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+
+ 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):
+ ============================
+
+ 1), kernel(driver):
+ - calls request_firmware(&fw_entry, $FIRMWARE, device)
+ - kernel searchs the fimware image with name $FIRMWARE directly
+ in the below search path of root filesystem:
+ User customized search path by module parameter 'path'[1]
+ "/lib/firmware/updates/" UTS_RELEASE,
+ "/lib/firmware/updates",
+ "/lib/firmware/" UTS_RELEASE,
+ "/lib/firmware"
+ - If found, goto 7), else goto 2)
+
+ [1], the 'path' is a string parameter which length should be less
+ than 256, user should pass 'firmware_class.path=$CUSTOMIZED_PATH'
+ if firmware_class is built in kernel(the general situation)
+
+ 2), 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
+
+ 3), kernel: Discard any previous partial load.
+
+ 4), userspace:
+ - hotplug: cat appropriate_firmware_image > \
+ /sys/class/firmware/xxx/data
+
+ 5), kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
+ comes in.
+
+ 6), userspace:
+ - hotplug: echo 0 > /sys/class/firmware/xxx/loading
+
+ 7), 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.
+
+ 8), 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
+ user contexts to request firmware asynchronously, but can't be called
+ in atomic 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.
+
+ about firmware cache:
+ --------------------
+ After firmware cache mechanism is introduced during system sleep,
+ request_firmware can be called safely inside device's suspend and
+ resume callback, and callers need't cache the firmware by
+ themselves any more for dealing with firmware loss during system
+ resume.