Re: [PATCH 15/15] drm/i915/display: Use optimized guardband to set vblank start

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

On Sun, Sep 28, 2025 at 12:35:40PM +0530, Ankit Nautiyal wrote:
> +static void intel_crtc_vblank_delay(struct intel_atomic_state *state,
> +				    struct intel_crtc *crtc)
> +{
> +	struct intel_crtc_state *crtc_state =
> +		intel_atomic_get_new_crtc_state(state, crtc);
> +	struct drm_display_mode *adjusted_mode =
> +		&crtc_state->hw.adjusted_mode;
> +	int vblank_delay = 0;
> +
> +	vblank_delay = intel_crtc_min_guardband_delay(state, crtc);
> +
> +	adjusted_mode->crtc_vblank_start += vblank_delay;

The situation with crtc_vblank_start is already kinda broken,
and I think we need to fix that first somehow.

Currently crtc_vblank_start is assumed to be the vblank_start
for the fixed refresh rate case. That value can be different
from the variable refresh rate case whenever
always_use_vrr_tg()==false. On icl/tgl it's always different
due to the extra vblank delay, and also on adl+ it could be
different if we were to use an optimized guardband.

I think there are a few options how we might solve this:
1. keep crtc_vblank_start as is, and make sure every user of it
   gets adjusted to also deal with the vrr case correctly
2. enable always_use_vrr_tg() whenever there might be switch
   between vrr and fixed refresh rate, which I think would mean
   crtc_state->vrr.in_range==true.

I kinda like option 2 because then we'll be doing the vrr vs.
fixed refresh rate always the same way. However we haven't really
tested that mode of operation on the older platforms, so I'd
rather not bet all your work on that working. If we later run
into problems with that then we'd have to revert everything.

So I think we should start with option 1, adjust all the
crtc_vblank_start users approriately (I don't think there are
too many of them), and adjust crtc_vblank_start to match
the guardband only when always_use_vrr_tg()==true.

After that I think we might still have some potential issues/race
conditions around the actual vrr <-> fixed refresh rate switch.
Those might require more work later, or if we decide at that point
to try option 2 maybe we could sidestep some/all of them.

-- 
Ville Syrjälä
Intel