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