Re: Safety of opening up /dev/dma_heap/* to physically present users (udev uaccess tag) ?

Dmitry Baryshkov
2024-05-07T19:59:53Z

On Tue, 7 May 2024 at 21:40, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> 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