linux-kernel - Re: [PATCH v2 4/4] dma-buf: heaps: add chunk heap to dmabuf heaps

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Date:   Tue, 1 Dec 2020 15:38:14 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Hyesoo Yu <hyesoo.yu@...sung.com>,
        Matthew Wilcox <willy@...radead.org>, david@...hat.com,
        iamjoonsoo.kim@....com, vbabka@...e.cz,
        Suren Baghdasaryan <surenb@...gle.com>,
        KyongHo Cho <pullip.cho@...sung.com>,
        John Dias <joaodias@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Brian Starkey <Brian.Starkey@....com>,
        linux-media <linux-media@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh@...nel.org>,
        Christian Koenig <christian.koenig@....com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [PATCH v2 4/4] dma-buf: heaps: add chunk heap to dmabuf heaps

On Tue, Dec 1, 2020 at 2:55 PM Minchan Kim <minchan@...nel.org> wrote:
> On Tue, Dec 01, 2020 at 11:48:15AM -0800, John Stultz wrote:
> > On Tue, Dec 1, 2020 at 9:51 AM Minchan Kim <minchan@...nel.org> wrote:
> >
> > Thanks for reworking and resending this!
> >
> > ...
> > > +static int __init chunk_heap_init(void)
> > > +{
> > > +       struct cma *default_cma = dev_get_cma_area(NULL);
> > > +       struct dma_heap_export_info exp_info;
> > > +       struct chunk_heap *chunk_heap;
> > > +
> > > +       if (!default_cma)
> > > +               return 0;
> > > +
> > > +       chunk_heap = kzalloc(sizeof(*chunk_heap), GFP_KERNEL);
> > > +       if (!chunk_heap)
> > > +               return -ENOMEM;
> > > +
> > > +       chunk_heap->order = CHUNK_HEAP_ORDER;
> > > +       chunk_heap->cma = default_cma;
> > > +
> > > +       exp_info.name = cma_get_name(default_cma);
> >
> > So, this would create a chunk heap name with the default CMA name,
> > which would be indistinguishable from the heap name used for the plain
> > CMA heap.
> >
> > Probably a good idea to prefix it with "chunk-" so the heap device
> > names are unique?
>
> That will give an impression to user that they are using different CMA
> area but that's not true. IMHO, let's be honest at this moment.

I disagree.  The dmabuf heaps provide an abstraction for allocating a
type of memory, and while your heap is pulling from CMA, you aren't
"just" allocating CMA as the existing CMA heap would suffice for that.

Since you need a slightly different method to allocate high order
pages in bulk, we really should have a unique way to name the
allocator interface. That's why I'd suggest the "chunk-" prefix to the
heap name.

thanks
-john