On 2026-09-22 14:04, Matthew Schwartz wrote:
> On 9/22/26 10:50 AM, Leo Li wrote:
>>
>>
>> On 2026-09-21 15:47, Matthew Schwartz wrote:
>>> Hi George,
>>>
>>> After updating to Linux 7.2.x, I noticed gamescope's frame limiter falling
>>> below the requested rate. The reproducer was a Navi 33 driving 4K120
>>> through a DP-HDMI PCON, with VRR disabled. A 40fps limit was producing
>>> roughly 37fps.
>>>
>>> Gamescope was pacing against the fixed 120Hz refresh interval, but we
>>> observed frame intervals stretching to around 9.34ms instead of 8.33ms.
>>> The OTG vtotal also increased from 2249 to 2521. This was enough to
>>> disrupt the limiter's pacing despite userspace leaving VRR_ENABLED=0.
>>
>> Hi Matt,
>>
>> I wonder if this has to do with re-introducing 2-frame vblank off for NV3+
>> DGPUs, which made it into 7.2:
>>
>> fba211b078d6 ("Revert "drm/amd/display: Restore 5s vbl offdelay for NV3x+
>> DGPUs"")
>>
>> Since gamescope is only updating every 3rd frame, it's possible vblanks were
>> turned off during those two frames, signaling driver to enable idle
>> optimizations.
>>
>> To test this idea, does bumping this line to something like 5 frames
>> ((u64)50 *) help?
>> https://elixir.bootlin.com/linux/v7.3-rc3/source/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c#L3377
>
> Tested this by reverting my own patch in the kernel and replacing it with
> this, and unfortunately the issue
> still reproduces here. In Hades 2, a 40FPS limit is locking to 37FPS
> erroneously.
Interesting... Maybe try reverting fba211b078d6 completely as well.
If that doesn't help, could you try isolating FPO by force disabling it
(without applying this fix, of course)?
Set `disable_fpo_optimizations = true` in both dcn32_resource.c and
dcn321_resource.c
- Leo
>
> Matt
>
>>
>> Thanks,
>> Leo
>>>
>>> Tracing this led to allow_freesync remaining true for a VRR-capable
>>> sink even when the FreeSync state was inactive. That allowed FPO to
>>> stretch vblank around UCLK switches. The PCON whitelist change exposed
>>> this on our setup by making the sink eligible for that path.
>>>
>>> The intent is to keep the frame period fixed when userspace has
>>> disabled VRR, while retaining FPO eligibility when variable refresh is
>>> actually requested. With the patch, the original setup holds the
>>> requested 40fps again. I also tested VRR toggling on a Legion Go 2's
>>> internal eDP OLED panel and did not observe a regression.
>>>
>>> Thanks,
>>> Matt
>>
>
