From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v6 4/5] dma-buf: heaps: Add CMA heap to dmabuf heaps
Date: Thu, 25 Jul 2019 07:04:48 -0700
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To: "Andrew F. Davis" <afd@ti.com>
Cc: Christoph Hellwig <hch@infradead.org>, John Stultz <john.stultz@linaro.org>, lkml <linux-kernel@vger.kernel.org>, Laura Abbott <labbott@redhat.com>, Benjamin Gaignard <benjamin.gaignard@linaro.org>, 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>, 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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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.