Messages in this thread | | | Date | Thu, 16 May 2013 14:31:23 -0700 | From | Saravana Kannan <> | Subject | Re: [PATCH v2] clk: Fix race condition between clk_set_parent and clk_enable() |
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On 05/16/2013 01:44 PM, Mike Turquette wrote: > Quoting Saravana Kannan (2013-05-15 21:07:24) >> Without this patch, the following race condition is possible. >> * clk-A has two parents - clk-X and clk-Y. >> * All three are disabled and clk-X is current parent. >> * Thread A: clk_set_parent(clk-A, clk-Y). >> * Thread A: <snip execution flow> >> * Thread A: Grabs enable lock. >> * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y. >> * Thread A: Updates clk-A SW parent to clk-Y >> * Thread A: Releases enable lock. >> * Thread B: clk_enable(clk-A). >> * Thread B: clk_enable() enables clk-Y, then enabled clk-A and returns. >> >> clk-A is now enabled in software, but not clocking in hardware since the >> hardware parent is still clk-X. >> >> The only way to avoid race conditions between clk_set_parent() and >> clk_enable/disable() is to ensure that clk_enable/disable() calls don't >> require changes to hardware enable state between changes to software clock >> topology and hardware clock topology. >> >> The options to achieve the above are: >> 1. Grab the enable lock before changing software/hardware topology and >> release it afterwards. >> 2. Keep the clock enabled for the duration of software/hardware topology >> change so that any additional enable/disable calls don't try to change >> the hardware state. Once the topology change is complete, the clock can >> be put back in its original enable state. >> >> Option (1) is not an acceptable solution since the set_parent() ops might >> need to sleep. >> >> Therefore, this patch implements option (2). >> >> This patch doesn't violate any API semantics. clk_disable() doesn't >> guarantee that the clock is actually disabled. So, no clients of a clock >> can assume that a clock is disabled after their last call to clk_disable(). >> So, enabling the clock during a parent change is not a violation of any API >> semantics. >> >> This also has the nice side effect of simplifying the error handling code. >> >> Signed-off-by: Saravana Kannan <skannan@codeaurora.org> > > Updated to this version in clk-next. >
Thanks Mike. I forgot to add the Ack by Ulf. Would be nice if you can put that in.
Btw, I did send this email to the list. But looks like this mail is wedged in the series of tubes in the arm mailing list.
-Saravana
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