From: Thomas Zimmermann <[email protected]> Sent: Tuesday, September 30, 2025 8:04 AM > > Hi > > Am 16.09.25 um 17:00 schrieb Michael Kelley: > > From: Thomas Zimmermann <[email protected]> Sent: Tuesday, September 16, > > 2025 1:31 AM > >> Hi > >> > >> Am 09.09.25 um 05:29 schrieb Michael Kelley: > >>> From: Michael Kelley Sent: Thursday, September 4, 2025 10:36 PM > >>>> From: Thomas Zimmermann <[email protected]> Sent: Thursday, September > >>>> 4, 2025 7:56 AM > >>>>> Compositors often depend on vblanks to limit their display-update > >>>>> rate. Without, they see vblank events ASAP, which breaks the rate- > >>>>> limit feature. This creates high CPU overhead. It is especially a > >>>>> problem with virtual devices with fast framebuffer access. > >>>>> > >>>>> The series moves vkms' vblank timer to DRM and converts the hyperv > >>>>> DRM driver. An earlier version of this series contains examples of > >>>>> other updated drivers. In principle, any DRM driver without vblank > >>>>> hardware can use the timer. > >>>> I've tested this patch set in a Hyper-V guest against the > >>>> linux-next20250829 > >>>> kernel. All looks good. Results and perf are the same as reported here > >>>> [4]. > >>>> So far I haven't seen the "vblank timer overrun" error, which is > >>>> consistent > >>>> with the changes you made since my earlier testing. I'll keep running > >>>> this > >>>> test kernel for a while to see if anything anomalous occurs. > >>> As I continued to run with this patch set, I got a single occurrence of > >>> this > >>> WARN_ON. I can't associate it with any particular action as I didn't > >>> notice > >>> it until well after it occurred. > >> I've investigated. The stack trace comes from the kernel console's > >> display update. It runs concurrently to the vblank timeout. What likely > >> happens here is that the update code reads two values and in between, > >> the vblank timeout updates them. So the update then compares an old and > >> a new value; leading to an incorrect result with triggers the warning. > >> > >> I'll include a fix in the series' next iteration. But I also think that > >> it's not critical. DRM's vblank helpers can usually deal with such > >> problems. > > Thanks! I'm giving your v4 series a try now. Good that the underlying > > problem is not critical. But I was seeing the WARN_ON() output in > > dmesg every few days (a total of 4 times now), and that's not really > > acceptable even if everything continues to work correctly. > > So it's probably a good sign that I haven't heard from you recently. :) > If you haven't seen any warnings with v4, I'd like to merge the series soon. > > Best regards > Thomas >
Yes, all is good. I was delayed a bit due to some travel, but the test system has been running for a week now with no warnings or other issues. From my standpoint, the patch series is good to merge. Michael
