Locking in the clk API, part 2: clk_prepare/clk_unprepare
From: Richard Zhao <hidden>
Date: 2011-02-04 09:55:31
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On Tue, Feb 01, 2011 at 09:24:09PM +0000, Russell King - ARM Linux wrote:
On Tue, Feb 01, 2011 at 12:59:11PM -0800, Stephen Boyd wrote:quoted
On 02/01/2011 07:24 AM, Russell King - ARM Linux wrote:quoted
I'd also be tempted at this stage to build-in a no-op dummy clock, that being the NULL clk: int clk_prepare(struct clk *clk) { int ret = 0; if (clk) { mutex_lock(&clk->mutex); if (clk->prepared == 0) ret = clk->ops->prepare(clk); if (ret == 0) clk->prepared++; mutex_unlock(&clk->mutex); } return ret; }I'm afraid this will hide enable/disable imbalances on some targets and then expose them on others. Maybe its not a big problem though since this also elegantly handles the root(s) of the tree.You can't catch enable/disable imbalances in the prepare code, and you can't really catch them in the unprepare code either. Consider two drivers sharing the same struct clk. When the second driver prepares the clock, the enable count could well be non-zero, caused by the first driver. Ditto for when the second driver is removed, and it calls unprepare - the enable count may well be non-zero. The only thing you can check is that when the prepare count is zero, the enable count is also zero. You can also check in clk_enable() and clk_disable() that the prepare count is non-zero.
but how can we check prepare count without mutex lock? Even if prepare count is atomic_t, it can not guarantee the clock is actually prepared or unprepared. So it's important for driver writer to maintain the call sequence. Thanks Richard
If you want tigher checking than that, you need to somehow identify and match up the clk_prepare/clk_enable/clk_disable/clk_unprepare calls from a particular driver instance. Addresses of the functions don't work as you can't be certain that driver code will be co-located within a certain range. Adding an additional argument to these functions which is driver instance specific seems to be horrible too. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel at lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel