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Date: Tue, 7 May 2024 18:09:13 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
To: Hans de Goede <hdegoede@redhat.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>, 
	Benjamin Gaignard <benjamin.gaignard@collabora.com>, Brian Starkey <Brian.Starkey@arm.com>, 
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	Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>, Lennart Poettering <mzxreary@0pointer.de>, 
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	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Bryan O'Donoghue <bryan.odonoghue@linaro.org>, 
	Milan Zamazal <mzamazal@redhat.com>, Maxime Ripard <mripard@redhat.com>, 
	Andrey Konovalov <andrey.konovalov.ynk@gmail.com>
Subject: Re: Safety of opening up /dev/dma_heap/* to physically present users
 (udev uaccess tag) ?
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In-Reply-To: <3c0c7e7e-1530-411b-b7a4-9f13e0ff1f9e@redhat.com>

On Tue, May 07, 2024 at 04:34:24PM +0200, Hans de Goede wrote:
> Hi Dmitry,
> 
> On 5/7/24 3:32 PM, Dmitry Baryshkov wrote:
> > On Mon, May 06, 2024 at 01:49:17PM +0200, Hans de Goede wrote:
> >> Hi dma-buf maintainers, et.al.,
> >>
> >> Various people have been working on making complex/MIPI cameras work OOTB
> >> with mainline Linux kernels and an opensource userspace stack.
> >>
> >> The generic solution adds a software ISP (for Debayering and 3A) to
> >> libcamera. Libcamera's API guarantees that buffers handed to applications
> >> using it are dma-bufs so that these can be passed to e.g. a video encoder.
> >>
> >> In order to meet this API guarantee the libcamera software ISP allocates
> >> dma-bufs from userspace through one of the /dev/dma_heap/* heaps. For
> >> the Fedora COPR repo for the PoC of this:
> >> https://hansdegoede.dreamwidth.org/28153.html
> > 
> > Is there any reason for allocating DMA buffers for libcamera through
> > /dev/dma_heap/ rather than allocating them via corresponding media
> > device and then giving them away to DRM / VPU / etc?
> > 
> > At least this should solve the permission usecase: if the app can open
> > camera device, it can allocate a buffer.
> 
> This is with a software ISP, the input buffers with raw bayer data
> come from a v4l2 device, but the output buffers with the processed
> data are purely userspace managed in this case.

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. If the data is further processed by
a userspace app, then it should not require DMA memory at all.

My main concern is that dma-heaps is both too generic and too limiting
for the generic library implementation.

-- 
With best wishes
Dmitry