firmware: revamp firmware documentation
Understanding this code is getting out of control without any notes. Give the firmware_class driver a much needed documentation love, and while at it convert it to the new sphinx documentation format. v2: typos and small fixes Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Luis R. Rodriguez
Greg Kroah-Hartman
parent
880444e214
commit
113ccc3837
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=================
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Built-in firmware
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=================
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Firmware can be built-in to the kernel, this means building the firmware
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into vmlinux directly, to enable avoiding having to look for firmware from
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the filesystem. Instead, firmware can be looked for inside the kernel
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directly. You can enable built-in firmware using the kernel configuration
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options:
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* CONFIG_EXTRA_FIRMWARE
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* CONFIG_EXTRA_FIRMWARE_DIR
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This should not be confused with CONFIG_FIRMWARE_IN_KERNEL, this is for drivers
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which enables firmware to be built as part of the kernel build process. This
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option, CONFIG_FIRMWARE_IN_KERNEL, will build all firmware for all drivers
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enabled which ship its firmware inside the Linux kernel source tree.
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There are a few reasons why you might want to consider building your firmware
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into the kernel with CONFIG_EXTRA_FIRMWARE though:
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* Speed
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* Firmware is needed for accessing the boot device, and the user doesn't
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want to stuff the firmware into the boot initramfs.
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Even if you have these needs there are a few reasons why you may not be
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able to make use of built-in firmware:
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* Legalese - firmware is non-GPL compatible
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* Some firmware may be optional
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* Firmware upgrades are possible, therefore a new firmware would implicate
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a complete kernel rebuild.
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* Some firmware files may be really large in size. The remote-proc subsystem
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is an example subsystem which deals with these sorts of firmware
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* The firmware may need to be scraped out from some device specific location
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dynamically, an example is calibration data for for some WiFi chipsets. This
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calibration data can be unique per sold device.
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==========================
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Firmware API core features
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==========================
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The firmware API has a rich set of core features available. This section
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documents these features.
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.. toctree::
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fw_search_path
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built-in-fw
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firmware_cache
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direct-fs-lookup
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fallback-mechanisms
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lookup-order
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========================
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Direct filesystem lookup
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========================
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Direct filesystem lookup is the most common form of firmware lookup performed
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by the kernel. The kernel looks for the firmware directly on the root
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filesystem in the paths documented in the section 'Firmware search paths'.
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The filesystem lookup is implemented in fw_get_filesystem_firmware(), it
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uses common core kernel file loader facility kernel_read_file_from_path().
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The max path allowed is PATH_MAX -- currently this is 4096 characters.
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It is recommended you keep /lib/firmware paths on your root filesystem,
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avoid having a separate partition for them in order to avoid possible
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races with lookups and avoid uses of the custom fallback mechanisms
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documented below.
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Firmware and initramfs
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----------------------
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Drivers which are built-in to the kernel should have the firmware integrated
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also as part of the initramfs used to boot the kernel given that otherwise
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a race is possible with loading the driver and the real rootfs not yet being
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available. Stuffing the firmware into initramfs resolves this race issue,
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however note that using initrd does not suffice to address the same race.
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There are circumstances that justify not wanting to include firmware into
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initramfs, such as dealing with large firmware firmware files for the
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remote-proc subsystem. For such cases using a userspace fallback mechanism
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is currently the only viable solution as only userspace can know for sure
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when the real rootfs is ready and mounted.
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===================
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Fallback mechanisms
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===================
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A fallback mechanism is supported to allow to overcome failures to do a direct
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filesystem lookup on the root filesystem or when the firmware simply cannot be
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installed for practical reasons on the root filesystem. The kernel
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configuration options related to supporting the firmware fallback mechanism are:
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* CONFIG_FW_LOADER_USER_HELPER: enables building the firmware fallback
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mechanism. Most distributions enable this option today. If enabled but
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CONFIG_FW_LOADER_USER_HELPER_FALLBACK is disabled, only the custom fallback
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mechanism is available and for the request_firmware_nowait() call.
