From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: semantics of dma_map_single() Date: Mon, 17 Jul 2017 09:46:30 -0700 Message-ID: <1500309990.3244.2.camel@HansenPartnership.com> References: <20170717064220.GA15807@lst.de> <23203d16-da54-99c7-0eba-c082eba120d7@synopsys.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Return-path: Received: from bedivere.hansenpartnership.com ([66.63.167.143]:55698 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751314AbdGQQqd (ORCPT ); Mon, 17 Jul 2017 12:46:33 -0400 In-Reply-To: <23203d16-da54-99c7-0eba-c082eba120d7@synopsys.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Vineet Gupta , Christoph Hellwig Cc: "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , bart.vanassche@sandisk.com, Alexander Duyck , Krzysztof Kozlowski , Andrew Morton , lkml , arcml 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