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>

Documentation/driver-api/firmware/built-in-fw.rst

+=================
+Built-in firmware
+=================
+
+Firmware can be built-in to the kernel, this means building the firmware
+into vmlinux directly, to enable avoiding having to look for firmware from
+the filesystem. Instead, firmware can be looked for inside the kernel
+directly. You can enable built-in firmware using the kernel configuration
+options:
+
+  * CONFIG_EXTRA_FIRMWARE
+  * CONFIG_EXTRA_FIRMWARE_DIR
+
+This should not be confused with CONFIG_FIRMWARE_IN_KERNEL, this is for drivers
+which enables firmware to be built as part of the kernel build process. This
+option, CONFIG_FIRMWARE_IN_KERNEL, will build all firmware for all drivers
+enabled which ship its firmware inside the Linux kernel source tree.
+
+There are a few reasons why you might want to consider building your firmware
+into the kernel with CONFIG_EXTRA_FIRMWARE though:
+
+* Speed
+* Firmware is needed for accessing the boot device, and the user doesn't
+  want to stuff the firmware into the boot initramfs.
+
+Even if you have these needs there are a few reasons why you may not be
+able to make use of built-in firmware:
+
+* Legalese - firmware is non-GPL compatible
+* Some firmware may be optional
+* Firmware upgrades are possible, therefore a new firmware would implicate
+  a complete kernel rebuild.
+* Some firmware files may be really large in size. The remote-proc subsystem
+  is an example subsystem which deals with these sorts of firmware
+* The firmware may need to be scraped out from some device specific location
+  dynamically, an example is calibration data for for some WiFi chipsets. This
+  calibration data can be unique per sold device.
+

Documentation/driver-api/firmware/core.rst

+==========================
+Firmware API core features
+==========================
+
+The firmware API has a rich set of core features available. This section
+documents these features.
+
+.. toctree::
+
+   fw_search_path
+   built-in-fw
+   firmware_cache
+   direct-fs-lookup
+   fallback-mechanisms
+   lookup-order
+

Documentation/driver-api/firmware/direct-fs-lookup.rst

+========================
+Direct filesystem lookup
+========================
+
+Direct filesystem lookup is the most common form of firmware lookup performed
+by the kernel. The kernel looks for the firmware directly on the root
+filesystem in the paths documented in the section 'Firmware search paths'.
+The filesystem lookup is implemented in fw_get_filesystem_firmware(), it
+uses common core kernel file loader facility kernel_read_file_from_path().
+The max path allowed is PATH_MAX -- currently this is 4096 characters.
+
+It is recommended you keep /lib/firmware paths on your root filesystem,
+avoid having a separate partition for them in order to avoid possible
+races with lookups and avoid uses of the custom fallback mechanisms
+documented below.
+
+Firmware and initramfs
+----------------------
+
+Drivers which are built-in to the kernel should have the firmware integrated
+also as part of the initramfs used to boot the kernel given that otherwise
+a race is possible with loading the driver and the real rootfs not yet being
+available. Stuffing the firmware into initramfs resolves this race issue,
+however note that using initrd does not suffice to address the same race.
+
+There are circumstances that justify not wanting to include firmware into
+initramfs, such as dealing with large firmware firmware files for the
+remote-proc subsystem. For such cases using a userspace fallback mechanism
+is currently the only viable solution as only userspace can know for sure
+when the real rootfs is ready and mounted.

Documentation/driver-api/firmware/fallback-mechanisms.rst

+===================
+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 (firmare_class) and group (fw_dev_attr_groups).
+This is actually where the original firmware_class.c file name comes from,
+as originally the only firmware loading mechanism available was the
+mechanism we now use as a fallback mechanism.
+
+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 cat 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 fw_load_from_user_helper().  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 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 _request_firmware_load(), 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. Variables passwdd with a kobject add event:
+
+* 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.

