Re: [PATCH] drm/i915/display: Ensure a non-zero safe window from PTL onwards

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 7/21/2026 8:12 PM, Borah, Chaitanya Kumar wrote:
>
>
> On 7/20/2026 1:48 PM, Ankit Nautiyal wrote:
>>  From PTL onwards the DSB waits on the VRR safe window instead of vblank
>> (chicken bit 14 selects the safe window signal from the DPT unit to
>
> bit [15:14]

Thats right.


>
> It is not entirely true that Safe window signal was used only from PTL 
> onwards. Pre-PTL it was still used but just for VRR.

Yes right I missed to add that we are waiting for safe signal for Fixed 
RR case. I will reframe this.

Thanks for pointing these out, will fix these in v2.

Regards,

Ankit

>
>> start the DMA engine). The safe window starts at (undelayed vblank
>> start - set context latency) and ends at the delayed vblank start.
>>
>> With modes that have a smaller vblank region, the computed guardband is
>> clamped to the vblank length, which makes the undelayed and delayed
>> vblank coincide. If the set context latency (SCL) is also 0, the safe
>> window collapses to 0. The DSB, configured to wait for the safe window,
>> then stalls forever since the hardware never signals it, leading to:
>>
>>    *ERROR* [CRTC:159:pipe A] flip_done timed out
>>    *ERROR* [CRTC:159:pipe A] DSB 0 timed out waiting for idle
>>
>> Clamp the set context latency to a minimum of 1 from PTL onwards so the
>> safe window always keeps a non-zero width.
>>
>
> I could not thoroughly check if there are any side effects of this on 
> the optimized guardband (and therefore on PSR, LOBF etc.) but the 
> change looks safe enough to me.
>
> Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
>
>> Fixes: 4a68c7516c57 ("drm/i915/dsb: Use safe window path when VRR TG 
>> is used")
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
>> Assisted-by: GitHub-Copilot:Claude-Opus-4.8
>> ---
>>   drivers/gpu/drm/i915/display/intel_display.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_display.c 
>> b/drivers/gpu/drm/i915/display/intel_display.c
>> index 38763a6802c5..0822a2194c7f 100644
>> --- a/drivers/gpu/drm/i915/display/intel_display.c
>> +++ b/drivers/gpu/drm/i915/display/intel_display.c
>> @@ -2459,6 +2459,19 @@ static int 
>> intel_crtc_set_context_latency(struct intel_crtc_state *crtc_state)
>>       set_context_latency = max(set_context_latency,
>> intel_psr_min_set_context_latency(crtc_state));
>>   +    /*
>> +     * From PTL onwards, the set context latency can be in the vactive
>> +     * region, letting the safe window start some lines before the 
>> vblank
>> +     * start. With modes that have a smaller vblank region, the 
>> computed
>> +     * guardband is clamped to the vblank length, making the 
>> undelayed and
>> +     * delayed vblank coincide. If the SCL is also 0, the 'safe window'
>> +     * becomes effectively 0, and the DSB configured to wait for it 
>> gets
>> +     * stalled, since the hardware never signals the safe window. 
>> Keep the
>> +     * set context latency at a minimum of 1 to avoid this.
>> +     */
>> +    if (DISPLAY_VER(display) >= 30)
>> +        set_context_latency = max(1, set_context_latency);
>> +
>>       return set_context_latency;
>>   }
>