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Diffstat (limited to 'kernel/Documentation/firmware_class/README')
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diff --git a/kernel/Documentation/firmware_class/README b/kernel/Documentation/firmware_class/README new file mode 100644 index 000000000..71f86859d --- /dev/null +++ b/kernel/Documentation/firmware_class/README @@ -0,0 +1,128 @@ + + 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_firmware(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 > /sys/$DEVPATH/data + echo 0 > /sys/$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 needn't cache the firmware by + themselves any more for dealing with firmware loss during system + resume. |