On Tue, Jul 18, 2017 at 9:07 AM, Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > On Mon, Jul 17, 2017 at 02:57:19PM +0800, Chen-Yu Tsai wrote: >> On Mon, Jul 17, 2017 at 2:55 PM, Maxime Ripard >> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: >> > On Fri, Jul 14, 2017 at 04:56:01PM +0800, Chen-Yu Tsai wrote: >> >> Hi, >> >> >> >> On Thu, Jul 13, 2017 at 10:41 PM, Maxime Ripard >> >> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: >> >> > In the earlier display engine designs, any register access while a commit >> >> > is pending is forbidden. >> >> > >> >> > One of the symptoms is that reading a register will return another, random, >> >> > register value which can lead to register corruptions if we ever do a >> >> > read/modify/write cycle. >> >> >> >> Alternatively, if changes to the backend (layers) are guaranteed to happen >> >> while the CRTC is disabled (which seems to be the case after looking at >> >> drm_atomic_helper_commit_planes and drm_atomic_helper_commit_tail), we >> >> could just turn on register auto-commit all the time and not deal with >> >> this. >> > >> > As far as I understand, it will only be the case if we need a new >> > modeset or we changed the active CRTC or connectors. But if you change >> > only the format, buffers or properties it won't be the case, and we'll >> > need to commit. >> >> So in other words, if someone were to use it for actual compositing and >> moved the upper composited layer around, we would need commit support to be >> safe. >> >> Sounds more or less like something a video player would do. > > Not only that. A change of buffer will happen every frame or so, and > we can change the format whenever we want too (even if it's usually > going to be in sync with a new buffer). Changing a property can happen > any time too (like zpos for example). You can upgrade any property change to an atomic modeset by e.g. setting connector->mode_changed (and then making sure to call check_modeset() helper again perhaps). This is for cases where your hw can't handle a property change within 1 vblank. The default is just the solution for most common hw. The other way round works too, you can clear these flags in your atomic_check callbacks. But that requires a bit more care (to make sure you never clear it when there's something else also changing that still needs a full modeset sequence to commit to hw). -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch
