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Date: Tue, 7 May 2024 19:36:59 +0200
From: Daniel Vetter <daniel@ffwll.ch>
To: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Hans de Goede <hdegoede@redhat.com>,
	Sumit Semwal <sumit.semwal@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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In-Reply-To: <d2a512b2-e6b1-4675-b406-478074bbbe95@linaro.org>

On Tue, May 07, 2024 at 04:15:05PM +0100, 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.

Well ... that's an entirely different issue. And it's unsolved.

Currently the approach is to allocate where the constraints are usually
most severe (like display, if you need that, or the camera module for
input) and then just pray the stack works out without too much copying.
All userspace (whether the generic glue or the userspace driver depends a
bit upon the exact api) does need to have a copy fallback for these
sharing cases, ideally with the copying accelerated by hw.

If you try to solve this by just preemptive allocating everything as cma
buffers, then you'll make the situation substantially worse (because now
you're wasting tons of cma memory where you might not even need it).
And without really solving the problem, since for some gpus that memory
might be unusable (because you cannot scan that out on any discrete gpu,
and sometimes not even on an integrated one).
-Sima

> 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.
> 
> ---
> bod

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch