Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper

From	Russell King - ARM Linux <linux@arm.linux.org.uk>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper
Date	2016-03-31 16:20 +0200
Message-ID	<riLC3-6ib-33@gated-at.bofh.it> (permalink)
References	<riK3g-531-23@gated-at.bofh.it> <riK3h-531-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

Thread

[PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-03-31 14:40 +0200
  Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-03-31 16:20 +0200
    Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-03-31 16:50 +0200
      Re: [PATCH 2/4] scatterlist: add sg_alloc_table_from_buf() helper Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-03-31 17:20 +0200

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