From: Russell King - ARM Linux <linux@arm.linux.org.uk>
Subject: Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper
Date: Thu, 31 Mar 2016 15:14:13 +0100
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References: <1459427384-21374-1-git-send-email-boris.brezillon@free-electrons.com>
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To: Boris Brezillon <boris.brezillon@free-electrons.com>
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On Thu, Mar 31, 2016 at 02:29:42PM +0200, Boris Brezillon wrote:
> sg_alloc_table_from_buf() provides an easy solution to create an sg_table
> from a virtual address pointer. This function takes care of dealing with
> vmallocated buffers, buffer alignment, or DMA engine limitations (maximum
> DMA transfer size).

Please note that the DMA API does not take account of coherency of memory
regions other than non-high/lowmem - there are specific extensions to
deal with this.

What this means is that having an API that takes any virtual address
pointer, converts it to a scatterlist which is then DMA mapped, is
unsafe.

It'll be okay for PIPT and non-aliasing VIPT cache architectures, but
for other cache architectures this will hide this problem and make
review harder.

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