Re: [PATCH 11/15] drm/i915/display: Use vrr.guardband to derive vblank_start

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

On Sun, Sep 14, 2025 at 11:29:10AM +0530, Nautiyal, Ankit K wrote:
> 
> On 9/11/2025 7:55 PM, Ville Syrjälä wrote:
> > On Thu, Sep 11, 2025 at 08:15:50AM +0530, Ankit Nautiyal wrote:
> >> When VRR TG is always enabled and an optimized guardband is used, the pipe
> >> vblank start is derived from the guardband.
> >> Currently TRANS_SET_CONTEXT_LATENCY is programmed with crtc_vblank_start -
> >> crtc_vdisplay, which is ~1 when guardband matches the vblank length.
> >> With shorter guardband this become a large window.
> >>
> >> To avoid misprogramming TRANS_SET_CONTEXT_LATENCY, clamp the scl value to 1
> >> when using optimized guardband.
> >>
> >> Also update the VRR get config logic to set crtc_vblank_start based on
> >> vtotal - guardband, during readback.
> >>
> >> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/display/intel_display.c | 36 ++++++++++++++++----
> >>   drivers/gpu/drm/i915/display/intel_vrr.c     |  9 ++++-
> >>   2 files changed, 38 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> >> index 55bea1374dc4..73aec6d4686a 100644
> >> --- a/drivers/gpu/drm/i915/display/intel_display.c
> >> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> >> @@ -2638,6 +2638,30 @@ transcoder_has_vrr(const struct intel_crtc_state *crtc_state)
> >>   	return HAS_VRR(display) && !transcoder_is_dsi(cpu_transcoder);
> >>   }
> >>   
> >> +static int intel_set_context_latency(const struct intel_crtc_state *crtc_state,
> >> +				     int crtc_vblank_start,
> >> +				     int crtc_vdisplay)
> >> +{
> >> +	struct intel_display *display = to_intel_display(crtc_state);
> >> +
> >> +	/*
> >> +	 * When VRR TG is always on and optimized guardband is used,
> >> +	 * the pipe vblank start is based on the guardband,
> >> +	 * TRANS_SET_CONTEXT_LATENCY cannot be used to configure it.
> >> +	 */
> >> +	if (intel_vrr_always_use_vrr_tg(display))
> >> +		return clamp(crtc_vblank_start - crtc_vdisplay, 0, 1);
> > What are you trying to achieve with this? As in what problem are you
> > seeing with the current SCL programming?
> 
> In VRR TG mode with optimized guardband, the guardband is shortened and 
> vblank start is moved to match the delayed vblank position.
> 
> The SCL value which we are currently writing as difference between 
> delayed vblank and undelayed vblank becomes quite large.
> 
> With this large SCL, the flipline decision boundary which is given by 
> delayed vblank start and SCL lines is same as the undelayed vblank.

Everything should match the undelayed vblank.

> 
> It seems that intel_dsb_wait_vblank_delay() (in turn 
> intel_dsb_wait_usec()) does not behave correctly within the W2 window 
> (between flipdone decision boundary and delayed vblank start).
> 
> It seems to return prematurely. Since the push bit hasn’t cleared yet, 
> this leads to DSB poll errors.

That doesn't make any sense. That command is supposed to simply wait
for the specifid number of microseconds. It should not care at all
what is happening with the scanout. If that is not the case then we
need to write a synthetic test to reproduce it, and report the
problem to the hardware folks.

> AFAIU we are not using the SCL (Set Context Latency) lines to write 
> registers via DSB.
> 
> The evasion logic ensures we write within a separate window, making the 
> actual SCL value less critical for register programming.
> 
> So I have clamped the SCL value to (0,1). With this after the push is 
> sent the send push bit is cleared after (0,1) lines.
> 
> But we still need to wait for the delayed vblank. For this we need 
> either intel_dsb_wait_vblank_delay() or dsb_wait_for_scanline_in().
> 
> 
> Do you have any ideas, what could have been going wrong or if anything 
> we might have been missing?

Was your crtc_vblank_start even correct (== undelayed vblank)
when you were testing that?

-- 
Ville Syrjälä
Intel