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From:   Benjamin Gaignard <benjamin.gaignard@linaro.org>
Date:   Thu, 25 Jul 2019 16:46:55 +0200
Message-ID: <CA+M3ks5s7GKdgUA1J3WDuRCdmQJkQ2t_CzqknAd5+S9FVynnfg@mail.gmail.com>
Subject: Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps
To:     Christoph Hellwig <hch@infradead.org>
Cc:     John Stultz <john.stultz@linaro.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Laura Abbott <labbott@redhat.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Liam Mark <lmark@codeaurora.org>,
        Pratik Patel <pratikp@codeaurora.org>,
        Brian Starkey <Brian.Starkey@arm.com>,
        Vincent Donnefort <Vincent.Donnefort@arm.com>,
        Sudipto Paul <Sudipto.Paul@arm.com>,
        "Andrew F . Davis" <afd@ti.com>,
        Xu YiPing <xuyiping@hisilicon.com>,
        "Chenfeng (puck)" <puck.chen@hisilicon.com>,
        butao <butao@hisilicon.com>,
        "Xiaqing (A)" <saberlily.xia@hisilicon.com>,
        Yudongbin <yudongbin@hisilicon.com>,
        Chenbo Feng <fengc@google.com>,
        Alistair Strachan <astrachan@google.com>,
        dri-devel <dri-devel@lists.freedesktop.org>
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Le jeu. 25 juil. 2019 =C3=A0 16:33, Christoph Hellwig <hch@infradead.org> a=
 =C3=A9crit :
>
> On Thu, Jul 25, 2019 at 03:20:11PM +0200, Benjamin Gaignard wrote:
> > > But that just means we need a flag that memory needs to be contiguous=
,
> > > which totally makes sense at the API level.  But CMA is not the only
> > > source of contigous memory, so we should not conflate the two.
> >
> > We have one file descriptor per heap to be able to add access control
> > on each heap.
> > That wasn't possible with ION because the heap was selected given the
> > flags in ioctl
> > structure and we can't do access control based on that. If we put flag
> > to select the
> > allocation mechanism (system, CMA, other) in ioctl we come back to ION =
status.
> > For me one allocation mechanism =3D one heap.
>
> Well, I agree with your split for different fundamentally different
> allocators.  But the point is that CMA (at least the system cma area)
> fundamentally isn't a different allocator.  The per-device CMA area
> still are kinda the same, but you can just have one fd for each
> per-device CMA area to make your life simple.

Form my point of view default cma area is a specific allocator because
it could migrate page to give them in priority to contigous allocation. It =
is
also one of the last region where system page are going to be allocated.
I don't think that system allocator does have the same features, either we
would have use it rather develop CMA years ago ;-). In short CMA had
solved lot of problems on our side so it had to have it own heap.