Re: [PATCH 5/7] drm/amd/display: Reset plane for anything that's not a FAST update

[daniel@ffwll.ch]

On Thu, Jul 30, 2020 at 04:36:40PM -0400, Nicholas Kazlauskas wrote:
> [Why]
> MEDIUM or FULL updates can require global validation or affect
> bandwidth. By treating these all simply as surface updates we aren't
> actually passing this through DC global validation.
> 
> [How]
> There's currently no way to pass surface updates through DC global
> validation, nor do I think it's a good idea to change the interface
> to accept these.
> 
> DC global validation itself is currently stateless, and we can move
> our update type checking to be stateless as well by duplicating DC
> surface checks in DM based on DRM properties.
> 
> We wanted to rely on DC automatically determining this since DC knows
> best, but DM is ultimately what fills in everything into DC plane
> state so it does need to know as well.
> 
> There are basically only three paths that we exercise in DM today:
> 
> 1) Cursor (async update)
> 2) Pageflip (fast update)
> 3) Full pipe programming (medium/full updates)
> 
> Which means that anything that's more than a pageflip really needs to
> go down path #3.
> 
> So this change duplicates all the surface update checks based on DRM
> state instead inside of should_reset_plane().
> 
> Next step is dropping dm_determine_update_type_for_commit and we no
> longer require the old DC state at all for global validation.

I think we do something similar in i915, where we have a "nothing except
base address changed" fast path, but for anything else we fully compute a
new state. Obviously you should try to keep global state synchronization
to a minimum for this step, so it's not entirely only 2 options.

Once we have the states, we compare them and figure out whether we can get
away with a fast modeset operation (maybe what you guys call medium
update). Anyway I think being slightly more aggressive with computing full
state, and then falling back to more optimized update again is a good
approach. Only risk is if we you have too much synchronization in your
locking (e.g. modern compositors do like to change tiling and stuff,
especially once you have modifiers enabled, so this shouldn't cause a sync
across crtc except when absolutely needed).
-Daniel

> 
> Optimization can come later so we don't reset DC planes at all for
> MEDIUM udpates and avoid validation, but we might require some extra
> checks in DM to achieve this.
> 
> Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
> Cc: Hersen Wu <hersenxs.wu@amd.com>
> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> ---
>  .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 25 +++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index 0d5f45742bb5..2cbb29199e61 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -8336,6 +8336,31 @@ static bool should_reset_plane(struct drm_atomic_state *state,
>  		if (old_other_state->crtc != new_other_state->crtc)
>  			return true;
>  
> +		/* Src/dst size and scaling updates. */
> +		if (old_other_state->src_w != new_other_state->src_w ||
> +		    old_other_state->src_h != new_other_state->src_h ||
> +		    old_other_state->crtc_w != new_other_state->crtc_w ||
> +		    old_other_state->crtc_h != new_other_state->crtc_h)
> +			return true;
> +
> +		/* Rotation / mirroring updates. */
> +		if (old_other_state->rotation != new_other_state->rotation)
> +			return true;
> +
> +		/* Blending updates. */
> +		if (old_other_state->pixel_blend_mode !=
> +		    new_other_state->pixel_blend_mode)
> +			return true;
> +
> +		/* Alpha updates. */
> +		if (old_other_state->alpha != new_other_state->alpha)
> +			return true;
> +
> +		/* Colorspace changes. */
> +		if (old_other_state->color_range != new_other_state->color_range ||
> +		    old_other_state->color_encoding != new_other_state->color_encoding)
> +			return true;
> +
>  		/* Framebuffer checks fall at the end. */
>  		if (!old_other_state->fb || !new_other_state->fb)
>  			continue;
> -- 
> 2.25.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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