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* CONFIG_FW_LOADER_USER_HELPER_FALLBACK: force enables each request to
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enable the kobject uevent fallback mechanism on all firmware API calls
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except request_firmware_direct(). Most distributions disable this option
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today. The call request_firmware_nowait() allows for one alternative
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fallback mechanism: if this kconfig option is enabled and your second
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argument to request_firmware_nowait(), uevent, is set to false you are
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informing the kernel that you have a custom fallback mechanism and it will
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manually load the firmware. Read below for more details.
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Note that this means when having this configuration:
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CONFIG_FW_LOADER_USER_HELPER=y
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CONFIG_FW_LOADER_USER_HELPER_FALLBACK=n
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the kobject uevent fallback mechanism will never take effect even
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for request_firmware_nowait() when uevent is set to true.
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Justifying the firmware fallback mechanism
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==========================================
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Direct filesystem lookups may fail for a variety of reasons. Known reasons for
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this are worth itemizing and documenting as it justifies the need for the
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fallback mechanism:
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* Race against access with the root filesystem upon bootup.
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* Races upon resume from suspend. This is resolved by the firmware cache, but
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the firmware cache is only supported if you use uevents, and its not
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supported for request_firmware_into_buf().
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* Firmware is not accessible through typical means:
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* It cannot be installed into the root filesystem
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* The firmware provides very unique device specific data tailored for
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the unit gathered with local information. An example is calibration
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data for WiFi chipsets for mobile devices. This calibration data is
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not common to all units, but tailored per unit. Such information may
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be installed on a separate flash partition other than where the root
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filesystem is provided.
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Types of fallback mechanisms
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============================
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There are really two fallback mechanisms available using one shared sysfs
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interface as a loading facility:
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* Kobject uevent fallback mechanism
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* Custom fallback mechanism
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First lets document the shared sysfs loading facility.
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Firmware sysfs loading facility
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===============================
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In order to help device drivers upload firmware using a fallback mechanism
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the firmware infrastructure creates a sysfs interface to enable userspace
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to load and indicate when firmware is ready. The sysfs directory is created
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via fw_create_instance(). This call creates a new struct device named after
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the firmware requested, and establishes it in the device hierarchy by
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associating the device used to make the request as the device's parent.
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The sysfs directory's file attributes are defined and controlled through
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the new device's class (firmare_class) and group (fw_dev_attr_groups).
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This is actually where the original firmware_class.c file name comes from,
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as originally the only firmware loading mechanism available was the
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mechanism we now use as a fallback mechanism.
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To load firmware using the sysfs interface we expose a loading indicator,
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and a file upload firmware into:
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* /sys/$DEVPATH/loading
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* /sys/$DEVPATH/data
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To upload firmware you will echo 1 onto the loading file to indicate
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you are loading firmware. You then cat the firmware into the data file,
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and you notify the kernel the firmware is ready by echo'ing 0 onto
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the loading file.
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The firmware device used to help load firmware using sysfs is only created if
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direct firmware loading fails and if the fallback mechanism is enabled for your
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firmware request, this is set up with fw_load_from_user_helper(). It is
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important to re-iterate that no device is created if a direct filesystem lookup
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succeeded.
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Using::
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echo 1 > /sys/$DEVPATH/loading
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Will clean any previous partial load at once and make the firmware API
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return an error. When loading firmware the firmware_class grows a buffer
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for the firmware in PAGE_SIZE increments to hold the image as it comes in.
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firmware_data_read() and firmware_loading_show() are just provided for the
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test_firmware driver for testing, they are not called in normal use or
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expected to be used regularly by userspace.
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Firmware kobject uevent fallback mechanism
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==========================================
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Since a device is created for the sysfs interface to help load firmware as a
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fallback mechanism userspace can be informed of the addition of the device by
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relying on kobject uevents. The addition of the device into the device
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hierarchy means the fallback mechanism for firmware loading has been initiated.
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For details of implementation refer to _request_firmware_load(), in particular
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on the use of dev_set_uevent_suppress() and kobject_uevent().
