[PATCH 2/3] drm/i915/dsb: Fix DSB vblank waits when using VRR

[Manna, Animesh]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Ville
> Syrjala
> Sent: Wednesday, March 6, 2024 9:38 AM
> To: intel-gfx at lists.freedesktop.org
> Cc: stable at vger.kernel.org
> Subject: [PATCH 2/3] drm/i915/dsb: Fix DSB vblank waits when using VRR
> 
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Looks like the undelayed vblank gets signalled exactly when the active period
> ends. That is a problem for DSB+VRR when we are already in vblank and
> expect DSB to start executing as soon as we send the push. Instead of
> starting the DSB just keeps on waiting for the undelayed vblank which won't
> signal until the end of the next frame's active period, which is far too late.
> 
> The end result is that DSB won't have even started executing by the time the
> flips/etc. have completed.
> We then wait for an extra 1ms, after which we terminate the DSB and report
> a timeout:
> [drm] *ERROR* [CRTC:80:pipe A] DSB 0 timed out waiting for idle (current
> head=0xfedf4000, head=0x0, tail=0x1080)
> 
> To fix this let's configure DSB to use the so called VRR "safe window" instead
> of the undelayed vblank to trigger the DSB vblank logic, when VRR is enabled.
> 
> Cc: stable at vger.kernel.org
> Fixes: 34d8311f4a1c ("drm/i915/dsb: Re-instate DSB for LUT updates")
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9927
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

LGTM.
Reviewed-by: Animesh Manna <animesh.manna at intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dsb.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c
> b/drivers/gpu/drm/i915/display/intel_dsb.c
> index d62e050185e7..e4515bf92038 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> @@ -340,6 +340,17 @@ static int intel_dsb_dewake_scanline(const struct
> intel_crtc_state *crtc_state)
>  	return max(0, vblank_start -
> intel_usecs_to_scanlines(adjusted_mode, latency));  }
> 
> +static u32 dsb_chicken(struct intel_crtc *crtc) {
> +	if (crtc->mode_flags & I915_MODE_FLAG_VRR)
> +		return DSB_CTRL_WAIT_SAFE_WINDOW |
> +			DSB_CTRL_NO_WAIT_VBLANK |
> +			DSB_INST_WAIT_SAFE_WINDOW |
> +			DSB_INST_NO_WAIT_VBLANK;
> +	else
> +		return 0;
> +}
> +
>  static void _intel_dsb_commit(struct intel_dsb *dsb, u32 ctrl,
>  			      int dewake_scanline)
>  {
> @@ -361,6 +372,9 @@ static void _intel_dsb_commit(struct intel_dsb *dsb,
> u32 ctrl,
>  	intel_de_write_fw(dev_priv, DSB_CTRL(pipe, dsb->id),
>  			  ctrl | DSB_ENABLE);
> 
> +	intel_de_write_fw(dev_priv, DSB_CHICKEN(pipe, dsb->id),
> +			  dsb_chicken(crtc));
> +
>  	intel_de_write_fw(dev_priv, DSB_HEAD(pipe, dsb->id),
>  			  intel_dsb_buffer_ggtt_offset(&dsb->dsb_buf));
> 
> --
> 2.43.0