Re: [PATCH 0/4] vkms: Switch to shadow-buffered plane state

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 05.07.21 um 16:20 schrieb Daniel Vetter:
> On Mon, Jul 05, 2021 at 12:05:28PM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 05.07.21 um 11:27 schrieb Daniel Vetter:
>>> On Mon, Jul 05, 2021 at 09:46:29AM +0200, Thomas Zimmermann wrote:
>>>> Vkms copies each plane's framebuffer into the output buffer; essentially
>>>> using a shadow buffer. DRM provides struct drm_shadow_plane_state, which
>>>> handles the details of mapping/unmapping shadow buffers into memory for
>>>> active planes.
>>>>
>>>> Convert vkms to the helpers. Makes vkms use shared code and gives more
>>>> test exposure to shadow-plane helpers.
>>>>
>>>> Thomas Zimmermann (4):
>>>>     drm/gem: Export implementation of shadow-plane helpers
>>>>     drm/vkms: Inherit plane state from struct drm_shadow_plane_state
>>>>     drm/vkms: Let shadow-plane helpers prepare the plane's FB
>>>>     drm/vkms: Use dma-buf mapping from shadow-plane state for composing
>>>
>>> So I think right now this fits, but I think it'll mismit going forward: We
>>> don't really have a shadow-plane that we then toss to the hw, it's a
>>> shadow-crtc-area. Right now there's no difference, because we don't
>>> support positioning/scaling the primary plane. But that's all kinda stuff
>>> that's on the table.
>>>
>>> But conceptually at least the compositioning buffer should bet part of the
>>> crtc, not of the primary plane.
>>>
>>> So not sure what to do, but also coffee hasn't kicked in yet, so maybe I'm
>>> just confused.
>>
>> I'm not sure if I understand your concern. Can you elaborate? The
>> compositing output buffer is not affected by this patchset. Only the input
>> frambuffers of the planes. Those are shadow buffers. AFAICT the composer
>> code memcpy's the primary plane and then blends the other planes on top.
>> Supporting transformation of the primary plane doesn't really change much
>> wrt to the vmaping of input fbs.
> 
> Yeah that's the current implementation, because that's easier. But
> fundamentally we don't need a copy of the input shadow plane, we need a
> scratch area that's sized for the crtc.

I stll don't understand. Are you talking about the whole patchset or 
just patch 4? Because if you want to do anything with vkms planes, you 
have to vmap the framebuffer BOs into memory. (right?) That's all that 
shadow-buffer planes do. Patches 1 to 3 have nothing to do with memcpy. 
Admittedly, Patch 4 does some minor refactoring, but only because it 
became a low-hanging fruit.

Best regards
Thomas


-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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