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

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

On Tue, Sep 16, 2025 at 08:08:01PM +0530, Nautiyal, Ankit K wrote:
> 
> On 9/16/2025 8:00 PM, Nautiyal, Ankit K wrote:
> >
> > On 9/15/2025 6:02 PM, Ville Syrjälä wrote:
> >> 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.
> >
> > You are right, on debugging further I noticed that 
> > intel_dsb_wait_usec() and intel_dsb_wait_vblank_delay() are working 
> > correctly.
> >
> > Due to large SCL, the the intel_dsb_wait_vblanks() is not waiting till 
> > the undelayed vblank but the safe window, apparently undelayed vblank 
> > - SCL lines.
> >
> > We are setting DSB_CHICKEN_REG bits 14-15 : which says: Wait for 
> > Vblank instruction will use only safe window signal from dptunit in 
> > DSB HW to complete the wait for vblank instruction.
> >
> > I am not exactly sure if its mentioned in Bspec that safe window start 
> > = undelayed vblank start - SCL lines.
> >
> > Observation:
> >
> > For example with eDP panel VRR range 40-60 and below mode:
> >
> > Mode: "2880x1800": 60 347710 2880 2928 2960 3040 1800 1803 1809 1906
> >
> > Before optimization:
> >
> > guardband = vblank length = 106; Undelayed vblank start =1800; Delayed 
> > vblank start = 1906 - 106 = 1800
> >
> > SCL = 1800 - 1800 = 0
> >
> > Flipline decision boundary is = 1800
> >
> > After optimization:
> >
> > vblank length = 106; guardband = 38; Undelayed Vblank start = 1800; 
> > Delayed Vblank start = 1868 (1906 - 38)
> >
> > SCL = 1868 - 1800 = 68
> >
> > Flipline decision boundary = 1868 - 68 = 1800
> >
> > Consider lines in  intel_atomic_dsb_finish() :
> >
> >  intel_dsb_wait_vblanks(new_crtc_state->dsb_commit, 1);         /* If 
> > flip is earlier than 1732 (1800 - 68) this waits till 1732.*/
> >
> >                 intel_vrr_send_push(new_crtc_state->dsb_commit, 
> > new_crtc_state);    /* Push happens immediately*/
> >                 intel_dsb_wait_vblank_delay(state, 
> > new_crtc_state->dsb_commit);        /* Waits for duration (delayed 
> > vblank start - undelayed vblank start) ie. 68 lines ie. till we reach 
> > 1732 + 68 = 1800*/
> > intel_vrr_check_push_sent(new_crtc_state->dsb_commit,     /* Push is 
> > not clear yet as delayed vblank start (1868) is not reach yet, we get 
> > DSB POLL error */
> >                                           new_crtc_state);
> > intel_dsb_interrupt(new_crtc_state->dsb_commit);       /* DSB 
> > interrupt is fired earlier */
> 
> 
> Sorry for the bad formatting, perhaps this will be more readable:
> 
> 
>   intel_dsb_wait_vblanks(new_crtc_state->dsb_commit, 1);
> 
> /* If flip is earlier than 1732 (1800 - 68) this waits till 1732.*/

That does not seem right, or at least it's not how it works on LNL.
I just hacked some DSB_STATUS stuff into intel_display_poller [1],
and when running that on LNL the safe window always starts at the
undelayed vblank.

So if we look at the vmax case then I think the diagram should look
like this:

    udelayed vblank 
    ^               vmax decision boudnary
    |               ^           delayed vblank
    |               |           ^                 vmax
    |               |           |                 ^
    | <- stretch -> | <- scl -> | <- guardband - >|
     _______________
..._/               \______________________________... safe window

... push affects curent frame ->|<- push affects next frame ...
                                |
				v
				push send bit clears if set

And then for the maximum vrefresh case (defined by flipline instead
of vnax) the "stretch" part is something between 0 and 
delayed_vblank-undelayed_vblank, depending on how we configure SCL.

Additionally if a push is sent during the scl window just
after the vmax decisioun bondary, said push will still affect
the current frame (ie. such a frame will not have a full
scl/w2 window). Only a push sent after the delayed vblank
will in fact get deferred to the next frame. That particular
scenatio isn't really described in the bspec timing diagrams.
Though since we always precede the push with a "wait for safe
window" for us the push would get deferred to the next frame
anyway.

Now, I suppose PTL may have changed how the safe window works to 
support that "SCL during vactive" stuff. I think a bit more poking
might be needed to get to the bottom of this...

[1] https://github.com/vsyrjala/intel-gpu-tools.git dsb_status_poller

-- 
Ville Syrjälä
Intel