On Mon, Jun 17, 2013 at 01:20:56AM +0100, Magnus Damm wrote:
> From: Magnus Damm <[email protected]>
>
> 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 <[email protected]>
> ---
>
> 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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