On 1/4/23 03:11, Brian Norris wrote: > On Tue, Jan 03, 2023 at 07:04:00PM +0100, Michel Dänzer wrote: >> On 12/21/22 23:02, Brian Norris wrote: > >>> 3. leave vblank enabled even in the presence of PSR > > I'm leaning toward this. If this means vblank interrupts will arrive as expected even while in PSR, that may be the ideal solution indeed. >> 5. Go/stay out of PSR while vblank interrupts are enabled (probably want to make sure vblankoffdelay is set up such that vblank interrupts are disabled ASAP) > > That seems not extremely nice, to waste power. Based on the earlier > downstream implementation (which left vblank interrupts enabled), I'd > wager there's a much larger power win from PSR (on the display-transport > and memory-bandwidth costs), relative to the power cost of vblank > interrupts. > > Also, messing with vblankoffdelay sounds likely to trigger the races > mentioned in the drm_vblank.c kerneldoc. Not sure how likely that is, quite a few drivers are setting dev->vblank_disable_immediate = true. With that, vblank interrupts should generally be enabled only while there are screen updates as well[0], in which case PSR shouldn't be possible anyway. [0] There may be user space which uses the vblank ioctls even while there are no screen updates though, which would prevent PSR in this case. >>> [1] Or is it? I don't really know the DRM_IOCTL_WAIT_VBLANK ABI >>> contract in the presence of PSR, but I believe the upstream PSR >>> support has always worked this way. I could imagine >>> DRM_IOCTL_WAIT_VBLANK users might not love seeing EINVAL here >>> though. >> >> Yeah, that's pretty likely to cause issues with existing real-world user space. > > OK. Any hints as to what kind of user space uses this? I don't have any specific example, just thinking about how user space could respond to the vblank ioctls returning an error, and it would seem to be generally either of: * Just run non-throttled, which might negate any energy savings from PSR * Fail to work altogether > I'm in part simply curious, but I'm also wondering what the > error-handling and reliability (e.g., what if vblanks don't come?) > expectations might be in practice. If vblank events never come, user space might freeze. -- Earthling Michel Dänzer | https://redhat.com Libre software enthusiast | Mesa and Xwayland developer
