linux/Documentation/firmware_class/README
<<
>>
Prefs
   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
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.