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). Maxime -- Maxime Ripard, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com
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