Jetpack/kernel/kernel-4.9/Documentation/firmware_class

 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.