On 05/14/2013 11:54 AM, Mike Turquette wrote:
Quoting Saravana Kannan (2013-04-30 21:42:08)Without this patch, the following race conditions are possible. Race condition 1: * 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. There are options to achieve the above: 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@xxxxxxxxxxxxxx>I've taken this patch into clk-next for testing. The code itself looks fine.
Thanks Mike. I'll send it out again with some typo/grammar corrections.
The only thing that remains to be seen is if any platforms have a problem with disabled clocks getting turned on during a reparent operation.
I would think that would be a general issue with the clock APIs since disable doesn't guarantee a disable (since it's ref counted).
Also, those clocks could be marked as CLK_SET_PARENT_GATE if it's a real issue.
On platforms that I have worked on this is OK, but I suppose there could be some platform out there where a clock is prepared and disabled, and briefly enabling the clock during the reparent operation somehow puts the hardware in a bad state.
I can't think of any either, but as I mentioned, we have CLK_SET_PARENT_GATE for that.
Thanks, Saravana -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
