linux-kernel - Re: [RFC][PATCH 1/4] devicetree: bindings: Add linux,cma-heap tag for reserved memory

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Message-ID: <CALAqxLWnEj-c3CYGC6p23cwMqce-MV6pJOzGbp+ptWFB0NQoog@mail.gmail.com>
Date:   Wed, 6 May 2020 09:30:16 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     "Andrew F. Davis" <afd@...com>
Cc:     Brian Starkey <brian.starkey@....com>,
        lkml <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Pratik Patel <pratikp@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-mm <linux-mm@...ck.org>, nd <nd@....com>
Subject: Re: [RFC][PATCH 1/4] devicetree: bindings: Add linux,cma-heap tag for
 reserved memory

On Wed, May 6, 2020 at 9:04 AM Andrew F. Davis <afd@...com> wrote:
> On 5/4/20 4:50 AM, Brian Starkey wrote:
> > On Fri, May 01, 2020 at 11:40:16AM -0700, John Stultz wrote:
> >> So the name we expose is the CMA name itself. So with dt it will be
> >> the name of the reserved memory node that the flag property is added
> >> to.
> >>
> >
> > Yeah I'm just wondering if that's "stable" so we can say "the heap
> > will use the node name", or if saying that would cause us a headache
> > in the future.
>
>
> The issue is going to be this causes the node name in DT to become a
> kind of ABI. Right now until we have some userspace lib that enumerates
> the heaps in a stable way programs will hard-code the full heap name,
> which right now would look like:
>
> char *heap = "/dev/dma_heap/dma_heap_mem@...00000";
>

If that's what the device chose to export.

> Yuk.. we might want to look into exporting heap properties to make them
> searchable based on something other than name here soon. Or this will be
> a mess to cleanup in the future.

Eh. I don't see this as such an issue. On different systems we have
different device nodes. Some boards have more or fewer NICs, or
various partitions, etc. There has to be some device specific userland
config that determines which partitions are mounted where (this is my
"gralloc is fstab" thesis :)

I think with the heaps, qualities other than name are going to be
poorly specified or unenumerable, so any generic query interface is
going to fall down there (and be awful to use).

thanks
-john