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

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Date: Tue, 7 May 2024 18:19:18 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: "Bryan O'Donoghue" <bryan.odonoghue@...aro.org>
Cc: Hans de Goede <hdegoede@...hat.com>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T.J. Mercier" <tjmercier@...gle.com>, 
	Christian König <christian.koenig@....com>, 
	Lennart Poettering <mzxreary@...inter.de>, Robert Mader <robert.mader@...labora.com>, 
	Sebastien Bacher <sebastien.bacher@...onical.com>, 
	Linux Media Mailing List <linux-media@...r.kernel.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, linaro-mm-sig@...ts.linaro.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Milan Zamazal <mzamazal@...hat.com>, 
	Maxime Ripard <mripard@...hat.com>, Andrey Konovalov <andrey.konovalov.ynk@...il.com>
Subject: Re: Safety of opening up /dev/dma_heap/* to physically present users
 (udev uaccess tag) ?

On Tue, 7 May 2024 at 18:15, Bryan O'Donoghue
<bryan.odonoghue@...aro.org> 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. Applications should be able to allocate
the buffer out of the generic memory. 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.

-- 
With best wishes
Dmitry