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

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Message-ID: <656423737618913a19633b4b4c39e2e8de54546d.camel@collabora.com>
Date: Wed, 22 May 2024 09:02:21 -0400
From: Nicolas Dufresne <nicolas.dufresne@...labora.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>, Simon Ser
	 <contact@...rsion.fr>
Cc: Maxime Ripard <mripard@...hat.com>, Bryan O'Donoghue
 <bryan.odonoghue@...aro.org>, Dmitry Baryshkov
 <dmitry.baryshkov@...aro.org>,  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>, Andrey Konovalov <andrey.konovalov.ynk@...il.com>
Subject: Re: Safety of opening up /dev/dma_heap/* to physically present
 users (udev uaccess tag) ?

Le jeudi 16 mai 2024 à 14:20 +0300, Laurent Pinchart a écrit :
> On Thu, May 16, 2024 at 07:00:31AM +0000, Simon Ser wrote:
> > On Tuesday, May 14th, 2024 at 22:42, Laurent Pinchart wrote:
> > 
> > > My experience on Arm platforms is that the KMS drivers offer allocation
> > > for scanout buffers, not render buffers, and mostly using the dumb
> > > allocator API. If the KMS device can scan out YUV natively, YUV buffer
> > > allocation should be supported. Am I missing something here ?
> > 
> > Note that dumb buffers are only intended for simple software-rendering
> > use-cases. Anything more complicated (e.g. involving GPU rendering)
> > should use another mechanism.
> 
> Sure. Even if dumb buffers may work for GPU rendering in some cases,
> there's no guarantee they will, so they shouldn't be used.
> 
> My comment was related to scanout buffers, as I was puzzled by Nicolas
> mentioning how "KMS drivers only offer allocation for render buffers".
> On Arm platforms the render buffers are allocated on the GPU's DRM
> device as far as I understand, while the KMS drivers allocate scanout
> buffers using the dumb buffers API.
> 

The message is getting distorted. I'm saying that not all supported formats have
an allocation API in DRM/KMS drivers. Most YUV formats needed for media handling
(GPU or scannout) are not supported.

Nicolas

p.s. I feel like commenters thinks its evident for userspace application to know
if they are doing scanout or GPU ... while in reality, they offload their
allocated buffer to a compositor which will have to dynamically juggle between
these two with its own heuristic.