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diff --git a/Documentation/driver-api/firmware/fallback-mechanisms.rst b/Documentation/driver-api/firmware/fallback-mechanisms.rst new file mode 100644 index 000000000..5f04c3bcd --- /dev/null +++ b/Documentation/driver-api/firmware/fallback-mechanisms.rst @@ -0,0 +1,308 @@ +=================== +Fallback mechanisms +=================== + +A fallback mechanism is supported to allow to overcome failures to do a direct +filesystem lookup on the root filesystem or when the firmware simply cannot be +installed for practical reasons on the root filesystem. The kernel +configuration options related to supporting the firmware fallback mechanism are: + + * CONFIG_FW_LOADER_USER_HELPER: enables building the firmware fallback + mechanism. Most distributions enable this option today. If enabled but + CONFIG_FW_LOADER_USER_HELPER_FALLBACK is disabled, only the custom fallback + mechanism is available and for the request_firmware_nowait() call. + * CONFIG_FW_LOADER_USER_HELPER_FALLBACK: force enables each request to + enable the kobject uevent fallback mechanism on all firmware API calls + except request_firmware_direct(). Most distributions disable this option + today. The call request_firmware_nowait() allows for one alternative + fallback mechanism: if this kconfig option is enabled and your second + argument to request_firmware_nowait(), uevent, is set to false you are + informing the kernel that you have a custom fallback mechanism and it will + manually load the firmware. Read below for more details. + +Note that this means when having this configuration: + +CONFIG_FW_LOADER_USER_HELPER=y +CONFIG_FW_LOADER_USER_HELPER_FALLBACK=n + +the kobject uevent fallback mechanism will never take effect even +for request_firmware_nowait() when uevent is set to true. + +Justifying the firmware fallback mechanism +========================================== + +Direct filesystem lookups may fail for a variety of reasons. Known reasons for +this are worth itemizing and documenting as it justifies the need for the +fallback mechanism: + +* Race against access with the root filesystem upon bootup. + +* Races upon resume from suspend. This is resolved by the firmware cache, but + the firmware cache is only supported if you use uevents, and its not + supported for request_firmware_into_buf(). + +* Firmware is not accessible through typical means: + + * It cannot be installed into the root filesystem + * The firmware provides very unique device specific data tailored for + the unit gathered with local information. An example is calibration + data for WiFi chipsets for mobile devices. This calibration data is + not common to all units, but tailored per unit. Such information may + be installed on a separate flash partition other than where the root + filesystem is provided. + +Types of fallback mechanisms +============================ + +There are really two fallback mechanisms available using one shared sysfs +interface as a loading facility: + +* Kobject uevent fallback mechanism +* Custom fallback mechanism + +First lets document the shared sysfs loading facility. + +Firmware sysfs loading facility +=============================== + +In order to help device drivers upload firmware using a fallback mechanism +the firmware infrastructure creates a sysfs interface to enable userspace +to load and indicate when firmware is ready. The sysfs directory is created +via fw_create_instance(). This call creates a new struct device named after +the firmware requested, and establishes it in the device hierarchy by +associating the device used to make the request as the device's parent. +The sysfs directory's file attributes are defined and controlled through +the new device's class (firmware_class) and group (fw_dev_attr_groups). +This is actually where the original firmware_class module name came from, +given that originally the only firmware loading mechanism available was the +mechanism we now use as a fallback mechanism, which registers a struct class +firmware_class. Because the attributes exposed are part of the module name, the +module name firmware_class cannot be renamed in the future, to ensure backward +compatibility with old userspace. + +To load firmware using the sysfs interface we expose a loading indicator, +and a file upload firmware into: + + * /sys/$DEVPATH/loading + * /sys/$DEVPATH/data + +To upload firmware you will echo 1 onto the loading file to indicate +you are loading firmware. You then write the firmware into the data file, +and you notify the kernel the firmware is ready by echo'ing 0 onto +the loading file. + +The firmware device used to help load firmware using sysfs is only created if +direct firmware loading fails and if the fallback mechanism is enabled for your +firmware request, this is set up with :c:func:`firmware_fallback_sysfs`. It is +important to re-iterate that no device is created if a direct filesystem lookup +succeeded. + +Using:: + + echo 1 > /sys/$DEVPATH/loading + +Will clean any previous partial load at once and make the firmware