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Date: Thu, 9 May 2024 00:54:50 +0300
From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Bryan O'Donoghue <bryan.odonoghue@linaro.org>,
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Subject: Re: Safety of opening up /dev/dma_heap/* to physically present users
 (udev uaccess tag) ?
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On Wed, May 08, 2024 at 10:39:58AM +0200, Daniel Vetter wrote:
> On Tue, May 07, 2024 at 10:59:42PM +0300, Dmitry Baryshkov wrote:
> > On Tue, 7 May 2024 at 21:40, Laurent Pinchart 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.
> 
> I think there's two reasons:
> 
> - It's a really hard problem with many aspects. Where you need to allocate
>   the buffer is just one of the myriad of issues a common allocator needs
>   to solve.

The other large problem is picking up an optimal pixel format. I wonder
if that could be decoupled from the allocation. That could help moving
forward.

> - Every linux-based os has their own solution for these, and the one that
>   suffers most has an entirely different one from everyone else: Android
>   uses binder services to allow apps to make these allocations, keep track
>   of them and make sure there's no abuse. And if there is, it can just
>   nuke the app.

-- 
Regards,

Laurent Pinchart