Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754437AbbG0OLG (ORCPT ); Mon, 27 Jul 2015 10:11:06 -0400 Received: from foss.arm.com ([217.140.101.70]:53588 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754083AbbG0OLD (ORCPT ); Mon, 27 Jul 2015 10:11:03 -0400 Date: Mon, 27 Jul 2015 15:11:02 +0100 From: Will Deacon To: Robin Murphy Cc: Yong Wu , Joerg Roedel , Thierry Reding , Mark Rutland , Matthias Brugger , Daniel Kurtz , Tomasz Figa , Lucas Stach , Rob Herring , Catalin Marinas , "linux-mediatek@lists.infradead.org" , Sasha Hauer , "srv_heupstream@mediatek.com" , "devicetree@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "iommu@lists.linux-foundation.org" , "pebolle@tiscali.nl" , "arnd@arndb.de" , "mitchelh@codeaurora.org" , "cloud.chou@mediatek.com" , "frederic.chen@mediatek.com" Subject: Re: [PATCH v3 3/6] iommu: add ARM short descriptor page table allocator. Message-ID: <20150727141102.GJ3358@arm.com> References: <1437037475-9065-1-git-send-email-yong.wu@mediatek.com> <1437037475-9065-4-git-send-email-yong.wu@mediatek.com> <20150721171101.GN31095@arm.com> <1437715466.23932.68.camel@mhfsdcap03> <20150724165325.GC21177@arm.com> <1437970868.25925.20.camel@mhfsdcap03> <55B63AB2.4050006@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55B63AB2.4050006@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1956 Lines: 41 On Mon, Jul 27, 2015 at 03:05:38PM +0100, Robin Murphy wrote: > On 27/07/15 05:21, Yong Wu wrote: > >>>>> + } else { /* page or largepage */ > >>>>> + if (quirk & IO_PGTABLE_QUIRK_SHORT_MTK) { > >>>>> + if (large) { /* special Bit */ > >>>> > >>>> This definitely needs a better comment! What exactly are you doing here > >>>> and what is that quirk all about? > >>> > >>> I use this quirk is for MTK Special Bit as we don't have the XN bit in > >>> pagetable. > >> > >> I'm still not really clear about what this is. > > > > There is some difference between the standard spec and MTK HW, > > Our hw don't implement some bits, like XN and AP. > > So I add a quirk for MTK special. > > When you say it doesn't implement these bits, do you mean that having > them set will lead to Bad Things happening in the hardware, or that it > will simply ignore them and not enforce any of the protections they > imply? The former case would definitely want clearly documenting > somewhere, whereas for the latter case I'm not sure it's even worth the > complication of having a quirk - if the value doesn't matter there seems > little point in doing a special dance just for the sake of semantic > correctness of the in-memory PTEs, in my opinion. Agreed. We should only use quirks if the current (architecturally compliant) code causes real issues with the hardware. Then the quirk can be used to either avoid the problematic routines or to take extra steps to make things work as the architecture intended. I've asked how this IOMMU differs from the architecture on a number of occasions, but I'm still yet to receive a response other than "it's special". Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/