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@free-electrons.com 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@free-electrons.com 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