[Intel-gfx] [PATCH 1/3] drm/i915/gt: Prefer soft-rc6 over RPS DOWN_TIMEOUT

[Andi Shyti]

Hi Chris,

On Wed, Apr 22, 2020 at 01:17:01AM +0100, Chris Wilson wrote:
> The RPS DOWN_TIMEOUT interrupt is signaled after a period of rc6, and
> upon receipt of that interrupt we reprogram the GPU clocks down to the
> next idle notch [to help convserve power during rc6]. However, on
> execlists, we benefit from soft-rc6 immediately parking the GPU and
> setting idle frequencies upon idling [within a jiffie], and here the
> interrupt prevents us from restarting from our last frequency.
> 
> In the process, we can simply opt for a static pm_events mask and rely
> on the enable/disable interrupts to flush the worker on parking.
> 
> This will reduce the amount of oscillation observed during steady
> workloads with microsleeps, as each time the rc6 timeout occurs we
> immediately follow with a waitboost for a dropped frame.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/gt/intel_rps.c | 41 +++++++++++++----------------
>  1 file changed, 18 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_rps.c b/drivers/gpu/drm/i915/gt/intel_rps.c
> index 4dcfae16a7ce..785cd58fba76 100644
> --- a/drivers/gpu/drm/i915/gt/intel_rps.c
> +++ b/drivers/gpu/drm/i915/gt/intel_rps.c
> @@ -57,7 +57,7 @@ static u32 rps_pm_mask(struct intel_rps *rps, u8 val)
>  	if (val < rps->max_freq_softlimit)
>  		mask |= GEN6_PM_RP_UP_EI_EXPIRED | GEN6_PM_RP_UP_THRESHOLD;
>  
> -	mask &= READ_ONCE(rps->pm_events);
> +	mask &= rps->pm_events;

we are giving up the read/write ordering here because we thing
that removing the down interval we won't have anymore
inconsistent reads?

Andi