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The kernel's kobject uevent mechanism is implemented in lib/kobject_uevent.c,
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it issues uevents to userspace. As a supplement to kobject uevents Linux
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distributions could also enable CONFIG_UEVENT_HELPER_PATH, which makes use of
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core kernel's usermode helper (UMH) functionality to call out to a userspace
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helper for kobject uevents. In practice though no standard distribution has
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ever used the CONFIG_UEVENT_HELPER_PATH. If CONFIG_UEVENT_HELPER_PATH is
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enabled this binary would be called each time kobject_uevent_env() gets called
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in the kernel for each kobject uevent triggered.
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Different implementations have been supported in userspace to take advantage of
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this fallback mechanism. When firmware loading was only possible using the
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sysfs mechanism the userspace component "hotplug" provided the functionality of
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monitoring for kobject events. Historically this was superseded be systemd's
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udev, however firmware loading support was removed from udev as of systemd
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commit be2ea723b1d0 ("udev: remove userspace firmware loading support")
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as of v217 on August, 2014. This means most Linux distributions today are
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not using or taking advantage of the firmware fallback mechanism provided
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by kobject uevents. This is specially exacerbated due to the fact that most
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distributions today disable CONFIG_FW_LOADER_USER_HELPER_FALLBACK.
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Refer to do_firmware_uevent() for details of the kobject event variables
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setup. Variables passwdd with a kobject add event:
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* FIRMWARE=firmware name
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* TIMEOUT=timeout value
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* ASYNC=whether or not the API request was asynchronous
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By default DEVPATH is set by the internal kernel kobject infrastructure.
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Below is an example simple kobject uevent script::
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# Both $DEVPATH and $FIRMWARE are already provided in the environment.
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MY_FW_DIR=/lib/firmware/
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echo 1 > /sys/$DEVPATH/loading
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cat $MY_FW_DIR/$FIRMWARE > /sys/$DEVPATH/data
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echo 0 > /sys/$DEVPATH/loading
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Firmware custom fallback mechanism
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==================================
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Users of the request_firmware_nowait() call have yet another option available
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at their disposal: rely on the sysfs fallback mechanism but request that no
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kobject uevents be issued to userspace. The original logic behind this
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was that utilities other than udev might be required to lookup firmware
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in non-traditional paths -- paths outside of the listing documented in the
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section 'Direct filesystem lookup'. This option is not available to any of
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the other API calls as uevents are always forced for them.
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Since uevents are only meaningful if the fallback mechanism is enabled
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in your kernel it would seem odd to enable uevents with kernels that do not
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have the fallback mechanism enabled in their kernels. Unfortunately we also
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rely on the uevent flag which can be disabled by request_firmware_nowait() to
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also setup the firmware cache for firmware requests. As documented above,
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the firmware cache is only set up if uevent is enabled for an API call.
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Although this can disable the firmware cache for request_firmware_nowait()
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calls, users of this API should not use it for the purposes of disabling
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the cache as that was not the original purpose of the flag. Not setting
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the uevent flag means you want to opt-in for the firmware fallback mechanism
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but you want to suppress kobject uevents, as you have a custom solution which
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will monitor for your device addition into the device hierarchy somehow and
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load firmware for you through a custom path.
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Firmware fallback timeout
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=========================
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The firmware fallback mechanism has a timeout. If firmware is not loaded
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onto the sysfs interface by the timeout value an error is sent to the
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driver. By default the timeout is set to 60 seconds if uevents are
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desirable, otherwise MAX_JIFFY_OFFSET is used (max timeout possible).
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The logic behind using MAX_JIFFY_OFFSET for non-uevents is that a custom
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solution will have as much time as it needs to load firmware.
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You can customize the firmware timeout by echo'ing your desired timeout into
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the following file:
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* /sys/class/firmware/timeout
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If you echo 0 into it means MAX_JIFFY_OFFSET will be used. The data type
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for the timeout is an int.
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==============
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Firmware cache
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==============
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When Linux resumes from suspend some device drivers require firmware lookups to
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re-initialize devices. During resume there may be a period of time during which
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firmware lookups are not possible, during this short period of time firmware
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requests will fail. Time is of essence though, and delaying drivers to wait for
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the root filesystem for firmware delays user experience with device
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functionality. In order to support these requirements the firmware
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infrastructure implements a firmware cache for device drivers for most API
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calls, automatically behind the scenes.
