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    <p>From the kernel's perspective, see <a href="http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/firmware_class/README;hb=HEAD" rel="noreferrer">/usr/src/linux/Documentation/firmware_class/README</a>:</p> <pre> 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 &gt; /sys/class/firmware/xxx/loading kernel: Discard any previous partial load. userspace: - hotplug: cat appropriate_firmware_image &gt; \ /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 &gt; /sys/class/firmware/xxx/loading kernel: request_firmware() returns and the driver has the firmware image in fw_entry-&gt;{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. </pre> <p>The kernel doesn't actually load any firmware at all. It simply informs userspace, "I want a firmware by the name of <em>xxx</em>", and waits for userspace to pipe the firmware image back to the kernel.</p> <p>Now, on Ubuntu 8.04,</p> <pre> $ grep firmware /etc/udev/rules.d/80-program.rules # Load firmware on demand SUBSYSTEM=="firmware", ACTION=="add", RUN+="firmware_helper" </pre> <p>so as you've discovered, <code>udev</code> is configured to run <code>firmware_helper</code> when the kernel asks for firmware.</p> <pre> $ 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 </pre> <p>If you read the source, you'll find that Ubuntu wrote a <code>firmware_helper</code> which is hard-coded to first look for <code>/lib/modules/$(uname -r)/$FIRMWARE</code>, then <code>/lib/modules/$FIRMWARE</code>, and no other locations. Translating it to <code>sh</code>, it does approximately this:</p> <pre><code>echo -n 1 &gt; /sys/$DEVPATH/loading cat /lib/firmware/$(uname -r)/$FIRMWARE &gt; /sys/$DEVPATH/data \ || cat /lib/firmware/$FIRMWARE &gt; /sys/$DEVPATH/data if [ $? = 0 ]; then echo -n 1 &gt; /sys/$DEVPATH/loading echo -n -1 &gt; /sys/$DEVPATH/loading fi </code></pre> <p>which is exactly the format the kernel expects.</p> <hr> <p>To make a long story short: Ubuntu's <code>udev</code> package has customizations that always look in <code>/lib/firmware/$(uname -r)</code> first. This policy is being handled in userspace.</p>
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    1. POHow does Linux Kernel know where to look for driver firmware?
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    1. POHow does Linux Kernel know where to look for driver firmware?
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    1. COWonderful answer, thanks! I was looking through the udev source pulling my hair out trying to figure out where Ubuntu's `firmware_helper` was coming from. Should have just done a recursive grep for firmware in the udev source tree.
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      1. USRobert S. Barnes
    2. COIt's actually probably from the udev-extras tree, some parts of which Ubuntu has decided to modify and bundle with their udev package.
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    3. CO`apt-get source` downloads and unpacks the Debian (or Ubuntu) source tree for the package, which includes a `debian` directory with packaging information and distribution-specific setup, which sometimes includes patches which aren't found upstream. This is a case of that.
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