[ Dropping intel-gfx list from CC, as it automatically rejects posts from non-subscribers ] On Mit, 2011-11-16 at 21:39 -0800, Andy Lutomirski wrote: > There are two possible races when disabling vblanks. If the IRQ > fired but the hardware didn't update its counter yet, then we store > too low a hardware counter. (Sensible hardware never does this. > Apparently not all hardware is sensible.) The thing is, 'the' IRQ and 'the' hardware counter aren't necessarily about the same thing. We want an IRQ which triggers at the beginning of vertical blank, but the Radeon hardware counters increment at the beginning of scanout, i.e. at the end of vertical blank. Does that make the hardware 'broken' or 'not sensible'? > If, on the other hand, the counter updated but the IRQ didn't fire > yet, we store too high a counter. > > We handled the former case with a heuristic based on timestamps and > we did not handle the latter case. By saving a little more state, > we can handle both cases exactly: all we need to do is watch for > changes in the difference between the hardware and software vblank > counts. I'm afraid that can't work: Some (AFAIR also Intel) hardware resets the counter to 0 when the CRTC is disabled / enabled (e.g. for DPMS, or a modeset). That's why we ended up only counting interrupts while the IRQ is enabled, and only using the hardware counter to fill in while the IRQ is disabled. The hardware counter cannot be assumed to have any defined behaviour between enabling and disabling the IRQ. To compensate for this, the drivers call drm_vblank_pre_modeset (which enables the IRQ, which also updates the virtual counter from the hardware counter) before disabling the CRTC and drm_vblank_post_modeset (which disables the IRQ, which also records the hardware counter) after enabling the CRTC. > This compiles but is not very well tested, because I don't know what > tests to run. Not sure there are any good tests yet. Mario, would it be possible to extract something exercising the various corner cases from your toolkit? -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Debian, X and DRI developer _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