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The firmware cache makes using certain firmware API calls safe during a device
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driver's suspend and resume callback. Users of these API calls needn't cache
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the firmware by themselves for dealing with firmware loss during system resume.
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The firmware cache works by requesting for firmware prior to suspend and
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caching it in memory. Upon resume device drivers using the firmware API will
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have access to the firmware immediately, without having to wait for the root
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filesystem to mount or dealing with possible race issues with lookups as the
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root filesystem mounts.
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Some implementation details about the firmware cache setup:
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* The firmware cache is setup by adding a devres entry for each device that
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uses all synchronous call except :c:func:`request_firmware_into_buf`.
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* If an asynchronous call is used the firmware cache is only set up for a
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device if if the second argument (uevent) to request_firmware_nowait() is
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true. When uevent is true it requests that a kobject uevent be sent to
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userspace for the firmware request. For details refer to the Fackback
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mechanism documented below.
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* If the firmware cache is determined to be needed as per the above two
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criteria the firmware cache is setup by adding a devres entry for the
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device making the firmware request.
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* The firmware devres entry is maintained throughout the lifetime of the
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device. This means that even if you release_firmware() the firmware cache
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will still be used on resume from suspend.
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* The timeout for the fallback mechanism is temporarily reduced to 10 seconds
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as the firmware cache is set up during suspend, the timeout is set back to
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the old value you had configured after the cache is set up.
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* Upon suspend any pending non-uevent firmware requests are killed to avoid
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stalling the kernel, this is done with kill_requests_without_uevent(). Kernel
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calls requiring the non-uevent therefore need to implement their own firmware
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cache mechanism but must not use the firmware API on suspend.
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=====================
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Firmware search paths
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=====================
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The following search paths are used to look for firmware on your
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root filesystem.
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* fw_path_para - module parameter - default is empty so this is ignored
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* /lib/firmware/updates/UTS_RELEASE/
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* /lib/firmware/updates/
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* /lib/firmware/UTS_RELEASE/
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* /lib/firmware/
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The module parameter ''path'' can be passed to the firmware_class module
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to activate the first optional custom fw_path_para. The custom path can
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only be up to 256 characters long. The kernel parameter passed would be:
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* 'firmware_class.path=$CUSTOMIZED_PATH'
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There is an alternative to customize the path at run time after bootup, you
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can use the file:
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* /sys/module/firmware_class/parameters/path
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You would echo into it your custom path and firmware requested will be
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searched for there first.
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==================
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Linux Firmware API
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==================
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.. toctree::
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|
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introduction
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core
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request_firmware
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.. only:: subproject and html
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Indices
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=======
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* :ref:`genindex`
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============
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Introduction
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============
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The firmware API enables kernel code to request files required
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for functionality from userspace, the uses vary:
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* Microcode for CPU errata
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* Device driver firmware, required to be loaded onto device
|
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microcontrollers
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* Device driver information data (calibration data, EEPROM overrides),
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some of which can be completely optional.
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Types of firmware requests
|
||||
==========================
|
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||||
There are two types of calls:
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* Synchronous
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* Asynchronous
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Which one you use vary depending on your requirements, the rule of thumb
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however is you should strive to use the asynchronous APIs unless you also
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are already using asynchronous initialization mechanisms which will not
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stall or delay boot. Even if loading firmware does not take a lot of time
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processing firmware might, and this can still delay boot or initialization,
|
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as such mechanisms such as asynchronous probe can help supplement drivers.
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|
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=====================
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Firmware lookup order
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=====================
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||||
Different functionality is available to enable firmware to be found.
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Below is chronological order of how firmware will be looked for once
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a driver issues a firmware API call.
