Hi Ville Am 20.09.22 um 16:31 schrieb Ville Syrjälä:
On Tue, Sep 20, 2022 at 03:56:18PM +0200, Thomas Zimmermann wrote:Set partial updates on a plane if the framebuffer has not been changed on an atomic commit. If such a plane has damage clips, the driver will use them; otherwise the update is effectively empty. Planes that change their framebuffer still perform a full update.I have a feeling you're sort of papering over some inefficient userspace that's asking updates on planes that don't actually need them. I'm not sure adding more code for that is a particularly great idea. Wouldn't it be better to just fix the userspace code?
Some more context might be in order:The ast driver currently uses VRAM helpers, which leads to many problems; including blank screens from the low amount of video memory. The best solution is to switch SHMEM with BOs on system meory. The video memory is only the internal buffer for scanout. This update involves a mempcy from the BO to video memory.
Ast's hardware is really slow, so it makes sense to reduce the updates to video memory to a minimum. There's support for cursor planes, which really makes a difference here.
But doing any cursor-plane update (e.g., moving, animation) with SHMEM and the current damage helpers always results in a full-screen memcpy from the BO to the video memory for the primary plane. As the ast hardware is slow, performance goes down to a an estimated 5 frame per seconds. After moving the mouse, one can watch the mouse cursor follow along the screen for the next seconds. Userspace sends the movement information and DRM slowly processes them. The same can be observed for cursor animation.
The problem is that each change to the cursor plane results in an atomic_update call for the primary plane. Not having damage information for the plane just means 'update everything'. Not a problem if redrawing consists of reprogramming the scanout address. For a memcpy it's not feasible.
So can this be fixed in userspace? No realistically IMHO. I've seen this problem on Weston, Wayland-Gnome and X11-Gnome. And they are all problematic in their own way. (That's why there are multiple patches needed.) For example, X11 uses the legacy mouse ioctl, which one of the patches fixes. The other userspace needs the other heuristics. A potential userspace fix would mean to always set empty-damage information on all planes that don't get an update. And I don't consider X11 fixable TBH.
Any property change on the plane could need a full plane update as well (eg. some color mangement stuff etc.). So you'd have to keep adding exceptions to that list whenever new properties are added.
It's not about the occasional change of a property. It's the constant sluggish redraw when the interface is supposed to be snappy, such as mouse interaction.
And I think the semantics of the ioctl(s) has so far been that basically any page flip (whether or not it actually changes the fb) still ends up doing whatever flushing is needed to guarantee the fb contents are up to date on the screen (if someone sneaked in some front buffer rendering in between). Ie. you could even use basically a nop page flip in place of dirtyfb.
That's why full updates are still the default. Only in cases where it's save to avoid an update of unaffected planes, we do so.
I know that we don't give performance guarantees to userspace. But using cursor/overlay planes should be faster then not using them. I think that's a reasonable assumption for userspace to make.
Another thing the ioctls have always done is actually perform the commit whether anything changed or nor. That is, they still do all the same the same vblanky stuff and the commit takes the same amount of time. Not sure if your idea is to potentially short circuit the entire thing and make it take no time at all?
The patches don't change the overall commit logics. All they do is to set damage updates to a size of 0 if a plane reliably does not need an update. A driver's atomic_update still runs, but the damage iterator does not return any damaged areas.
Best regards Thomas
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Ivo Totev
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