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    copied!<p>Wow this is very useful information and it led me to the solution for my problem when making a custom USB kernel module for a device requiring firmware.</p> <p>Basically, every Ubuntu brings a new rehash of hal,sysfs,devfs,udev,and so on...and things just change. In fact I read they stopped using hal.</p> <p>So let's reverse engineer this yet again so it's pertinent to the latest [Ubuntu] systems.</p> <p>On Ubuntu Lucid (the latest at time of writing), <code>/lib/udev/rules.d/50-firmware.rules</code> is used. This file calls the binary <code>/lib/udev/firmware</code>, where magic happens.</p> <p>Listing: /lib/udev/rules.d/50-firmware.rules</p> <pre><code># firmware-class requests, copies files into the kernel SUBSYSTEM=="firmware", ACTION=="add", RUN+="firmware --firmware=$env{FIRMWARE} --devpath=$env{DEVPATH}" </code></pre> <p>The magic should be something along these lines (source: <a href="http://lwn.net/Kernel/LDD3/" rel="noreferrer"><em>Linux Device Drivers, 3rd Ed., Ch. 14: The Linux Device Model</a></em>):</p> <ul> <li>echo 1 to <code>loading</code></li> <li>copy firmware to <code>data</code></li> <li>on failure, echo -1 to <code>loading</code> and halt firmware loading process</li> <li>echo 0 to <code>loading</code> (signal the kernel)</li> <li>then, a specific kernel module receives the data and pushes it to the device</li> </ul> <p>If you look at Lucid's source page for udev, in <code>udev-151/extras/firmware/firmware.c</code>, the source for that firmware /lib/udev/firmware binary, that's exactly what goes on.</p> <p>Excerpt: Lucid source, udev-151/extras/firmware/firmware.c</p> <pre><code> util_strscpyl(datapath, sizeof(datapath), udev_get_sys_path(udev), devpath, "/data", NULL); if (!copy_firmware(udev, fwpath, datapath, statbuf.st_size)) { err(udev, "error sending firmware '%s' to device\n", firmware); set_loading(udev, loadpath, "-1"); rc = 4; goto exit; }; set_loading(udev, loadpath, "0"); </code></pre> <p>Additionally, many devices use an Intel HEX format (textish files containing checksum and other stuff) (wiki it i have no reputation and no ability to link). The kernel program ihex2fw (called from Makefile in kernel_source/lib/firmware on .HEX files) converts these HEX files to an arbitrary-designed binary format that the Linux kernel then picks up with <code>request_ihex_firmware</code>, because they thought reading text files in the kernel was silly (it would slow things down).</p>
 

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