On Tue, May 17, 2016 at 06:43:24PM +0300, Mika Kuoppala wrote:
> Usually the condition we are after appears within very short time.
> Spin few times before going into sleep. With this approximately
> half of the wait_for in init path will take the fast path without
> sleeping.
>
> Signed-off-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/intel_drv.h | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 488141929a7a..c225605c727c 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -39,7 +39,7 @@
> #include <drm/drm_atomic.h>
>
> /**
> - * _wait_for_ms - magic (register) wait macro
> + * __wait_for_ms - magic (register) wait macro
> *
> * Does the right thing for modeset paths when run under kdgb or similar atomic
> * contexts. Note that it's important that we check the condition again after
> @@ -50,17 +50,22 @@
> * drm_can_sleep() can be removed and in_atomic()/!in_atomic() asserts
> * added.
> */
> -#define _wait_for_ms(COND, TIMEOUT_MS, SLEEP_US) ({ \
> +#define __wait_for_ms(COND, TIMEOUT_MS, SLEEP_US, SPIN_COUNT) ({ \
> const unsigned long timeout__ = \
> jiffies + msecs_to_jiffies(TIMEOUT_MS) + 1; \
> + unsigned int c__ = 0; \
> int ret__ = 0; \
> + \
> while (!(COND)) { \
> if (time_after(jiffies, timeout__)) { \
> if (!(COND)) \
> ret__ = -ETIMEDOUT; \
> break; \
> } \
> - if ((SLEEP_US) && drm_can_sleep()) { \
> + \
> + if (++c__ > (SPIN_COUNT) && \
> + (SLEEP_US) && \
> + drm_can_sleep()) { \
> usleep_range((SLEEP_US), (SLEEP_US) * 2); \
> } else { \
> cpu_relax(); \
> @@ -69,7 +74,8 @@
> ret__; \
> })
>
> -#define wait_for(COND, MS) _wait_for_ms((COND), (MS), 1 * USEC_PER_MSEC)
> +#define wait_for(COND, MS) __wait_for_ms((COND), (MS), 1 * USEC_PER_MSEC, 5)
> +#define _wait_for_ms(COND, MS, US) __wait_for_ms((COND), (MS), (US), 5)
Why 5? I did some histrograms on wait_for() on my BSW (just looking
at modeset paths really), and that showed ~66% of the time we got it
right on the first check, and ~33% was completed after one sleep
iteration. The sleep duration didn't make much of a difference here,
so the current 1-2 msec is probably not at all optimal. I also tested
doing a double check intially, and IIRC that reduced the second bin
by about half. I didn't check whether further spinnign would have
helped more.
>
> /* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
> #if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
> --
> 2.5.0
>
> _______________________________________________
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> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx
--
Ville Syrjälä
Intel OTC
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