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 18:09:13 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Hans de Goede <hdegoede@...hat.com>
Cc: 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>, Bryan O'Donoghue <bryan.odonoghue@...aro.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, 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