Re: [PATCH] drm/i915/vrr: Fix transmission line for Adaptive Sync SDP

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

On Thu, Oct 23, 2025 at 06:08:47PM +0300, Ville Syrjälä wrote:
> On Thu, Oct 23, 2025 at 05:40:09PM +0300, Ville Syrjälä wrote:
> > On Thu, Oct 23, 2025 at 05:57:02PM +0530, Nautiyal, Ankit K wrote:
> > > 
> > > On 10/23/2025 5:34 PM, Ville Syrjälä wrote:
> > > > On Thu, Oct 23, 2025 at 01:46:14PM +0530, Ankit Nautiyal wrote:
> > > >> Currently the EMP_AS_SDP_TL is set to vrr.vsync_start which is
> > > >> incorrect.
> > > >>
> > > >> As per Bspec:71197 the transmission line must be within the SCL +
> > > >> guardband region. Before guardband optimization, guradband was same as
> > > >> vblank length so EMP_AS_SDP_TL set with vrr.sync_start was falling in
> > > >> this region and it was not giving an issue.
> > > >>
> > > >> Now with optimized guardband, this is falling outside the SCL +
> > > >> guardband region and since the same transmission line is used by VSC SDP
> > > >> also, this results in PSR timeout issues.
> > > >>
> > > >> Further restrictions on the position of the transmission line:
> > > >> For DP/eDP, if there is a set context latency (SCL) window, then it
> > > >> cannot be the first line of SCL
> > > >> For DP/eDP, if there is no SCL window, then it cannot be the first line of
> > > >> the Delayed V. Blank
> > > >>
> > > >> Fix the EMP_AS_SDP_TL to VTOTAL - (delayed vblank_start - SCL + 1)
> > > >> Internally the HW computes the value as VTOTAL - EMP_AS_SDP_TL.
> > > >>
> > > >> Fixes: e1123e617e51 ("drm/i915/vrr: Program EMP_AS_SDP_TL for DP AS SDP")
> > > >> Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> > > >> Cc: Jouni Högander <jouni.hogander@intel.com>
> > > >> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> > > >> ---
> > > >>   drivers/gpu/drm/i915/display/intel_vrr.c | 12 +++++++++---
> > > >>   1 file changed, 9 insertions(+), 3 deletions(-)
> > > >>
> > > >> diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
> > > >> index 92fb72b56f16..dd81d2133aba 100644
> > > >> --- a/drivers/gpu/drm/i915/display/intel_vrr.c
> > > >> +++ b/drivers/gpu/drm/i915/display/intel_vrr.c
> > > >> @@ -655,18 +655,24 @@ void intel_vrr_set_db_point_and_transmission_line(const struct intel_crtc_state
> > > >>   {
> > > >>   	struct intel_display *display = to_intel_display(crtc_state);
> > > >>   	enum transcoder cpu_transcoder = crtc_state->cpu_transcoder;
> > > >> +	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> > > >> +	int transmission_line;
> > > >>   
> > > >>   	/*
> > > >>   	 * For BMG and LNL+ onwards the EMP_AS_SDP_TL is used for programming
> > > >>   	 * double buffering point and transmission line for VRR packets for
> > > >>   	 * HDMI2.1/DP/eDP/DP->HDMI2.1 PCON.
> > > >>   	 * Since currently we support VRR only for DP/eDP, so this is programmed
> > > >> -	 * to for Adaptive Sync SDP to Vsync start.
> > > >> +	 * for Adaptive Sync SDP.
> > > >>   	 */
> > > >> -	if (DISPLAY_VERx100(display) == 1401 || DISPLAY_VER(display) >= 20)
> > > >> +	if (DISPLAY_VERx100(display) == 1401 || DISPLAY_VER(display) >= 20) {
> > > >> +		transmission_line = adjusted_mode->crtc_vtotal - (adjusted_mode->crtc_vblank_start -
> > > >> +								  crtc_state->set_context_latency +
> > > >> +								  1);
> > > >>   		intel_de_write(display,
> > > >>   			       EMP_AS_SDP_TL(display, cpu_transcoder),
> > > >> -			       EMP_AS_SDP_DB_TL(crtc_state->vrr.vsync_start));
> > > >> +			       EMP_AS_SDP_DB_TL(transmission_line));
> > > >> +	}
> > > > Pretty sure we are expected to send it at vsync_start.
> > > 
> > > Hmm.. then do we need to move vsync_start too similar to vblank_start 
> > > for optimized guardband?
> > 
> > The vsync pulse location is dictated by the timings.
> > 
> > > 
> > > If we do not move vsync_start, and set the transmission line to 
> > > vsync_start, it will never fall in the region SCL + guardband with a 
> > > reduced guardband.
> > 
> > Only if the vsync pulse is early in the vblank. That's up to the
> > display.
> 
> Oh and I think we should get rid of that 'assume_all_enabled' stuff
> for the AS SDP, and account for it only when actually needed.
> Which I *think* means PCON or panel replay with AUX-less ALPM.
> 
> There's also that t1 vs. t2 setup time thing for the panel replay,
> which seems to be telling me that we could sometimes transmit the
> AS SDP later. But if I'm reading that right we have to switch to the
> t1 (vsync) transmission line whenever we switch to live frame mode,
> which I presume can happen basically at any time. So maybe we can't
> actually use that t2 transmission line.

Ah, apparently we have to always put t1 into EMP_AS_SDP_TL, and 
there's some other logic for panel replay to use t2 when possible.
But since it's all dynamic we have to use t1 when calculating the
guardband/SCL.

-- 
Ville Syrjälä
Intel