Re: [PATCH 3/9] drm/i915/display: Use VBLANK_START to get the vblank delay for TGL

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Sun, Sep 21, 2025 at 10:05:29AM +0530, Ankit Nautiyal wrote:
> For TGL the TRANS_SET_CONTEXT_LATENCY doesn't exist to account for SCL.
> However, the VBLANK_START-VACTIVE diffence plays an identical role here ie.
> it can be used to create the SCL window ahead of the undelayed vblank.
> 
> While readback there is no specific register to read out the SCL, so use
> the same helper intel_crtc_compute_scl() and reduce the vblank_start by
> this amount.
> 
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index 5a7794387ea2..455bbebb50a5 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -2822,6 +2822,16 @@ static void intel_get_transcoder_timings(struct intel_crtc *crtc,
>  		adjusted_mode->crtc_vblank_end += 1;
>  	}
>  
> +	if (DISPLAY_VER(display) == 12) {
> +		/*
> +		 * There is no specific register for SCL for TGL.
> +		 * Derive the value from the helper intel_crtc_set_context_latency().
> +		 * crtc_vblank_start needs to be reduced with this amount.
> +		 */
> +		pipe_config->set_context_latency = intel_crtc_set_context_latency(pipe_config);

That thing *computes* the SCL, it doesn't teel us what the
current hw value is.

> +		adjusted_mode->crtc_vblank_start -= pipe_config->set_context_latency;
> +	}
> +
>  	if (DISPLAY_VER(display) >= 13 && !transcoder_is_dsi(cpu_transcoder)) {
>  		pipe_config->set_context_latency =
>  			intel_de_read(display,

I think this whole thing needs to look something like this:

if (DISPLAY_VER >= 13 && !dsi) {
	set_context_latency = read(TRANS_SET_CONTEXT_LATENCY);
	crtc_vblank_start = crtc_vdisplay + set_set_context_latency;
} else if (DISPLAY_VER >= 12) {
	set_context_latency = crtc_vblank_start - crtc_vdisplay;
}

That'll also cover DSI (not that the DSI code itself is actually
ready for SCL...).

And I think you need to squash this into the set_context_latency
patch. Otherwise we'll get state checker mismatches on TGL in
between.

-- 
Ville Syrjälä
Intel