Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757434AbcCRL0A (ORCPT ); Fri, 18 Mar 2016 07:26:00 -0400 Received: from foss.arm.com ([217.140.101.70]:53649 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755081AbcCRLZw (ORCPT ); Fri, 18 Mar 2016 07:25:52 -0400 Subject: Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with dma_map_single To: Boris Brezillon , Russell King - ARM Linux References: <1458252137-24497-1-git-send-email-okaya@codeaurora.org> <20160317225459.GA19428@n2100.arm.linux.org.uk> <80402e2f82651814b1411a25b1740288@codeaurora.org> <20160317235020.GB19428@n2100.arm.linux.org.uk> <20160318103028.4be15b78@bbrezillon> Cc: linux-arm-kernel@lists.infradead.org, Herbert Xu , timur@codeaurora.org, Arnaud Ebalard , linux-kernel@vger.kernel.org, okaya@codeaurora.org, cov@codeaurora.org, "David S. Miller" , nwatters@codeaurora.org, linux-crypto@vger.kernel.org From: Robin Murphy Message-ID: <56EBE5BC.6000000@arm.com> Date: Fri, 18 Mar 2016 11:25:48 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <20160318103028.4be15b78@bbrezillon> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1919 Lines: 43 On 18/03/16 09:30, Boris Brezillon wrote: > On Thu, 17 Mar 2016 23:50:20 +0000 > Russell King - ARM Linux wrote: > >> On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya@codeaurora.org wrote: >>> What is the correct way? I don't want to write engine->sram_dma = sram >> >> Well, what the driver _is_ wanting to do is to go from a CPU physical >> address to a device DMA address. phys_to_dma() looks like the correct >> thing there to me, but I guess that's just an offset and doesn't take >> account of any IOMMU that may be in the way. >> >> If you have an IOMMU, then the whole phys_to_dma() thing is a total >> failure as it only does a linear translation, and there are no >> interfaces in the kernel to take account of an IOMMU in the way. So, >> it needs something designed for the job, implemented and discussed by >> the normal methods of proposing a new cross-arch interface for drivers >> to use. >> >> What I'm certain of, though, is that the change proposed in this patch >> will break current users of this driver: virt_to_page() on an address >> returned by ioremap() is completely undefined, and will result in >> either a kernel oops, or if not poking at memory which isn't a struct >> page, ultimately resulting in something that isn't SRAM being pointed >> to by "engine->sram_dma". >> > > Or we could just do > > engine->sram_dma = res->start; > > which is pretty much what the SRAM/genalloc code is doing already. As Russell points out this is yet another type of "set up a DMA master to access something other than kernel RAM" - there's already discussion in progress over how to handle this for dmaengine slaves[1], so gathering more use-cases might help distil exactly what the design of not-strictly-DMA-but-so-closely-coupled-it-can't-really-live-anywhere-else needs to be. Robin. [1]:http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414422.html