[Intel-gfx] [PATCH] drm/i915: Program LUT before intel_color_commit() if LUT was not previously set

[Ville Syrjälä]

On Sun, Oct 20, 2019 at 08:19:33PM +0200, Hans de Goede wrote:
> Since commit 051a6d8d3ca0 ("drm/i915: Move LUT programming to happen after
> vblank waits"), I am seeing an ugly colored flash of the first few display
> lines on 2 Cherry Trail devices when the gamma table gets set for the first
> time. A blue flash on a GPD win and a yellow flash on an Asus T100HA.
> 
> The problem is that since this change, the LUT is programmed after the
> write *and latching* of the double-buffered register which causes the LUT
> to be used starting at the next frame. This means that the old LUT is still
> used for the first couple of lines of the display. If no LUT was in use
> before then the LUT registers may contain bogus values. This leads to
> messed up colors until the new LUT values are written. At least on CHT DSI
> panels this causes messed up colors on the first few lines.
> 
> This commit fixes this by adding a load_lut_before_commit boolean,
> modifying intel_begin_crtc_commit to load the luts earlier if this is set,
> and setting this from intel_color_check when a LUT table was not in use
> before (and thus may contain bogus values), or when the table size
> changes.

The real solution is vblank workers, which I have somewhat implemented
here:
git://github.com/vsyrjala/linux.git vblank_worker_8_kthread

Though even with the qos tricks there we still probably can't quite make
it in time. Essentially we have a bit less than one scanline after start
of vblank to do the work before pixels start to flow through the pipe.
We might be extend that to almost four scanlines but that partocular
thing is documeted as debug feature so not sure we should really use it.
Also I don't think four scanlines is always enough either. So it's still
very much possible that we get the first 100 or so pixels with the old LUT.

> 
> Fixes: 051a6d8d3ca0 ("drm/i915: Move LUT programming to happen after vblank waits")
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
>  drivers/gpu/drm/i915/display/intel_color.c    | 26 +++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_display.c  |  7 +++++
>  .../drm/i915/display/intel_display_types.h    |  3 +++
>  3 files changed, 36 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_color.c b/drivers/gpu/drm/i915/display/intel_color.c
> index 71a0201437a9..0da6dcc5bebd 100644
> --- a/drivers/gpu/drm/i915/display/intel_color.c
> +++ b/drivers/gpu/drm/i915/display/intel_color.c
> @@ -1052,6 +1052,32 @@ intel_color_add_affected_planes(struct intel_crtc_state *new_crtc_state)
>  		new_crtc_state->update_planes |= BIT(plane->id);
>  	}
>  
> +	/*
> +	 * Normally we load the LUTs after vblank / after the double-buffer
> +	 * registers written by commit have been latched, this avoids a
> +	 * gamma change mid-way the screen. This does mean that the first
> +	 * few lines of the display will (sometimes) still use the old
> +	 * table. This is fine when changing an existing LUT, but if this
> +	 * is the first time the LUT gets loaded, then the hw may contain
> +	 * random values, causing the first lines to have funky colors.
> +	 *
> +	 * So if were enabling a LUT for the first time or changing the table
> +	 * size, then we must do this before the commit to avoid corrupting
> +	 * the first lines of the display.
> +	 */
> +	if (!old_crtc_state->base.gamma_lut && new_crtc_state->base.gamma_lut)
> +		new_crtc_state->load_lut_before_commit = true;
> +	else if (!old_crtc_state->base.degamma_lut &&
> +		 new_crtc_state->base.degamma_lut)
> +		new_crtc_state->load_lut_before_commit = true;
> +	else if (old_crtc_state->base.gamma_lut &&
> +		 new_crtc_state->base.gamma_lut &&
> +		 lut_is_legacy(old_crtc_state->base.gamma_lut) !=
> +			lut_is_legacy(new_crtc_state->base.gamma_lut))
> +		new_crtc_state->load_lut_before_commit = true;
> +	else
> +		new_crtc_state->load_lut_before_commit = false;

The 'no gamma -> yes gamma' thing I might be willing to accept. The rest
not so much. I was already pondering about such optimizations for the
plane gamma/csc stuff in my vblank branch.

But for the fastboot case I think what we could do is just sanitize
the LUT(s) after readout if gamma wasn't enabled by the BIOS.

> +
>  	return 0;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index aa54bb22796d..21442b0dd134 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -14033,6 +14033,7 @@ static void intel_atomic_commit_tail(struct intel_atomic_state *state)
>  	for_each_new_intel_crtc_in_state(state, crtc, new_crtc_state, i) {
>  		if (new_crtc_state->base.active &&
>  		    !needs_modeset(new_crtc_state) &&
> +		    !new_crtc_state->load_lut_before_commit &&
>  		    (new_crtc_state->base.color_mgmt_changed ||
>  		     new_crtc_state->update_pipe))
>  			intel_color_load_luts(new_crtc_state);
> @@ -14529,6 +14530,12 @@ static void intel_begin_crtc_commit(struct intel_atomic_state *state,
>  		intel_atomic_get_new_crtc_state(state, crtc);
>  	bool modeset = needs_modeset(new_crtc_state);
>  
> +	if (!modeset &&
> +	    new_crtc_state->load_lut_before_commit &&
> +	    (new_crtc_state->base.color_mgmt_changed ||
> +	     new_crtc_state->update_pipe))
> +		intel_color_load_luts(new_crtc_state);
> +
>  	/* Perform vblank evasion around commit operation */
>  	intel_pipe_update_start(new_crtc_state);
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
> index 449abaea619f..bbdeb3be64e6 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -973,6 +973,9 @@ struct intel_crtc_state {
>  	/* enable pipe csc? */
>  	bool csc_enable;
>  
> +	/* load luts before color settings commit */
> +	bool load_lut_before_commit;
> +
>  	/* Display Stream compression state */
>  	struct {
>  		bool compression_enable;
> -- 
> 2.23.0

-- 
Ville Syrjälä
Intel