288 lines
12 KiB
ReStructuredText
288 lines
12 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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==========================
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The MSI Driver Guide HOWTO
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==========================
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:Authors: Tom L Nguyen; Martine Silbermann; Matthew Wilcox
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:Copyright: 2003, 2008 Intel Corporation
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About this guide
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================
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This guide describes the basics of Message Signaled Interrupts (MSIs),
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the advantages of using MSI over traditional interrupt mechanisms, how
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to change your driver to use MSI or MSI-X and some basic diagnostics to
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try if a device doesn't support MSIs.
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What are MSIs?
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==============
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A Message Signaled Interrupt is a write from the device to a special
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address which causes an interrupt to be received by the CPU.
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The MSI capability was first specified in PCI 2.2 and was later enhanced
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in PCI 3.0 to allow each interrupt to be masked individually. The MSI-X
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capability was also introduced with PCI 3.0. It supports more interrupts
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per device than MSI and allows interrupts to be independently configured.
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Devices may support both MSI and MSI-X, but only one can be enabled at
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a time.
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Why use MSIs?
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=============
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There are three reasons why using MSIs can give an advantage over
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traditional pin-based interrupts.
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Pin-based PCI interrupts are often shared amongst several devices.
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To support this, the kernel must call each interrupt handler associated
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with an interrupt, which leads to reduced performance for the system as
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a whole. MSIs are never shared, so this problem cannot arise.
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When a device writes data to memory, then raises a pin-based interrupt,
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it is possible that the interrupt may arrive before all the data has
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arrived in memory (this becomes more likely with devices behind PCI-PCI
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bridges). In order to ensure that all the data has arrived in memory,
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the interrupt handler must read a register on the device which raised
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the interrupt. PCI transaction ordering rules require that all the data
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arrive in memory before the value may be returned from the register.
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Using MSIs avoids this problem as the interrupt-generating write cannot
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pass the data writes, so by the time the interrupt is raised, the driver
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knows that all the data has arrived in memory.
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PCI devices can only support a single pin-based interrupt per function.
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Often drivers have to query the device to find out what event has
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occurred, slowing down interrupt handling for the common case. With
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MSIs, a device can support more interrupts, allowing each interrupt
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to be specialised to a different purpose. One possible design gives
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infrequent conditions (such as errors) their own interrupt which allows
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the driver to handle the normal interrupt handling path more efficiently.
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Other possible designs include giving one interrupt to each packet queue
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in a network card or each port in a storage controller.
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How to use MSIs
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===============
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PCI devices are initialised to use pin-based interrupts. The device
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driver has to set up the device to use MSI or MSI-X. Not all machines
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support MSIs correctly, and for those machines, the APIs described below
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will simply fail and the device will continue to use pin-based interrupts.
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Include kernel support for MSIs
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-------------------------------
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To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI
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option enabled. This option is only available on some architectures,
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and it may depend on some other options also being set. For example,
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on x86, you must also enable X86_UP_APIC or SMP in order to see the
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CONFIG_PCI_MSI option.
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Using MSI
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---------
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Most of the hard work is done for the driver in the PCI layer. The driver
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simply has to request that the PCI layer set up the MSI capability for this
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device.
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To automatically use MSI or MSI-X interrupt vectors, use the following
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function::
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int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
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unsigned int max_vecs, unsigned int flags);
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which allocates up to max_vecs interrupt vectors for a PCI device. It
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returns the number of vectors allocated or a negative error. If the device
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has a requirements for a minimum number of vectors the driver can pass a
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min_vecs argument set to this limit, and the PCI core will return -ENOSPC
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if it can't meet the minimum number of vectors.
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The flags argument is used to specify which type of interrupt can be used
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by the device and the driver (PCI_IRQ_LEGACY, PCI_IRQ_MSI, PCI_IRQ_MSIX).
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A convenient short-hand (PCI_IRQ_ALL_TYPES) is also available to ask for
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any possible kind of interrupt. If the PCI_IRQ_AFFINITY flag is set,
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pci_alloc_irq_vectors() will spread the interrupts around the available CPUs.
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To get the Linux IRQ numbers passed to request_irq() and free_irq() and the
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vectors, use the following function::
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int pci_irq_vector(struct pci_dev *dev, unsigned int nr);
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Any allocated resources should be freed before removing the device using
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the following function::
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void pci_free_irq_vectors(struct pci_dev *dev);
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If a device supports both MSI-X and MSI capabilities, this API will use the
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MSI-X facilities in preference to the MSI facilities. MSI-X supports any
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number of interrupts between 1 and 2048. In contrast, MSI is restricted to
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a maximum of 32 interrupts (and must be a power of two). In addition, the
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MSI interrupt vectors must be allocated consecutively, so the system might
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not be able to allocate as many vectors for MSI as it could for MSI-X. On
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some platforms, MSI interrupts must all be targeted at the same set of CPUs
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whereas MSI-X interrupts can all be targeted at different CPUs.
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If a device supports neither MSI-X or MSI it will fall back to a single
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legacy IRQ vector.
