linux-kernel - Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps

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Message-ID: <20190725140448.GA25010@infradead.org>
Date:   Thu, 25 Jul 2019 07:04:48 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Andrew F. Davis" <afd@...com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Pratik Patel <pratikp@...eaurora.org>,
        Brian Starkey <Brian.Starkey@....com>,
        Vincent Donnefort <Vincent.Donnefort@....com>,
        Sudipto Paul <Sudipto.Paul@....com>,
        Xu YiPing <xuyiping@...ilicon.com>,
        "Chenfeng (puck)" <puck.chen@...ilicon.com>,
        butao <butao@...ilicon.com>,
        "Xiaqing (A)" <saberlily.xia@...ilicon.com>,
        Yudongbin <yudongbin@...ilicon.com>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps

On Thu, Jul 25, 2019 at 09:31:50AM -0400, Andrew F. Davis wrote:
> But that's just it, dma-buf does not assume buffers are backed by normal
> kernel managed memory, it is up to the buffer exporter where and when to
> allocate the memory. The memory backed by this SRAM buffer does not have
> the normal struct page backing. So moving the map, sync, etc functions
> to common code would fail for this and many other heap types. This was a
> major problem with Ion that prompted this new design.

The code clearly shows it has page backing, e.g. this:

+	sg_set_page(table->sgl, pfn_to_page(PFN_DOWN(buffer->paddr)), buffer->len, 0);

and the fact that it (and the dma-buf API) uses scatterlists, which 
requires pages.