request_firmware() hotplug interface:
------------------------------------
Copyright (C) 2003 Manuel Estrada Sainz <ranty@debian.org>
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.
- Why register_firmware()+__init can be useful:
- For boot devices needing firmware.
- To make the transition easier:
The firmware can be declared __init and register_firmware()
called on module_init. Then the firmware is warranted to be
there even if "firmware hotplug userspace" is not there yet or
it doesn't yet provide the needed firmware.
Once the firmware is widely available in userspace, it can be
removed from the kernel. Or made optional (CONFIG_.*_FIRMWARE).
In either case, if firmware hotplug support is there, it can move the
firmware out of kernel memory into the real filesystem for later
usage.
Note: If persistence is implemented on top of initramfs,
register_firmware() may not be appropriate.