API +return an error. When loading firmware the firmware_class grows a buffer +for the firmware in PAGE_SIZE increments to hold the image as it comes in. + +firmware_data_read() and firmware_loading_show() are just provided for the +test_firmware driver for testing, they are not called in normal use or +expected to be used regularly by userspace. + +firmware_fallback_sysfs +----------------------- +.. kernel-doc:: drivers/base/firmware_loader/fallback.c + :functions: firmware_fallback_sysfs + +Firmware kobject uevent fallback mechanism +========================================== + +Since a device is created for the sysfs interface to help load firmware as a +fallback mechanism userspace can be informed of the addition of the device by +relying on kobject uevents. The addition of the device into the device +hierarchy means the fallback mechanism for firmware loading has been initiated. +For details of implementation refer to fw_load_sysfs_fallback(), in particular +on the use of dev_set_uevent_suppress() and kobject_uevent(). + +The kernel's kobject uevent mechanism is implemented in lib/kobject_uevent.c, +it issues uevents to userspace. As a supplement to kobject uevents Linux +distributions could also enable CONFIG_UEVENT_HELPER_PATH, which makes use of +core kernel's usermode helper (UMH) functionality to call out to a userspace +helper for kobject uevents. In practice though no standard distribution has +ever used the CONFIG_UEVENT_HELPER_PATH. If CONFIG_UEVENT_HELPER_PATH is +enabled this binary would be called each time kobject_uevent_env() gets called +in the kernel for each kobject uevent triggered. + +Different implementations have been supported in userspace to take advantage of +this fallback mechanism. When firmware loading was only possible using the +sysfs mechanism the userspace component "hotplug" provided the functionality of +monitoring for kobject events. Historically this was superseded be systemd's +udev, however firmware loading support was removed from udev as of systemd +commit be2ea723b1d0 ("udev: remove userspace firmware loading support") +as of v217 on August, 2014. This means most Linux distributions today are +not using or taking advantage of the firmware fallback mechanism provided +by kobject uevents. This is specially exacerbated due to the fact that most +distributions today disable CONFIG_FW_LOADER_USER_HELPER_FALLBACK. + +Refer to do_firmware_uevent() for details of the kobject event variables +setup. The variables currently passed to userspace with a "kobject add" +event are: + +* FIRMWARE=firmware name +* TIMEOUT=timeout value +* ASYNC=whether or not the API request was asynchronous + +By default DEVPATH is set by the internal kernel kobject infrastructure. +Below is an example simple kobject uevent script:: + + # Both $DEVPATH and $FIRMWARE are already provided in the environment. + MY_FW_DIR=/lib/firmware/ + echo 1 > /sys/$DEVPATH/loading + cat $MY_FW_DIR/$FIRMWARE > /sys/$DEVPATH/data + echo 0 > /sys/$DEVPATH/loading + +Firmware custom fallback mechanism +================================== + +Users of the request_firmware_nowait() call have yet another option available +at their disposal: rely on the sysfs fallback mechanism but request that no +kobject uevents be issued to userspace. The original logic behind this +was that utilities other than udev might be required to lookup firmware +in non-traditional paths -- paths outside of the listing documented in the +section 'Direct filesystem lookup'. This option is not available to any of +the other API calls as uevents are always forced for them. + +Since uevents are only meaningful if the fallback mechanism is enabled +in your kernel it would seem odd to enable uevents with kernels that do not +have the fallback mechanism enabled in their kernels. Unfortunately we also +rely on the uevent flag which can be disabled by request_firmware_nowait() to +also setup the firmware cache for firmware requests. As documented above, +the firmware cache is only set up if uevent is enabled for an API call. +Although this can disable the firmware cache for request_firmware_nowait() +calls, users of this API should not use it for the purposes of disabling +the cache as that was not the original purpose of the flag. Not setting +the uevent flag means you want to opt-in for the firmware fallback mechanism +but you want to suppress kobject uevents, as you have a custom solution which +will monitor for your device addition into the device hierarchy somehow and +load firmware for you through a custom path. + +Firmware fallback timeout +========================= + +The firmware fallback mechanism has a timeout. If firmware is not loaded +onto the sysfs interface by the timeout value an error is sent to the +driver. By default the timeout is set to 60 seconds if uevents are +desirable, otherwise MAX_JIFFY_OFFSET is used (max timeout possible). +The logic behind using MAX_JIFFY_OFFSET for non-uevents is that a custom +solution will have as much time as it needs to load firmware. + +You can customize the firmware timeout by