[net-next-2.6.git] / 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):
 ============================

 kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device)

 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

 kernel: Discard any previous partial load.

 userspace:
		- hotplug: cat appropriate_firmware_image > \
					/sys/class/firmware/xxx/data

 kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
	 comes in.

 userspace:
		- hotplug: echo 0 > /sys/class/firmware/xxx/loading

 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.

 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(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 > /sysfs/$DEVPATH/data
	echo 0 > /sys/$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.
Commit	Line	Data
1da177e4 LT	1
	2	request_firmware() hotplug interface:
	3	------------------------------------
87d37a4f	4	Copyright (C) 2003 Manuel Estrada Sainz
1da177e4 LT	5
	6	Why:
	7	---
	8
	9	Today, the most extended way to use firmware in the Linux kernel is linking
	10	it statically in a header file. Which has political and technical issues:
	11
	12	1) Some firmware is not legal to redistribute.
	13	2) The firmware occupies memory permanently, even though it often is just
	14	used once.
	15	3) Some people, like the Debian crowd, don't consider some firmware free
	16	enough and remove entire drivers (e.g.: keyspan).
	17
	18	High level behavior (mixed):
	19	============================
	20
	21	kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device)
	22
	23	userspace:
	24	- /sys/class/firmware/xxx/{loading,data} appear.
	25	- hotplug gets called with a firmware identifier in $FIRMWARE
	26	and the usual hotplug environment.
	27	- hotplug: echo 1 > /sys/class/firmware/xxx/loading
	28
	29	kernel: Discard any previous partial load.
	30
	31	userspace:
	32	- hotplug: cat appropriate_firmware_image > \
	33	/sys/class/firmware/xxx/data
	34
	35	kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
	36	comes in.
	37
	38	userspace:
	39	- hotplug: echo 0 > /sys/class/firmware/xxx/loading
	40
	41	kernel: request_firmware() returns and the driver has the firmware
	42	image in fw_entry->{data,size}. If something went wrong
	43	request_firmware() returns non-zero and fw_entry is set to
	44	NULL.
	45
	46	kernel(driver): Driver code calls release_firmware(fw_entry) releasing
	47	the firmware image and any related resource.
	48
	49	High level behavior (driver code):
	50	==================================
	51
	52	if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
	53	copy_fw_to_device(fw_entry->data, fw_entry->size);
	54	release(fw_entry);
	55
	56	Sample/simple hotplug script:
	57	============================
	58
	59	# Both $DEVPATH and $FIRMWARE are already provided in the environment.
	60
	61	HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/
	62
	63	echo 1 > /sys/$DEVPATH/loading
	64	cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data
	65	echo 0 > /sys/$DEVPATH/loading
	66
	67	Random notes:
	68	============
69
70	- "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
71	once and make request_firmware() return with error.
72
73	- firmware_data_read() and firmware_loading_show() are just provided
74	for testing and completeness, they are not called in normal use.
75
76	- There is also /sys/class/firmware/timeout which holds a timeout in
77	seconds for the whole load operation.
78
79	- request_firmware_nowait() is also provided for convenience in
7fcab099 ML	80	user contexts to request firmware asynchronously, but can't be called
7fcab099 ML	81	in atomic contexts.
1da177e4 LT	82
	83
	84	about in-kernel persistence:
	85	---------------------------
	86	Under some circumstances, as explained below, it would be interesting to keep
	87	firmware images in non-swappable kernel memory or even in the kernel image
	88	(probably within initramfs).
	89
	90	Note that this functionality has not been implemented.
	91
	92	- Why OPTIONAL in-kernel persistence may be a good idea sometimes:
	93
	94	- If the device that needs the firmware is needed to access the
	95	filesystem. When upon some error the device has to be reset and the
	96	firmware reloaded, it won't be possible to get it from userspace.
	97	e.g.:
	98	- A diskless client with a network card that needs firmware.
	99	- The filesystem is stored in a disk behind an scsi device
	100	that needs firmware.
	101	- Replacing buggy DSDT/SSDT ACPI tables on boot.
	102	Note: this would require the persistent objects to be included
	103	within the kernel image, probably within initramfs.
	104
	105	And the same device can be needed to access the filesystem or not depending
	106	on the setup, so I think that the choice on what firmware to make
	107	persistent should be left to userspace.
	108