linux-kernel - Re: semantics of dma_map

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Date:   Mon, 17 Jul 2017 09:06:29 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     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()

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.

-Vineet