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* The ''Built-in firmware'' is checked first, if the firmware is present we
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return it immediately
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* The ''Firmware cache'' is looked at next. If the firmware is found we
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return it immediately
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* The ''Direct filesystem lookup'' is performed next, if found we
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return it immediately
|
||||
* If no firmware has been found and the fallback mechanism was enabled
|
||||
the sysfs interface is created. After this either a kobject uevent
|
||||
is issued or the custom firmware loading is relied upon for firmware
|
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loading up to the timeout value.
|
||||
|
|
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|||
====================
|
||||
request_firmware API
|
||||
====================
|
||||
|
||||
You would typically load firmware and then load it into your device somehow.
|
||||
The typical firmware work flow is reflected below::
|
||||
|
||||
if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
|
||||
copy_fw_to_device(fw_entry->data, fw_entry->size);
|
||||
release_firmware(fw_entry);
|
||||
|
||||
Synchronous firmware requests
|
||||
=============================
|
||||
|
||||
Synchronous firmware requests will wait until the firmware is found or until
|
||||
an error is returned.
|
||||
|
||||
request_firmware
|
||||
----------------
|
||||
.. kernel-doc:: drivers/base/firmware_class.c
|
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:functions: request_firmware
|
||||
|
||||
request_firmware_direct
|
||||
-----------------------
|
||||
.. kernel-doc:: drivers/base/firmware_class.c
|
||||
:functions: request_firmware_direct
|
||||
|
||||
request_firmware_into_buf
|
||||
-------------------------
|
||||
.. kernel-doc:: drivers/base/firmware_class.c
|
||||
:functions: request_firmware_into_buf
|
||||
|
||||
Asynchronous firmware requests
|
||||
==============================
|
||||
|
||||
Asynchronous firmware requests allow driver code to not have to wait
|
||||
until the firmware or an error is returned. Function callbacks are
|
||||
provided so that when the firmware or an error is found the driver is
|
||||
informed through the callback. request_firmware_nowait() cannot be called
|
||||
in atomic contexts.
|
||||
|
||||
request_firmware_nowait
|
||||
-----------------------
|
||||
.. kernel-doc:: drivers/base/firmware_class.c
|
||||
:functions: request_firmware_nowait
|
||||
|
||||
request firmware API expected driver use
|
||||
========================================
|
||||
|
||||
Once an API call returns you process the firmware and then release the
|
||||
firmware. For example if you used request_firmware() and it returns,
|
||||
the driver has the firmware image accessible in fw_entry->{data,size}.
|
||||
If something went wrong request_firmware() returns non-zero and fw_entry
|
||||
is set to NULL. Once your driver is done with processing the firmware it
|
||||
can call call release_firmware(fw_entry) to release the firmware image
|
||||
and any related resource.
|
||||
|
|
@ -30,6 +30,7 @@ available subsections can be seen below.
|
|||
miscellaneous
|
||||
vme
|
||||
80211/index
|
||||
firmware/index
|
||||
|
||||
.. only:: subproject and html
|
||||
|
||||
|
|
|
|||
|
|
@ -1,128 +0,0 @@
|
|||
|
||||
request_firmware() hotplug interface:
|
||||
------------------------------------
|
||||
Copyright (C) 2003 Manuel Estrada Sainz
|
||||
|
||||
Why:
|
||||
---
|
||||
|
||||
Today, the most extended way to use firmware in the Linux kernel is linking
|
||||
it statically in a header file. Which has political and technical issues:
|
||||
|
||||
1) Some firmware is not legal to redistribute.
|
||||
2) The firmware occupies memory permanently, even though it often is just
|
||||
used once.
|
||||
3) Some people, like the Debian crowd, don't consider some firmware free
|
||||
enough and remove entire drivers (e.g.: keyspan).
|
||||
|
||||
High level behavior (mixed):
|
||||
============================
|
||||
|
||||
1), kernel(driver):
|
||||
- calls request_firmware(&fw_entry, $FIRMWARE, device)
|
||||
- kernel searches the firmware image with name $FIRMWARE directly
|
||||
in the below search path of root filesystem:
|
||||
User customized search path by module parameter 'path'[1]
|
||||
"/lib/firmware/updates/" UTS_RELEASE,
|
||||
"/lib/firmware/updates",
|
||||
"/lib/firmware/" UTS_RELEASE,
|
||||
"/lib/firmware"
|
||||
- If found, goto 7), else goto 2)
|
||||
|
||||
[1], the 'path' is a string parameter which length should be less
|
||||
than 256, user should pass 'firmware_class.path=$CUSTOMIZED_PATH'
|
||||
if firmware_class is built in kernel(the general situation)
|
||||
|
||||
2), userspace:
|
||||
- /sys/class/firmware/xxx/{loading,data} appear.
