Documentation/firmware_class/README

   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