[Mesa-dev] [PATCH 1/3] egl/android: Delete set_damage_region from egl dri vtbl

[Harish Krupo]

Eric Anholt <eric at anholt.net> writes:

> Harish Krupo <harish.krupo.kps at intel.com> writes:
>
>> Hi Eric,
>>
>> Eric Anholt <eric at anholt.net> writes:
>>
>>> Harish Krupo <harish.krupo.kps at intel.com> writes:
>>>
>>>> The intension of the KHR_partial_update was not to send the damage back
>>>> to the platform but to send the damage to the driver to ensure that the
>>>> following rendering could be restricted to those regions.
>>>> This patch removes the set_damage_region from the egl_dri vtbl and all
>>>> the platfrom_*.c files.
>>>> Then upcomming patches add a new dri2 interface for the drivers to
>>>> implement
>>>>
>>>> Signed-off-by: Harish Krupo <harish.krupo.kps at intel.com>
>>>
>>> Why shouldn't the platform know about the damage region in a swap, if
>>> it's available?  It looks like it was successfully used for Android, and
>>> we should be using it for Present as well.
>>
>> From the spec [1], the damage region referred to by partial_update spec is
>> the damaged part of the buffer when it is used again. The damage that the
>> compositor/platform needs to know is the damage between the (n-1)th
>> frame and the nth frame. Quoting from the spec:
>> "   The surface damage for frame n is the difference between frame n and frame
>>     (n-1), and represents the area that a compositor must recompose."
>> This is the damage referred to by the swap_buffers_with_damage spec [2],
>> whereas the partial_update damage region's objective is to restrict the subsequent
>> rendering operations on the back buffer, to only those regions which have changed since
>> that buffer was last used. This information is available as the buffer
>> age. Some more information: [3].
>
> OK, let's document that in the new internal API you're adding then.
> Things I'd want to know as an implementer of the hook:
>
> 1) Am I guaranteed that it's called before the frame is started?
>

No. When no damage region is set, the whole surface should be considered
damaged. As a matter of fact, the damage region is set to full surface
when the frame boundary is reached (i.e. swapbuffersXXX is called).

> 2) Is the behavior if the client draws outside of the partial update
> damage region defined?  (is it "the driver must not change pixels
> outside of the partial region" or "the driver might not change pixels
> outside of the partial region")
>

If I have understood the spec correctly, then the damage regions set are
a hint to the driver so that it can optimize the rendering by
restricting the client's drawing commands to only the damaged region.
In the current implementation, although the damage regions are sent back
to the compositor instead of sending it to the driver, no issues are
observed with the rendered output and it passes deqp tests. This
supports the argument that the damages are only a hint.

> 3) Is the client guaranteed to fully initialize pixels in the partial
> update region, or might it depend on previous contents?

If the above argument is right then it means that the client would
actually initialize the pixels of the full buffer but expect that the
driver renders only the damaged regions.

Thank you
Regards
Harish Krupo