[Intel-gfx] [PATCH 4/4] drm/i915: Prevent needless toggling of DC states during modesets

[Hogander, Jouni]

On Fri, 2023-06-16 at 21:51 +0300, Imre Deak wrote:
> DC states are disabled / re-enabled around each modeset, which may
> lead
> to a needless off->on->off toggling of the DC_off power well. This
> has
> some overhead as toggling DC states involves running a DMC firmware
> handler and also running a periodic firmware handler while DC states
> are
> enabled. The limit of when DC states have a benefit is at 30 FPS
> (using
> DC3co) and below 30 FPS (using DC5/6), where the firmware can
> actually
> disable clocks / power off power wells. Accordingly delay powering
> off
> the DC_off powerwell (which re-enables DC states) by 17 ms at the end
> of
> a modeset to avoid the above overhead at or above 60 FPS.

I'm wondering in which case this toggling would happen and how the
patch helps it. We have PSR idle frames configured > 6. I.e. Only after
6 frames without changes our HW is supposed to enter DC5/6. This is
purely with PSR glasses on. Do we have some other (than PSR) use-case
this is targeting where DC5/DC6 would be possible while display is on?


> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c
> b/drivers/gpu/drm/i915/display/intel_display.c
> index dd52c689267f4..de77d130d6f9f 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -7148,7 +7148,11 @@ static void intel_atomic_commit_tail(struct
> intel_atomic_state *state)
>                  */
>                 intel_uncore_arm_unclaimed_mmio_detection(&dev_priv-
> >uncore);
>         }
> -       intel_display_power_put(dev_priv, POWER_DOMAIN_DC_OFF,
> wakeref);
> +       /*
> +        * Delay re-enabling DC states by 17 ms to avoid the off->on-
> >off
> +        * toggling overhead at and above 60 FPS.
> +        */
> +       intel_display_power_put_async_delay(dev_priv,
> POWER_DOMAIN_DC_OFF, wakeref, 17);
>         intel_runtime_pm_put(&dev_priv->runtime_pm, state->wakeref);
>  
>         /*