Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755333AbcCPO4V (ORCPT ); Wed, 16 Mar 2016 10:56:21 -0400 Received: from foss.arm.com ([217.140.101.70]:42827 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752554AbcCPO4T (ORCPT ); Wed, 16 Mar 2016 10:56:19 -0400 Date: Wed, 16 Mar 2016 14:54:43 +0000 From: Mark Rutland To: Will Deacon Cc: Timur Tabi , Ganesh Mahendran , catalin.marinas@arm.com, linux-kernel@vger.kernel.org, stable@vger.kernel.org, rrichter@cavium.com, tchalamarla@cavium.com, Shanker Donthineni , apinski@cavium.com, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] Revert "arm64: Increase the max granular size" Message-ID: <20160316145406.GA19170@svinekod> References: <1458120743-12145-1-git-send-email-opensource.ganesh@gmail.com> <20160316100759.GA18387@arm.com> <56E95A4E.4050709@codeaurora.org> <20160316140334.GA17680@svinekod> <20160316143534.GI18387@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160316143534.GI18387@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1818 Lines: 36 On Wed, Mar 16, 2016 at 02:35:35PM +0000, Will Deacon wrote: > On Wed, Mar 16, 2016 at 02:03:35PM +0000, Mark Rutland wrote: > > If I understand correctly, the main reason that we need this for correctness is > > non-coherent DMA to/from SLAB caches. > > > > A more general approach (and more invasive, but perhaps less so than making > > ARCH_DMA_MINALIGN usage completely dynamic) would be to determine at runtime > > whether the CWG is larger than the configured ARCH_DMA_MINALIGN, and if so, > > force the use of bounce buffers (which could be padded to the architectural > > maximum of 2K) for non-coherent DMA. That nicely degrades to not mattering for > > the case of coherent DMA. > > > > I would consider NoSnoop a separate case. It's closer to "negatively coherent", > > and always required page-aligned buffer anyway due to MMU behaviour. > > What makes you say that? There are no such alignment requirements for > buffers that may be accessed with a NoSnoop transaction. On ARM, we'll > have a mismatched alias, but we'd need to solve that with explicit > cache maintenance (and my understanding is that's what things like GPU > drivers already do on x86). I was under the impression that NoSnoop transactions were permitted to be Cacheable, even if non-snooping (e.g. allowing them to allocate and hit in a system cache). If that is permitted, then data corruption could potentially occur in the presence of another cacheable alias due to things like line migration (e.g. a CPU making a speculative fetch and taking ownership of a line that was in the system cache). To avoid that, you'd have to remove any cachable alias, for which we only have page-granular control. If that is not permitted, then no-snoop is effectively non-cacheable and non-coherent, and my comment doesn't hold. Thanks, Mark.