1
2 request_firmware() hotplug interface:
3 ------------------------------------
4 Copyright (C) 2003 Manuel Estrada Sainz <ranty@debian.org>
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 non-user contexts.
81
82
83 about in-kernel persistence:
84 ---------------------------
85 Under some circumstances, as explained below, it would be interesting to keep
86 firmware images in non-swappable kernel memory or even in the kernel image
87 (probably within initramfs).
88
89 Note that this functionality has not been implemented.
90
91 - Why OPTIONAL in-kernel persistence may be a good idea sometimes:
92
93 - If the device that needs the firmware is needed to access the
94 filesystem. When upon some error the device has to be reset and the
95 firmware reloaded, it won't be possible to get it from userspace.
96 e.g.:
97 - A diskless client with a network card that needs firmware.
98 - The filesystem is stored in a disk behind an scsi device
99 that needs firmware.
100 - Replacing buggy DSDT/SSDT ACPI tables on boot.
101 Note: this would require the persistent objects to be included
102 within the kernel image, probably within initramfs.
103
104 And the same device can be needed to access the filesystem or not depending
105 on the setup, so I think that the choice on what firmware to make
106 persistent should be left to userspace.
107
108 - Why register_firmware()+__init can be useful:
109 - For boot devices needing firmware.
110 - To make the transition easier:
111 The firmware can be declared __init and register_firmware()
112 called on module_init. Then the firmware is warranted to be
113 there even if "firmware hotplug userspace" is not there yet or
114 it doesn't yet provide the needed firmware.
115 Once the firmware is widely available in userspace, it can be
116 removed from the kernel. Or made optional (CONFIG_.*_FIRMWARE).
117
118 In either case, if firmware hotplug support is there, it can move the
119 firmware out of kernel memory into the real filesystem for later
120 usage.
121
122 Note: If persistence is implemented on top of initramfs,
123 register_firmware() may not be appropriate.
124
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