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125 lines
4.3 KiB
Plaintext
125 lines
4.3 KiB
Plaintext
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request_firmware() hotplug interface:
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------------------------------------
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Copyright (C) 2003 Manuel Estrada Sainz <ranty@debian.org>
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Why:
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---
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Today, the most extended way to use firmware in the Linux kernel is linking
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it statically in a header file. Which has political and technical issues:
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1) Some firmware is not legal to redistribute.
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2) The firmware occupies memory permanently, even though it often is just
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used once.
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3) Some people, like the Debian crowd, don't consider some firmware free
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enough and remove entire drivers (e.g.: keyspan).
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High level behavior (mixed):
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============================
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kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device)
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userspace:
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- /sys/class/firmware/xxx/{loading,data} appear.
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- hotplug gets called with a firmware identifier in $FIRMWARE
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and the usual hotplug environment.
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- hotplug: echo 1 > /sys/class/firmware/xxx/loading
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kernel: Discard any previous partial load.
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userspace:
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- hotplug: cat appropriate_firmware_image > \
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/sys/class/firmware/xxx/data
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kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
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comes in.
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userspace:
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- hotplug: echo 0 > /sys/class/firmware/xxx/loading
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kernel: request_firmware() returns and the driver has the firmware
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image in fw_entry->{data,size}. If something went wrong
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request_firmware() returns non-zero and fw_entry is set to
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NULL.
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kernel(driver): Driver code calls release_firmware(fw_entry) releasing
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the firmware image and any related resource.
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High level behavior (driver code):
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==================================
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if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
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copy_fw_to_device(fw_entry->data, fw_entry->size);
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release(fw_entry);
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Sample/simple hotplug script:
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============================
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# Both $DEVPATH and $FIRMWARE are already provided in the environment.
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HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/
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echo 1 > /sys/$DEVPATH/loading
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cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data
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echo 0 > /sys/$DEVPATH/loading
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Random notes:
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============
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- "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
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once and make request_firmware() return with error.
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- firmware_data_read() and firmware_loading_show() are just provided
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for testing and completeness, they are not called in normal use.
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- There is also /sys/class/firmware/timeout which holds a timeout in
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seconds for the whole load operation.
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- request_firmware_nowait() is also provided for convenience in
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non-user contexts.
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about in-kernel persistence:
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---------------------------
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Under some circumstances, as explained below, it would be interesting to keep
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firmware images in non-swappable kernel memory or even in the kernel image
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(probably within initramfs).
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Note that this functionality has not been implemented.
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- Why OPTIONAL in-kernel persistence may be a good idea sometimes:
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- If the device that needs the firmware is needed to access the
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filesystem. When upon some error the device has to be reset and the
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firmware reloaded, it won't be possible to get it from userspace.
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e.g.:
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- A diskless client with a network card that needs firmware.
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- The filesystem is stored in a disk behind an scsi device
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that needs firmware.
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- Replacing buggy DSDT/SSDT ACPI tables on boot.
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Note: this would require the persistent objects to be included
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within the kernel image, probably within initramfs.
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And the same device can be needed to access the filesystem or not depending
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on the setup, so I think that the choice on what firmware to make
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persistent should be left to userspace.
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- Why register_firmware()+__init can be useful:
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- For boot devices needing firmware.
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- To make the transition easier:
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The firmware can be declared __init and register_firmware()
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called on module_init. Then the firmware is warranted to be
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there even if "firmware hotplug userspace" is not there yet or
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it doesn't yet provide the needed firmware.
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Once the firmware is widely available in userspace, it can be
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removed from the kernel. Or made optional (CONFIG_.*_FIRMWARE).
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In either case, if firmware hotplug support is there, it can move the
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firmware out of kernel memory into the real filesystem for later
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usage.
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Note: If persistence is implemented on top of initramfs,
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register_firmware() may not be appropriate.
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