dmabuf modifiers for HW overlays

[Scott Anderson]

On 2/08/19 10:19 pm, Martin Stransky wrote:
> On 8/2/19 12:04 PM, Martin Stransky wrote:
>>>> Also I wonder if it's feasible to use any modifiers as I need
>>>> plain/linear buffer to draw into by skia. I suspect when I create the
>>>> buffer with modifiers and then I map it to CPU memory for SW drawing,
>>>> intermediate buffer is created and then the pixels are de-composed back
>>>> to GPU memory.
>>>
>>> No, I don't believe there is any kind of intermediate buffer behind the
>>> scenes in GBM or dmabuf ioctls/mmap. OpenGL drivers may use
>>> intermediate buffers. Some EGLImage operations are allowed to create
>>> more buffers behind the scenes but I think implementations try to avoid
>>> it. Copies are bad for performance, and implicit copies are unexpected
>>> performance bottle-necks. So yes, I believe you very much need to
>>> ensure the buffer gets allocated as linear from the start.
>>>
>>> Some hardware may have hardware tiling units, that may be able to
>>> represent a linear CPU view into a tiled buffer, but I know very little
>>> of that. I think they might need driver-specific ioctls to use or
>>> something, and are a scarce resource.
> 
> The HW overlays is another use-case I'm working on. I want to draw 2D 
> data to dmabuf buffer by CPU and send it directly to GPU to render from 
> (bind as wl_buffer and attach it to wl_subsurface). I guess it also 
> depends on compositor how to handles that and how does that handle 
> mutter?. Does mutter such dmabuf buffer pass directly to GPU?
 >
> Also is it feasible to use *any* modifiers here and which flags should I 
> pass to gbm_bo_create()? I want to use plain DRM_FORMAT_ARGB8888 for the 
> dmabuf buffer as it's easy to draw into but I wonder if that buffer can 
> be used by GPU directly for compositing and how effective is tha >
> Thanks,
> ma
As the client, you don't need to try do the compositor's/driver's job 
for it. If you want CPU accessable buffers, just use wl_shm. Trying to 
push CPU data into a dmabuf directly can potentially hurt more than it 
helps. You should benchmark if that actually leads to increased 
performance, especially across different hardware/drivers.

Regarding overlays, a pending change to wp_linux_dmabuf [1] would give 
the client more information about how to allocate its GPU buffers so 
that's it's more likely to work with hardware overlays or be more 
efficient in general, but it's a complicated thing to get correct, and 
it'll never be perfect.

I'm also puzzled as to why you would need to use GBM directly. Some 
clients have a justified use for it, but most don't, and is mainly 
useful for display servers instead. Are the EGL/OpenGL APIs not sufficient?

Scott

[1]: 
https://lists.freedesktop.org/archives/wayland-devel/2018-November/039660.html