|
||||
- hotplug gets called with a firmware identifier in $FIRMWARE
|
||||
and the usual hotplug environment.
|
||||
- hotplug: echo 1 > /sys/class/firmware/xxx/loading
|
||||
|
||||
3), kernel: Discard any previous partial load.
|
||||
|
||||
4), userspace:
|
||||
- hotplug: cat appropriate_firmware_image > \
|
||||
/sys/class/firmware/xxx/data
|
||||
|
||||
5), kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
|
||||
comes in.
|
||||
|
||||
6), userspace:
|
||||
- hotplug: echo 0 > /sys/class/firmware/xxx/loading
|
||||
|
||||
7), kernel: request_firmware() returns and the driver has the firmware
|
||||
image in fw_entry->{data,size}. If something went wrong
|
||||
request_firmware() returns non-zero and fw_entry is set to
|
||||
NULL.
|
||||
|
||||
8), kernel(driver): Driver code calls release_firmware(fw_entry) releasing
|
||||
the firmware image and any related resource.
|
||||
|
||||
High level behavior (driver code):
|
||||
==================================
|
||||
|
||||
if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
|
||||
copy_fw_to_device(fw_entry->data, fw_entry->size);
|
||||
release_firmware(fw_entry);
|
||||
|
||||
Sample/simple hotplug script:
|
||||
============================
|
||||
|
||||
# Both $DEVPATH and $FIRMWARE are already provided in the environment.
|
||||
|
||||
HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/
|
||||
|
||||
echo 1 > /sys/$DEVPATH/loading
|
||||
cat $HOTPLUG_FW_DIR/$FIRMWARE > /sys/$DEVPATH/data
|
||||
echo 0 > /sys/$DEVPATH/loading
|
||||
|
||||
Random notes:
|
||||
============
|
||||
|
||||
- "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
|
||||
once and make request_firmware() return with error.
|
||||
|
||||
- firmware_data_read() and firmware_loading_show() are just provided
|
||||
for testing and completeness, they are not called in normal use.
|
||||
|
||||
- There is also /sys/class/firmware/timeout which holds a timeout in
|
||||
seconds for the whole load operation.
|
||||
|
||||
- request_firmware_nowait() is also provided for convenience in
|
||||
user contexts to request firmware asynchronously, but can't be called
|
||||
in atomic contexts.
|
||||
|
||||
|
||||
about in-kernel persistence:
|
||||
---------------------------
|
||||
Under some circumstances, as explained below, it would be interesting to keep
|
||||
firmware images in non-swappable kernel memory or even in the kernel image
|
||||
(probably within initramfs).
|
||||
|
||||
Note that this functionality has not been implemented.
|
||||
|
||||
- Why OPTIONAL in-kernel persistence may be a good idea sometimes:
|
||||
|
||||
- If the device that needs the firmware is needed to access the
|
||||
filesystem. When upon some error the device has to be reset and the
|
||||
firmware reloaded, it won't be possible to get it from userspace.
|
||||
e.g.:
|
||||
- A diskless client with a network card that needs firmware.
|
||||
- The filesystem is stored in a disk behind an scsi device
|
||||
that needs firmware.
|
||||
- Replacing buggy DSDT/SSDT ACPI tables on boot.
|
||||
Note: this would require the persistent objects to be included
|
||||
within the kernel image, probably within initramfs.
|
||||
|
||||
And the same device can be needed to access the filesystem or not depending
|
||||
on the setup, so I think that the choice on what firmware to make
|
||||
persistent should be left to userspace.
|
||||
|
||||
about firmware cache:
|
||||
--------------------
|
||||
After firmware cache mechanism is introduced during system sleep,
|
||||
request_firmware can be called safely inside device's suspend and
|
||||
resume callback, and callers needn't cache the firmware by
|
||||
themselves any more for dealing with firmware loss during system
|
||||
resume.
|
||||
Loading…
Reference in New Issue