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    <p>The generic Linux clock infra-structure is documented in <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/clk.txt" rel="nofollow">clk.txt</a>. For the ARM, Sasha Hauer created the <em>common clock</em> frame-work recently (last two years). The clocks are structured in a <em>parent/child</em> relation. Typical SOC (system on chip) has main clocks created from a crystal which are either scaled down (with a counter) or up with a PLL and maybe both. They hierarchy is important for power savings. Usually devices are only using one of the <em>lowest/youngest</em> clocks in the tree. When a device requests a clock, the infra-structure ensures that all parents are started.</p> <p>Previously (legacy), clocks were passed from the <em>machine file</em> (reference <code>arch/arm/Board***/</code>) to the driver/device via platform data; ultimately through <code>platform_device_register()</code>. Sometimes, the clocks were/are derived from the device name. For example the <em>fec</em> driver might use <em>fec-clk</em>. This did not work well for <strong>multiple machine</strong> configurations, so the <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-model/platform.txt" rel="nofollow">platform data</a> mechanism was introduced. Even newer machinery uses a <strong>dt</strong> (or device table). Here, there are no machine files, only a device table that is passed from the boot loader to the kernel. In this case, the <strong>dt</strong> tells the driver which clock to use. </p> <p>Originally, the <code>dev_id</code> and <code>con_id</code> were to relate clocks for a <strong>device</strong> and clocks that are <strong>connected</strong> (parent/child). Usually either <code>dev_id</code> or <code>con_id</code> are NULL as only one aspect is needed. I think that this view was found wanting; especially for starting an entire <strong>clock chain</strong>. So, depending on the Linux version, the answer varies. Even in the current source, some platforms (like <code>orion</code>) still use an older mechanism. I don't think <code>orion</code> supports <strong><a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree" rel="nofollow">device trees</a></strong>.</p> <p>Specific answers will depend on your Linux version and the machine (and possibly platform) in use.<br/>See also: <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clkdev.c" rel="nofollow">clkdev.c</a>, <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk.c" rel="nofollow">clk.c</a></p> <p><strong>Open source - There are many mutations. They all have a different plan.</strong></p> <p>Reference: <a href="http://lists.linaro.org/pipermail/linaro-dev/2011-October/007952.html" rel="nofollow">Russell Kings message on ARM clkdev</a>, original did not imply ordering.</p>
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    1. COSee: [LWN's ARM kernel consolidation](http://lwn.net/Articles/443510/), which has a good overview of some recent changes; how *clocks*, *device trees* and *drivers* work together.
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