1
2 request_firmware() hotplug interface:
3 ------------------------------------
4	Copyright (C) 2003 Manuel Estrada Sainz
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
80   user contexts to request firmware asynchronously, but can't be called
81   in atomic contexts.
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
109