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The typical usage of MSI or MSI-X interrupts is to allocate as many vectors
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as possible, likely up to the limit supported by the device. If nvec is
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larger than the number supported by the device it will automatically be
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capped to the supported limit, so there is no need to query the number of
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vectors supported beforehand::
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nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_ALL_TYPES)
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if (nvec < 0)
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goto out_err;
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If a driver is unable or unwilling to deal with a variable number of MSI
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interrupts it can request a particular number of interrupts by passing that
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number to pci_alloc_irq_vectors() function as both 'min_vecs' and
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'max_vecs' parameters::
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ret = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_ALL_TYPES);
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if (ret < 0)
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goto out_err;
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The most notorious example of the request type described above is enabling
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the single MSI mode for a device. It could be done by passing two 1s as
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'min_vecs' and 'max_vecs'::
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ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES);
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if (ret < 0)
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goto out_err;
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Some devices might not support using legacy line interrupts, in which case
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the driver can specify that only MSI or MSI-X is acceptable::
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nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_MSI | PCI_IRQ_MSIX);
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if (nvec < 0)
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goto out_err;
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Legacy APIs
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-----------
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The following old APIs to enable and disable MSI or MSI-X interrupts should
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not be used in new code::
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pci_enable_msi() /* deprecated */
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pci_disable_msi() /* deprecated */
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pci_enable_msix_range() /* deprecated */
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pci_enable_msix_exact() /* deprecated */
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pci_disable_msix() /* deprecated */
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Additionally there are APIs to provide the number of supported MSI or MSI-X
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vectors: pci_msi_vec_count() and pci_msix_vec_count(). In general these
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should be avoided in favor of letting pci_alloc_irq_vectors() cap the
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number of vectors. If you have a legitimate special use case for the count
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of vectors we might have to revisit that decision and add a
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pci_nr_irq_vectors() helper that handles MSI and MSI-X transparently.
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Considerations when using MSIs
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------------------------------
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Spinlocks
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~~~~~~~~~
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Most device drivers have a per-device spinlock which is taken in the
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interrupt handler. With pin-based interrupts or a single MSI, it is not
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necessary to disable interrupts (Linux guarantees the same interrupt will
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not be re-entered). If a device uses multiple interrupts, the driver
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must disable interrupts while the lock is held. If the device sends
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a different interrupt, the driver will deadlock trying to recursively
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acquire the spinlock. Such deadlocks can be avoided by using
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spin_lock_irqsave() or spin_lock_irq() which disable local interrupts
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and acquire the lock (see Documentation/kernel-hacking/locking.rst).
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How to tell whether MSI/MSI-X is enabled on a device
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----------------------------------------------------
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Using 'lspci -v' (as root) may show some devices with "MSI", "Message
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Signalled Interrupts" or "MSI-X" capabilities. Each of these capabilities
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has an 'Enable' flag which is followed with either "+" (enabled)
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or "-" (disabled).
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MSI quirks
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==========
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Several PCI chipsets or devices are known not to support MSIs.
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The PCI stack provides three ways to disable MSIs:
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1. globally
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2. on all devices behind a specific bridge
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3. on a single device
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Disabling MSIs globally
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-----------------------
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Some host chipsets simply don't support MSIs properly. If we're
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lucky, the manufacturer knows this and has indicated it in the ACPI
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FADT table. In this case, Linux automatically disables MSIs.
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Some boards don't include this information in the table and so we have
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to detect them ourselves. The complete list of these is found near the
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quirk_disable_all_msi() function in drivers/pci/quirks.c.
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If you have a board which has problems with MSIs, you can pass pci=nomsi
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on the kernel command line to disable MSIs on all devices. It would be
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in your best interests to report the problem to linux-pci@vger.kernel.org
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including a full 'lspci -v' so we can add the quirks to the kernel.
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Disabling MSIs below a bridge
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-----------------------------
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Some PCI bridges are not able to route MSIs between busses properly.
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In this case, MSIs must be disabled on all devices behind the bridge.
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Some bridges allow you to enable MSIs by changing some bits in their
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PCI configuration space (especially the Hypertransport chipsets such
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as the nVidia nForce and Serverworks HT2000). As with host chipsets,
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Linux mostly knows about them and automatically enables MSIs if it can.
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If you have a bridge unknown to Linux, you can enable
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MSIs in configuration space using whatever method you know works, then
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enable MSIs on that bridge by doing::
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echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
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where $bridge is the PCI address of the bridge you've enabled (eg
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0000:00:0e.0).
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To disable MSIs, echo 0 instead of 1. Changing this value should be
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done with caution as it could break interrupt handling for all devices
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below this bridge.
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Again, please notify linux-pci@vger.kernel.org of any bridges that need
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special handling.
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Disabling MSIs on a single device
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---------------------------------
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Some devices are known to have faulty MSI implementations. Usually this
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is handled in the individual device driver, but occasionally it's necessary
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to handle this with a quirk. Some drivers have an option to disable use
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of MSI. While this is a convenient workaround for the driver author,
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it is not good practice, and should not be emulated.
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Finding why MSIs are disabled on a device
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-----------------------------------------
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From the above three sections, you can see that there are many reasons
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why MSIs may not be enabled for a given device. Your first step should
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be to examine your dmesg carefully to determine whether MSIs are enabled
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for your machine. You should also check your .config to be sure you
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have enabled CONFIG_PCI_MSI.
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Then, 'lspci -t' gives the list of bridges above a device. Reading
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`/sys/bus/pci/devices/*/msi_bus` will tell you whether MSIs are enabled (1)
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or disabled (0). If 0 is found in any of the msi_bus files belonging
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to bridges between the PCI root and the device, MSIs are disabled.
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It is also worth checking the device driver to see whether it supports MSIs.
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For example, it may contain calls to pci_alloc_irq_vectors() with the
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PCI_IRQ_MSI or PCI_IRQ_MSIX flags.
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