Documentation/driver-api/firmware/firmware_cache.rst

+==============
+Firmware cache
+==============
+
+When Linux resumes from suspend some device drivers require firmware lookups to
+re-initialize devices. During resume there may be a period of time during which
+firmware lookups are not possible, during this short period of time firmware
+requests will fail. Time is of essence though, and delaying drivers to wait for
+the root filesystem for firmware delays user experience with device
+functionality. In order to support these requirements the firmware
+infrastructure implements a firmware cache for device drivers for most API
+calls, automatically behind the scenes.
+
+The firmware cache makes using certain firmware API calls safe during a device
+driver's suspend and resume callback.  Users of these API calls needn't cache
+the firmware by themselves for dealing with firmware loss during system resume.
+
+The firmware cache works by requesting for firmware prior to suspend and
+caching it in memory. Upon resume device drivers using the firmware API will
+have access to the firmware immediately, without having to wait for the root
+filesystem to mount or dealing with possible race issues with lookups as the
+root filesystem mounts.
+
+Some implementation details about the firmware cache setup:
+
+* The firmware cache is setup by adding a devres entry for each device that
+  uses all synchronous call except :c:func:`request_firmware_into_buf`.
+
+* If an asynchronous call is used the firmware cache is only set up for a
+  device if if the second argument (uevent) to request_firmware_nowait() is
+  true. When uevent is true it requests that a kobject uevent be sent to
+  userspace for the firmware request. For details refer to the Fackback
+  mechanism documented below.
+
+* If the firmware cache is determined to be needed as per the above two
+  criteria the firmware cache is setup by adding a devres entry for the
+  device making the firmware request.
+
+* The firmware devres entry is maintained throughout the lifetime of the
+  device. This means that even if you release_firmware() the firmware cache
+  will still be used on resume from suspend.
+
+* The timeout for the fallback mechanism is temporarily reduced to 10 seconds
+  as the firmware cache is set up during suspend, the timeout is set back to
+  the old value you had configured after the cache is set up.
+
+* Upon suspend any pending non-uevent firmware requests are killed to avoid
+  stalling the kernel, this is done with kill_requests_without_uevent(). Kernel
+  calls requiring the non-uevent therefore need to implement their own firmware
+  cache mechanism but must not use the firmware API on suspend.
+

Documentation/driver-api/firmware/fw_search_path.rst

+=====================
+Firmware search paths
+=====================
+
+The following search paths are used to look for firmware on your
+root filesystem.
+
+* fw_path_para - module parameter - default is empty so this is ignored
+* /lib/firmware/updates/UTS_RELEASE/
+* /lib/firmware/updates/
+* /lib/firmware/UTS_RELEASE/
+* /lib/firmware/
+
+The module parameter ''path'' can be passed to the firmware_class module
+to activate the first optional custom fw_path_para. The custom path can
+only be up to 256 characters long. The kernel parameter passed would be:
+
+* 'firmware_class.path=$CUSTOMIZED_PATH'
+
+There is an alternative to customize the path at run time after bootup, you
+can use the file:
+
+* /sys/module/firmware_class/parameters/path
+
+You would echo into it your custom path and firmware requested will be
+searched for there first.

Documentation/driver-api/firmware/index.rst

+==================
+Linux Firmware API
+==================
+
+.. toctree::
+
+   introduction
+   core
+   request_firmware
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/driver-api/firmware/introduction.rst

+============
+Introduction
+============
+
+The firmware API enables kernel code to request files required
+for functionality from userspace, the uses vary:
+
+* Microcode for CPU errata
+* Device driver firmware, required to be loaded onto device
+  microcontrollers
+* Device driver information data (calibration data, EEPROM overrides),
+  some of which can be completely optional.
+
+Types of firmware requests
+==========================
+
+There are two types of calls:
+
+* Synchronous
+* Asynchronous
+
+Which one you use vary depending on your requirements, the rule of thumb
+however is you should strive to use the asynchronous APIs unless you also
+are already using asynchronous initialization mechanisms which will not
+stall or delay boot. Even if loading firmware does not take a lot of time
+processing firmware might, and this can still delay boot or initialization,
+as such mechanisms such as asynchronous probe can help supplement drivers.

Documentation/driver-api/firmware/lookup-order.rst

+=====================
+Firmware lookup order
+=====================
+
+Different functionality is available to enable firmware to be found.
+Below is chronological order of how firmware will be looked for once
+a driver issues a firmware API call.
+
+* The ''Built-in firmware'' is checked first, if the firmware is present we
+  return it immediately
+* The ''Firmware cache'' is looked at next. If the firmware is found we
+  return it immediately
+* The ''Direct filesystem lookup'' is performed next, if found we
+  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
+  loading up to the timeout value.

Documentation/driver-api/firmware/request_firmware.rst

+====================
+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
+   :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.

Documentation/driver-api/index.rst

@@ -30,6 +30,7 @@ available subsections can be seen below.
    miscellaneous
    vme
    80211/index
+   firmware/index
 
 .. only::  subproject and html
 

Documentation/firmware_class/README

-
- 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.