dmabuf modifiers for HW overlays

[Martin Stransky]

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 that.

Thanks,
ma.

-- 
Martin Stransky
Software Engineer / Red Hat, Inc