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

Wed Sep 21 09:39:51 UTC 2022

Hi

Am 21.09.22 um 10:37 schrieb 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.

I try to resolve that in patch 5. The fb_dirty flag signals that the 
framebuffer's content changed in some way. Patches 4 and 5 could be seen 
as one change, but that might overload the resulting patch. (Maybe it's 
not well designed overall. :/)

> 
> 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

Assuming the specified fb to be damaged seems like a non-issue to me.

The problem is with the other framebuffers: if userspace sends us a 
CURSOR_MOVE ioctl, we can safely assume the cursor fb to be damaged. But 
we don't want the primary plane's fb to be damaged. Or else we'd redraw 
the full primary plane.

> 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.

The problem again is not in the damage information on planes that 
legitimately need a redraw. It's all the other planes that are being 
redrawn as well. Or is there some scenario that I don't see?

> 
> 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.

There is support for changing scanout coordinates in 
drm_atomic_helper_damage_iter_init() in patch 2. In general, maybe the 
heuristic needs to be stricter to only handle updates that only change 
damage.

For now, the problem only happens after converting ast to SHMEM. All the 
other SHMEM-based drivers use a single plane where the problem doesn't 
happen; or only reprogram the scanout address, which is fast. If the 
damage-handling logic imposes problems on other drivers, some of it 
could possibly be moved into ast itself.

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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