From the kernel’s perspective, see /usr/src/linux/Documentation/firmware_class/README:
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.
The kernel doesn’t actually load any firmware at all. It simply informs userspace, “I want a firmware by the name of xxx“, and waits for userspace to pipe the firmware image back to the kernel.
Now, on Ubuntu 8.04,
$ grep firmware /etc/udev/rules.d/80-program.rules # Load firmware on demand SUBSYSTEM=="firmware", ACTION=="add", RUN+="firmware_helper"
so as you’ve discovered, udev is configured to run firmware_helper when the kernel asks for firmware.
$ apt-get source udev Reading package lists... Done Building dependency tree Reading state information... Done Need to get 312kB of source archives. Get:1 http://us.archive.ubuntu.com hardy-security/main udev 117-8ubuntu0.2 (dsc) [716B] Get:2 http://us.archive.ubuntu.com hardy-security/main udev 117-8ubuntu0.2 (tar) [245kB] Get:3 http://us.archive.ubuntu.com hardy-security/main udev 117-8ubuntu0.2 (diff) [65.7kB] Fetched 312kB in 1s (223kB/s) gpg: Signature made Tue 14 Apr 2009 05:31:34 PM EDT using DSA key ID 17063E6D gpg: Can't check signature: public key not found dpkg-source: extracting udev in udev-117 dpkg-source: unpacking udev_117.orig.tar.gz dpkg-source: applying ./udev_117-8ubuntu0.2.diff.gz $ cd udev-117/ $ cat debian/patches/80-extras-firmware.patch
If you read the source, you’ll find that Ubuntu wrote a firmware_helper which is hard-coded to first look for /lib/modules/$(uname -r)/$FIRMWARE, then /lib/modules/$FIRMWARE, and no other locations. Translating it to sh, it does approximately this:
echo -n 1 > /sys/$DEVPATH/loading
cat /lib/firmware/$(uname -r)/$FIRMWARE > /sys/$DEVPATH/data \
|| cat /lib/firmware/$FIRMWARE > /sys/$DEVPATH/data
if [ $? = 0 ]; then
echo -n 1 > /sys/$DEVPATH/loading
echo -n -1 > /sys/$DEVPATH/loading
fi
which is exactly the format the kernel expects.
To make a long story short: Ubuntu’s udev package has customizations that always look in /lib/firmware/$(uname -r) first. This policy is being handled in userspace.