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 $FIRMWARE directly
  24        in the below search path of root filesystem:
  25                User customized search path by module parameter 'path'[1]
  26                "/lib/firmware/updates/" UTS_RELEASE,
  27                "/lib/firmware/updates",
  28                "/lib/firmware/" UTS_RELEASE,
  29                "/lib/firmware"
  30        - If found, goto 7), else goto 2)
  31
  32        [1], the 'path' is a string parameter which length should be less
  33        than 256, user should pass 'firmware_class.path=$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(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 > /sysfs/$DEVPATH/data
  78        echo 0 > /sys/$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 need't cache the firmware by
 127 themselves any more for dealing with firmware loss during system
 128 resume.
 129
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