Messages in this thread | | | Date | Mon, 17 Jun 2013 15:53:52 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH/RFC] arm: arch_timer: Do not set C3STOP in case CPU_IDLE=n |
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On Mon, Jun 17, 2013 at 01:20:56AM +0100, Magnus Damm wrote: > From: Magnus Damm <damm@opensource.se> > > Modify the ARM architected timer driver to not set C3STOP > in case CPU_IDLE is disabled. This is a short term fix that > allows use of high resolution timers even though no additional > clock event is registered. > > Not-really-Signed-off-by: Magnus Damm <damm@opensource.se> > --- > > If someone cares about this case then perhaps it should be > moved up to the clock event main code. The same issue should > in theory trigger on all architectures, perhaps x86 people > hunting for low latency may try to disable CPU_IDLE?
I think that changing tick_is_oneshot_capable and friends to only worry about C3STOP when CPU_IDLE is enabled would be a nicer solution. That way you enable all clock_event_devices with C3STOP to function as high resolution timers when CPU_IDLE's selected. Presenting the hardware differently depending on CPU_IDLE feels wrong.
Having some other clock_event_device would be a nicer solution still.
Thanks, Mark.
> > I propose carrying this patch locally to enable high resolution > timers until CPU_IDLE and an additional clock event is supported. > > Observed on r8a73a4 and APE6EVM. > > drivers/clocksource/arm_arch_timer.c | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > --- 0001/drivers/clocksource/arm_arch_timer.c > +++ work/drivers/clocksource/arm_arch_timer.c 2013-06-17 09:03:44.000000000 +0900 > @@ -125,7 +125,23 @@ static int arch_timer_set_next_event_phy > > static int __cpuinit arch_timer_setup(struct clock_event_device *clk) > { > - clk->features = CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_C3STOP; > + clk->features = CLOCK_EVT_FEAT_ONESHOT; > +#ifdef CONFIG_CPU_IDLE > + /* By not setting the C3STOP flag it is possible to let the > + * ARM architected timer to be the only clock event installed > + * on the system and have working high resolution timers. > + * > + * If the C3STOP flag is set unconditionally then the kernel > + * will always prevent using the high resoultion timer feature > + * unless an additional clock event is registered. > + * > + * In the case where CPU_IDLE is enabled then there is a chance > + * that deeper sleep states will be handled by software, but > + * if CPU_IDLE is disabled then deep sleep states cannot be > + * entered and the feature flagged by C3STOP is not needed. > + */ > + clk->features |= CLOCK_EVT_FEAT_C3STOP; > +#endif > clk->name = "arch_sys_timer"; > clk->rating = 450; > if (arch_timer_use_virtual) { >
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