linux-kernel - Re: semantics of dma_map

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Date:   Mon, 17 Jul 2017 09:46:30 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Christoph Hellwig <hch@....de>
Cc:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        bart.vanassche@...disk.com,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Krzysztof Kozlowski <k.kozlowski@...sung.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        arcml <linux-snps-arc@...ts.infradead.org>
Subject: Re: semantics of dma_map_single()

On Mon, 2017-07-17 at 09:06 -0700, Vineet Gupta wrote:
> Hi Christoph,
> 
> On 07/16/2017 11:42 PM, Christoph Hellwig wrote:
> > 
> > I would expect that it would support any contiguous range in
> > the kernel mapping (e.g. no vmalloc and friends).  But it's not
> > documented anywhere, and if no in kernel users makes use of that
> > fact at the moment it might be better to document a page size
> > limitation and add asserts to enforce it.
> 
> My first thought was indeed to add a BUG_ON for @size > PAGE_SIZE
> (also accounting for offset etc), but I have a feeling this will
> cause too many breakages. So perhaps it would be better to add the
> fact to Documentation that it can handle any physically contiguous
> range.

Actually, that's not historically right.  dma_map_single() was
originally designed to be called on any region that was kmalloc'd
meaning it was capable of mapping physically contiguous > PAGE_SIZE
regions.

For years (decades?) we've been eliminating the specialised
dma_map_single() calls in favour of dma_map_sg, so it's possible there
may not be any large region consumers anymore, so it *may* be safe to
enforce a PAGE_SIZE limit, but not without auditing the remaining
callers.

James