Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

 request_firmware() hotplug interface:
 ------------------------------------
        Copyright (C) 2003 Manuel Estrada Sainz

 Why:
 ---

 Today, the most extended way to use firmware in the Linux kernel is linking
 it statically in a header file. Which has political and technical issues:

  1) Some firmware is not legal to redistribute.
  2) The firmware occupies memory permanently, even though it often is just
     used once.
  3) Some people, like the Debian crowd, don't consider some firmware free
     enough and remove entire drivers (e.g.: keyspan).

 High level behavior (mixed):
 ============================

 kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device)

 userspace:
        - /sys/class/firmware/xxx/{loading,data} appear.
        - hotplug gets called with a firmware identifier in $FIRMWARE
          and the usual hotplug environment.
                - hotplug: echo 1 > /sys/class/firmware/xxx/loading

 kernel: Discard any previous partial load.

 userspace:
                - hotplug: cat appropriate_firmware_image > \
                                        /sys/class/firmware/xxx/data

 kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
         comes in.

 userspace:
                - hotplug: echo 0 > /sys/class/firmware/xxx/loading

 kernel: request_firmware() returns and the driver has the firmware
         image in fw_entry->{data,size}. If something went wrong
         request_firmware() returns non-zero and fw_entry is set to
         NULL.

 kernel(driver): Driver code calls release_firmware(fw_entry) releasing
                 the firmware image and any related resource.

 High level behavior (driver code):
 ==================================

         if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
                copy_fw_to_device(fw_entry->data, fw_entry->size);
         release(fw_entry);

 Sample/simple hotplug script:
 ============================

        # Both $DEVPATH and $FIRMWARE are already provided in the environment.

        HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/

        echo 1 > /sys/$DEVPATH/loading
        cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data
        echo 0 > /sys/$DEVPATH/loading

 Random notes:
 ============

 - "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
   once and make request_firmware() return with error.

 - firmware_data_read() and firmware_loading_show() are just provided
   for testing and completeness, they are not called in normal use.

 - There is also /sys/class/firmware/timeout which holds a timeout in
   seconds for the whole load operation.

 - request_firmware_nowait() is also provided for convenience in
   non-user contexts.


 about in-kernel persistence:
 ---------------------------
 Under some circumstances, as explained below, it would be interesting to keep
 firmware images in non-swappable kernel memory or even in the kernel image
 (probably within initramfs).

 Note that this functionality has not been implemented.

 - Why OPTIONAL in-kernel persistence may be a good idea sometimes:
 
        - If the device that needs the firmware is needed to access the
          filesystem. When upon some error the device has to be reset and the
          firmware reloaded, it won't be possible to get it from userspace.
          e.g.:
                - A diskless client with a network card that needs firmware.
                - The filesystem is stored in a disk behind an scsi device
                  that needs firmware.
        - Replacing buggy DSDT/SSDT ACPI tables on boot.
          Note: this would require the persistent objects to be included
          within the kernel image, probably within initramfs.
          
   And the same device can be needed to access the filesystem or not depending
   on the setup, so I think that the choice on what firmware to make
   persistent should be left to userspace.