[PATCH 4/5] drm/damage-helper: Do partial updates if framebuffer has not been changed

[Ville Syrjälä]

On Wed, Sep 21, 2022 at 09:59:10AM +0200, Thomas Zimmermann wrote:
> 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.

Umm. Maybe I misread the patch in my haste but it seemed 
that you consider the thing undamaged if the fb pointer
was unchanged. That goes against what I wrote above.

Though I don't really know if a there is software relying on
that behaviuor. I suppose one idea could be to keep that
behaviour for the legacy ioctls but not for atomic. Ee. any
fb directly specified in a legacy setcrtc/setplane/pageflip
is always considered fully damaged. But including an the same
fb in the atomic ioctl does not imply damage. That would
mean atomic has to rely on specifying damage explicitly, and
any userspace that doesn't do that will be broken.

Or we could introduce a client cap for it I guess, but that
would require (minimal) userspace changes.

> 
> 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.

What I gathered is that you seemed to only consider the fb 
contents as something that needs damage handling. That is not
the case for stuff like eDP PSR and DSI command mode displays
where we need to upload a new full frame whenever also the
non-damaged fb contents would get transformed by the hardware
on the way to the remote frambuffer. That would mean any change
in eg. scanout coordinates, color management, etc.

-- 
Ville Syrjälä
Intel