linux-kernel - Re: [PATCH v2 09/10] ARM: dts: sun7i-a20: Add Video Engine and reserved memory nodes

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Message-ID: <20180504091555.idgtzey53lozj2uh@flea>
Date:   Fri, 4 May 2018 11:15:55 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Chen-Yu Tsai <wens@...e.org>, Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Alexandre Courbot <acourbot@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v2 09/10] ARM: dts: sun7i-a20: Add Video Engine and
 reserved memory nodes

On Fri, May 04, 2018 at 10:47:44AM +0200, Paul Kocialkowski wrote:
> > > > > +			reg = <0x01c0e000 0x1000>;
> > > > > +			memory-region = <&ve_memory>;
> > > > 
> > > > Since you made the CMA region the default one, you don't need to
> > > > tie
> > > > it to that device in particular (and you can drop it being
> > > > mandatory
> > > > from your binding as well).
> > > 
> > > What if another driver (or the system) claims memory from that zone
> > > and
> > > that the reserved memory ends up not being available for the VPU
> > > anymore?
> > > 
> > > Acccording to the reserved-memory documentation, the reusable
> > > property
> > > (that we need for dmabuf) puts a limitation that the device driver
> > > owning the region must be able to reclaim it back.
> > > 
> > > How does that work out if the CMA region is not tied to a driver in
> > > particular?
> > 
> > I'm not sure to get what you're saying. You have the property
> > linux,cma-default in your reserved region, so the behaviour you
> > described is what you explicitly asked for.
> 
> My point is that I don't see how the driver can claim back (part of) the
> reserved area if the area is not explicitly attached to it.
> 
> Or is that mechanism made in a way that all drivers wishing to use the
> reserved memory area can claim it back from the system, but there is no
> priority (other than first-come first-served) for which drivers claims
> it back in case two want to use the same reserved region (in a scenario
> where there isn't enough memory to allow both drivers)?

This is indeed what happens. Reusable is to let the system use the
reserved memory for things like caches that can easily be dropped when
a driver wants to use the memory in that reserved area. Once that
memory has been allocated, there's no claiming back, unless that
memory segment was freed of course.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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