README
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 1), kernel(driver):
22 - calls request_firmware(&fw_entry, $FIRMWARE, device)
23 - kernel searchs the fimware image with name $FIRMWARE directly
24 in the below search path of root filesystem:
25 User customized search path by module parameter 'path'[1]
26 "/lib/firmware/updates/" UTS_RELEASE,
27 "/lib/firmware/updates",
28 "/lib/firmware/" UTS_RELEASE,
29 "/lib/firmware"
30 - If found, goto 7), else goto 2)
31
32 [1], the 'path' is a string parameter which length should be less
33 than 256, user should pass 'firmware_class.path=$CUSTOMIZED_PATH'
34 if firmware_class is built in kernel(the general situation)
35
36 2), userspace:
37 - /sys/class/firmware/xxx/{loading,data} appear.
38 - hotplug gets called with a firmware identifier in $FIRMWARE
39 and the usual hotplug environment.
40 - hotplug: echo 1 > /sys/class/firmware/xxx/loading
41
42 3), kernel: Discard any previous partial load.
43
44 4), userspace:
45 - hotplug: cat appropriate_firmware_image > \
46 /sys/class/firmware/xxx/data
47
48 5), kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
49 comes in.
50
51 6), userspace:
52 - hotplug: echo 0 > /sys/class/firmware/xxx/loading
53
54 7), kernel: request_firmware() returns and the driver has the firmware
55 image in fw_entry->{data,size}. If something went wrong
56 request_firmware() returns non-zero and fw_entry is set to
57 NULL.
58
59 8), kernel(driver): Driver code calls release_firmware(fw_entry) releasing
60 the firmware image and any related resource.
61
62 High level behavior (driver code):
63 ==================================
64
65 if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
66 copy_fw_to_device(fw_entry->data, fw_entry->size);
67 release_firmware(fw_entry);
68
69 Sample/simple hotplug script:
70 ============================
71
72 # Both $DEVPATH and $FIRMWARE are already provided in the environment.
73
74 HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/
75
76 echo 1 > /sys/$DEVPATH/loading
77 cat $HOTPLUG_FW_DIR/$FIRMWARE > /sys/$DEVPATH/data
78 echo 0 > /sys/$DEVPATH/loading
79
80 Random notes:
81 ============
82
83 - "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
84 once and make request_firmware() return with error.
85
86 - firmware_data_read() and firmware_loading_show() are just provided
87 for testing and completeness, they are not called in normal use.
88
89 - There is also /sys/class/firmware/timeout which holds a timeout in
90 seconds for the whole load operation.
91
92 - request_firmware_nowait() is also provided for convenience in
93 user contexts to request firmware asynchronously, but can't be called
94 in atomic contexts.
95
96
97 about in-kernel persistence:
98 ---------------------------
99 Under some circumstances, as explained below, it would be interesting to keep
100 firmware images in non-swappable kernel memory or even in the kernel image
101 (probably within initramfs).
102
103 Note that this functionality has not been implemented.
104
105 - Why OPTIONAL in-kernel persistence may be a good idea sometimes:
106
107 - If the device that needs the firmware is needed to access the
108 filesystem. When upon some error the device has to be reset and the
109 firmware reloaded, it won't be possible to get it from userspace.
110 e.g.:
111 - A diskless client with a network card that needs firmware.
112 - The filesystem is stored in a disk behind an scsi device
113 that needs firmware.
114 - Replacing buggy DSDT/SSDT ACPI tables on boot.
115 Note: this would require the persistent objects to be included
116 within the kernel image, probably within initramfs.
117
118 And the same device can be needed to access the filesystem or not depending
119 on the setup, so I think that the choice on what firmware to make
120 persistent should be left to userspace.
121
122 about firmware cache:
123 --------------------
124 After firmware cache mechanism is introduced during system sleep,
125 request_firmware can be called safely inside device's suspend and
126 resume callback, and callers needn't cache the firmware by
127 themselves any more for dealing with firmware loss during system
128 resume.
129