echo'ing your desired timeout into +the following file: + +* /sys/class/firmware/timeout + +If you echo 0 into it means MAX_JIFFY_OFFSET will be used. The data type +for the timeout is an int. + +EFI embedded firmware fallback mechanism +======================================== + +On some devices the system's EFI code / ROM may contain an embedded copy +of firmware for some of the system's integrated peripheral devices and +the peripheral's Linux device-driver needs to access this firmware. + +Device drivers which need such firmware can use the +firmware_request_platform() function for this, note that this is a +separate fallback mechanism from the other fallback mechanisms and +this does not use the sysfs interface. + +A device driver which needs this can describe the firmware it needs +using an efi_embedded_fw_desc struct: + +.. kernel-doc:: include/linux/efi_embedded_fw.h + :functions: efi_embedded_fw_desc + +The EFI embedded-fw code works by scanning all EFI_BOOT_SERVICES_CODE memory +segments for an eight byte sequence matching prefix; if the prefix is found it +then does a sha256 over length bytes and if that matches makes a copy of length +bytes and adds that to its list with found firmwares. + +To avoid doing this somewhat expensive scan on all systems, dmi matching is +used. Drivers are expected to export a dmi_system_id array, with each entries' +driver_data pointing to an efi_embedded_fw_desc. + +To register this array with the efi-embedded-fw code, a driver needs to: + +1. Always be builtin to the kernel or store the dmi_system_id array in a + separate object file which always gets builtin. + +2. Add an extern declaration for the dmi_system_id array to + include/linux/efi_embedded_fw.h. + +3. Add the dmi_system_id array to the embedded_fw_table in + drivers/firmware/efi/embedded-firmware.c wrapped in a #ifdef testing that + the driver is being builtin. + +4. Add "select EFI_EMBEDDED_FIRMWARE if EFI_STUB" to its Kconfig entry. + +The firmware_request_platform() function will always first try to load firmware +with the specified name directly from the disk, so the EFI embedded-fw can +always be overridden by placing a file under /lib/firmware. + +Note that: + +1. The code scanning for EFI embedded-firmware runs near the end + of start_kernel(), just before calling rest_init(). For normal drivers and + subsystems using subsys_initcall() to register themselves this does not + matter. This means that code running earlier cannot use EFI + embedded-firmware. + +2. At the moment the EFI embedded-fw code assumes that firmwares always start at + an offset which is a multiple of 8 bytes, if this is not true for your case + send in a patch to fix this. + +3. At the moment the EFI embedded-fw code only works on x86 because other archs + free EFI_BOOT_SERVICES_CODE before the EFI embedded-fw code gets a chance to + scan it. + +4. The current brute-force scanning of EFI_BOOT_SERVICES_CODE is an ad-hoc + brute-force solution. There has been discussion to use the UEFI Platform + Initialization (PI) spec's Firmware Volume protocol. This has been rejected + because the FV Protocol relies on *internal* interfaces of the PI spec, and: + 1. The PI spec does not define peripheral firmware at all + 2. The internal interfaces of the PI spec do not guarantee any backward + compatibility. Any implementation details in FV may be subject to change, + and may vary system to system. Supporting the FV Protocol would be + difficult as it is purposely ambiguous. + +Example how to check for and extract embedded firmware +------------------------------------------------------ + +To check for, for example Silead touchscreen controller embedded firmware, +do the following: + +1. Boot the system with efi=debug on the kernel commandline + +2. cp /sys/kernel/debug/efi/boot_services_code? to your home dir + +3. Open the boot_services_code? files in a hex-editor, search for the + magic prefix for Silead firmware: F0 00 00 00 02 00 00 00, this gives you + the beginning address of the firmware inside the boot_services_code? file. + +4. The firmware has a specific pattern, it starts with a 8 byte page-address, + typically F0 00 00 00 02 00 00 00 for the first page followed by 32-bit + word-address + 32-bit value pairs. With the word-address incrementing 4 + bytes (1 word) for each pair until a page is complete. A complete page is + followed by a new page-address, followed by more word + value pairs. This + leads to a very distinct pattern. Scroll down until this pattern stops, + this gives you the end of the firmware inside the boot_services_code? file. + +5. "dd if=boot_services_code? of=firmware bs=1 skip=<begin-addr> count=<len>" + will extract the firmware for you. Inspect the firmware file in a + hexeditor to make sure you got the dd parameters correct. + +6. Copy it to /lib/firmware under the expected name to test it. + +7. If the extracted firmware works, you can use the found info to fill an + efi_embedded_fw_desc struct to describe it, run "sha256sum firmware" + to get the sha256sum to put in the sha256 field. |