On Tue, 7 May 2024 at 21:40, Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > > On Tue, May 07, 2024 at 06:19:18PM +0300, Dmitry Baryshkov wrote: > > On Tue, 7 May 2024 at 18:15, Bryan O'Donoghue wrote: > > > On 07/05/2024 16:09, Dmitry Baryshkov wrote: > > > > Ah, I see. Then why do you require the DMA-ble buffer at all? If you are > > > > providing data to VPU or DRM, then you should be able to get the buffer > > > > from the data-consuming device. > > > > > > Because we don't necessarily know what the consuming device is, if any. > > > > > > Could be VPU, could be Zoom/Hangouts via pipewire, could for argument > > > sake be GPU or DSP. > > > > > > Also if we introduce a dependency on another device to allocate the > > > output buffers - say always taking the output buffer from the GPU, then > > > we've added another dependency which is more difficult to guarantee > > > across different arches. > > > > Yes. And it should be expected. It's a consumer who knows the > > restrictions on the buffer. As I wrote, Zoom/Hangouts should not > > require a DMA buffer at all. > > Why not ? If you want to capture to a buffer that you then compose on > the screen without copying data, dma-buf is the way to go. That's the > Linux solution for buffer sharing. Yes. But it should be allocated by the DRM driver. As Sima wrote, there is no guarantee that the buffer allocated from dma-heaps is accessible to the GPU. > > > Applications should be able to allocate > > the buffer out of the generic memory. > > If applications really want to copy data and degrade performance, they > are free to shoot themselves in the foot of course. Applications (or > compositors) need to support copying as a fallback in the worst case, > but all components should at least aim for the zero-copy case. I'd say that they should aim for the optimal case. It might include both zero-copying access from another DMA master or simple software processing of some kind. > > GPUs might also have different > > requirements. Consider GPUs with VRAM. It might be beneficial to > > allocate a buffer out of VRAM rather than generic DMA mem. > > Absolutely. For that we need a centralized device memory allocator in > userspace. An effort was started by James Jones in 2016, see [1]. It has > unfortunately stalled. If I didn't have a camera framework to develop, I > would try to tackle that issue :-) I'll review the talk. However the fact that the effort has stalled most likely means that 'one fits them all' approach didn't really fly well. We have too many usecases. > > [1] https://www.x.org/wiki/Events/XDC2016/Program/Unix_Device_Memory_Allocation.pdf -- With best wishes Dmitry
