a nice situation.
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This is another reason to use MSI or MSI-X if it's available.
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MSI and MSI-X are defined to be exclusive interrupts and thus
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are not susceptible to the "screaming interrupt" problem.
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Release the IRQ
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---------------
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Once the device is quiesced (no more IRQs), one can call free_irq().
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This function will return control once any pending IRQs are handled,
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"unhook" the drivers IRQ handler from that IRQ, and finally release
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the IRQ if no one else is using it.
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Stop all DMA activity
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---------------------
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It's extremely important to stop all DMA operations BEFORE attempting
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to deallocate DMA control data. Failure to do so can result in memory
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corruption, hangs, and on some chip-sets a hard crash.
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Stopping DMA after stopping the IRQs can avoid races where the
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IRQ handler might restart DMA engines.
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While this step sounds obvious and trivial, several "mature" drivers
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didn't get this step right in the past.
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Release DMA buffers
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-------------------
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Once DMA is stopped, clean up streaming DMA first.
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I.e. unmap data buffers and return buffers to "upstream"
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owners if there is one.
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Then clean up "coherent" buffers which contain the control data.
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See Documentation/core-api/dma-api.rst for details on unmapping interfaces.
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Unregister from other subsystems
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--------------------------------
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Most low level PCI device drivers support some other subsystem
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like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
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driver isn't losing resources from that other subsystem.
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If this happens, typically the symptom is an Oops (panic) when
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the subsystem attempts to call into a driver that has been unloaded.
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Disable Device from responding to MMIO/IO Port addresses
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--------------------------------------------------------
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io_unmap() MMIO or IO Port resources and then call pci_disable_device().
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This is the symmetric opposite of pci_enable_device().
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Do not access device registers after calling pci_disable_device().
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Release MMIO/IO Port Resource(s)
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--------------------------------
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Call pci_release_region() to mark the MMIO or IO Port range as available.
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Failure to do so usually results in the inability to reload the driver.
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How to access PCI config space
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==============================
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You can use `pci_(read|write)_config_(byte|word|dword)` to access the config
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space of a device represented by `struct pci_dev *`. All these functions return
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0 when successful or an error code (`PCIBIOS_...`) which can be translated to a
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text string by pcibios_strerror. Most drivers expect that accesses to valid PCI
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devices don't fail.
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If you don't have a struct pci_dev available, you can call
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`pci_bus_(read|write)_config_(byte|word|dword)` to access a given device
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and function on that bus.
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If you access fields in the standard portion of the config header, please
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use symbolic names of locations and bits declared in <linux/pci.h>.
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If you need to access Extended PCI Capability registers, just call
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pci_find_capability() for the particular capability and it will find the
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corresponding register block for you.
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Other interesting functions
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===========================
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============================= ================================================
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pci_get_domain_bus_and_slot() Find pci_dev corresponding to given domain,
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bus and slot and number. If the device is
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found, its reference count is increased.
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pci_set_power_state() Set PCI Power Management state (0=D0 ... 3=D3)
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pci_find_capability() Find specified capability in device's capability
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list.
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pci_resource_start() Returns bus start address for a given PCI region
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pci_resource_end() Returns bus end address for a given PCI region
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pci_resource_len() Returns the byte length of a PCI region
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pci_set_drvdata() Set private driver data pointer for a pci_dev
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pci_get_drvdata() Return private driver data pointer for a pci_dev
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pci_set_mwi() Enable Memory-Write-Invalidate transactions.
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pci_clear_mwi() Disable Memory-Write-Invalidate transactions.
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============================= ================================================
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Miscellaneous hints
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===================
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When displaying PCI device names to the user (for example when a driver wants
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to tell the user what card has it found), please use pci_name(pci_dev).
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Always refer to the PCI devices by a pointer to the pci_dev structure.
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All PCI layer functions use this identification and it's the only
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reasonable one. Don't use bus/slot/function numbers except for very
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special purposes -- on systems with multiple primary buses their semantics
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can be pretty complex.
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Don't try to turn on Fast Back to Back writes in your driver. All devices
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on the bus need to be capable of doing it, so this is something which needs
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to be handled by platform and generic code, not individual drivers.
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Vendor and device identifications
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=================================
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Do not add new device or vendor IDs to include/linux/pci_ids.h unless they
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are shared across multiple drivers. You can add private definitions in
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your driver if they're helpful, or just use plain hex constants.
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The device IDs are arbitrary hex numbers (vendor controlled) and normally used
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only in a single location, the pci_device_id table.
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Please DO submit new vendor/device IDs to https://pci-ids.ucw.cz/.
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There's a mirror of the pci.ids file at https://github.com/pciutils/pciids.
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Obsolete functions
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==================
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There are several functions which you might come across when trying to
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port an old driver to the new PCI interface. They are no longer present
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in the kernel as they aren't compatible with hotplug or PCI domains or
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having sane locking.
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================= ===========================================
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pci_find_device() Superseded by pci_get_device()
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pci_find_subsys() Superseded by pci_get_subsys()
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pci_find_slot() Superseded by pci_get_domain_bus_and_slot()
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pci_get_slot() Superseded by pci_get_domain_bus_and_slot()
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================= ===========================================
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The alternative is the traditional PCI device driver that walks PCI
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device lists. This is still possible but discouraged.
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MMIO Space and "Write Posting"
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==============================
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Converting a driver from using I/O Port space to using MMIO space
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often requires some additional changes. Specifically, "write posting"
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needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
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already do this. I/O Port space guarantees write transactions reach the PCI
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device before the CPU can continue. Writes to MMIO space allow the CPU
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to continue before the transaction reaches the PCI device. HW weenies
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call this "Write Posting" because the write completion is "posted" to
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the CPU before the transaction has reached its destination.
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Thus, timing sensitive code should add readl() where the CPU is
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expected to wait before doing other work. The classic "bit banging"
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sequence works fine for I/O Port space::
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for (i = 8; --i; val >>= 1) {
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outb(val & 1, ioport_reg); /* write bit */
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udelay(10);
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}
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The same sequence for MMIO space should be::
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for (i = 8; --i; val >>= 1) {
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writeb(val & 1, mmio_reg); /* write bit */
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readb(safe_mmio_reg); /* flush posted write */
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udelay(10);
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}
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It is important that "safe_mmio_reg" not have any side effects that
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interferes with the correct operation of the device.
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Another case to watch out for is when resetting a PCI device. Use PCI
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Configuration space reads to flush the writel(). This will gracefully
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handle the PCI master abort on all platforms if the PCI device is
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expected to not respond to a readl(). Most x86 platforms will allow
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MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
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(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
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