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

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Date: Wed, 8 May 2024 09:38:33 +0100
From: Daniel Stone <daniel@...ishbar.org>
To: Daniel Stone <daniel@...ishbar.org>, Hans de Goede <hdegoede@...hat.com>, 
	Maxime Ripard <mripard@...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>, 
	"Bryan O'Donoghue" <bryan.odonoghue@...aro.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) ?

On Wed, 8 May 2024 at 09:33, Daniel Vetter <daniel@...ll.ch> wrote:
> On Wed, May 08, 2024 at 06:46:53AM +0100, Daniel Stone wrote:
> > That would have the unfortunate side effect of making sandboxed apps
> > less efficient on some platforms, since they wouldn't be able to do
> > direct scanout anymore ...
>
> I was assuming that everyone goes through pipewire, and ideally that is
> the only one that can even get at these special chardev.
>
> If pipewire is only for sandboxed apps then yeah this aint great :-/

No, PipeWire is fine, I mean graphical apps.

Right now, if your platform requires CMA for display, then the app
needs access to the GPU render node and the display node too, in order
to allocate buffers which the compositor can scan out directly. If it
only has access to the render nodes and not the display node, it won't
be able to allocate correctly, so its content will need a composition
pass, i.e. performance penalty for sandboxing. But if it can allocate
correctly, then hey, it can exhaust CMA just like heaps can.

Personally I think we'd be better off just allowing access and
figuring out cgroups later. It's not like the OOM story is great
generally, and hey, you can get there with just render nodes ...

Cheers,
Daniel