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copied!<blockquote> <p>Why are there so many suspend and resume function pointers? </p> </blockquote> <ol> <li><code>i2c_driver</code> - legacy support.</li> <li><code>device_driver</code> - standard support. </li> <li><code>dev_pm_ops</code> - extended power management.</li> </ol> <p>Notice that they are all <em>function pointers</em>. There is sequencing for the <em>suspend</em> and <em>resume</em>. For instance, the <strong>i2c</strong> controller must be <em>suspended</em> after the devices but <em>resumed</em> before. </p> <blockquote> <p>Some kind of legacy thing?</p> </blockquote> <p>The <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/i2c/i2c-core.c?id=refs/tags/v3.12-rc5#n302" rel="nofollow"><code>struct i2c_driver</code></a> is a legacy mechism. It existed before the entire power infrastructure was created. As well, some configurations may exclude the full <code>struct dev_pm_ops</code> pointer, but have the <code>suspend</code> and <code>resume</code> driver hooks. The full <code>struct dev_pm_ops</code> supports <em>suspend to disk</em> and other features. <em>Suspend to memory</em> is more common and is given pointers space in <code>struct device_driver</code>. If <code>struct dev_pm_ops</code> is non-NULL, the two pointer will be the same. These two should call the same routine in your driver.</p> <blockquote> <p>Which one should I use for my driver?</p> </blockquote> <p>You probably shouldn't use any of them. It is probably more likely that your driver is part of some other sub-system. <sup>see <strong>Note</strong></sup> For instance, the <em>i2c</em> is used in <em>codecs</em> like the <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/sound/soc/codecs/wm8940.c?id=refs/tags/v3.12-rc5#n674" rel="nofollow">wm8940.c</a>. Generally, <em>i2c</em> is not the central controlling sub-system. It is a driver that is used by something else to control a chip-set. The <em>sound</em> sub-system will be suspended before <em>i2c</em> and it is better to put your hooks there. If your driver is pure <em>i2c</em>, then use the macros in <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/pm.h" rel="nofollow">pm.h</a>, like <code>SET_SYSTEM_SLEEP_PM_OPS</code> to conditionalize the setting of <code>dev_pm_ops</code>; so set both of them. Most likely the <code>device_driver</code> will be copied to <code>dev_pm_ops</code> if it exists, but doing it explicitly is better.</p> <p>The <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-model/" rel="nofollow">driver-model</a>, <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/i2c/writing-clients?id=refs/tags/v3.12-rc5#n277" rel="nofollow">i2c power management</a> and <a href="https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/power/devices.txt" rel="nofollow">power driver</a> documentation have more information on the structure and overviews.</p> <p><strong>Note:</strong> In this case, there are multiple <code>device_driver</code> structures. Usually the <code>i2c_driver</code> is managed by a controlling driver. The controlling driver should implement a <code>suspend</code>/<code>resume</code> for it's sub-system, which uses the <code>i2c_driver</code> interface.</p>

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