From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B56C3C32750 for ; Fri, 2 Aug 2019 08:14:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9222820679 for ; Fri, 2 Aug 2019 08:14:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390663AbfHBIOq (ORCPT ); Fri, 2 Aug 2019 04:14:46 -0400 Received: from verein.lst.de ([213.95.11.211]:50730 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726937AbfHBIOq (ORCPT ); Fri, 2 Aug 2019 04:14:46 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id 9F17E68B05; Fri, 2 Aug 2019 10:14:41 +0200 (CEST) Date: Fri, 2 Aug 2019 10:14:41 +0200 From: Christoph Hellwig To: Will Deacon Cc: Christoph Hellwig , iommu@lists.linux-foundation.org, Shawn Anastasio , Michael Ellerman , Russell King , Catalin Marinas , Robin Murphy , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_* Message-ID: <20190802081441.GA9725@lst.de> References: <20190801142118.21225-1-hch@lst.de> <20190801142118.21225-2-hch@lst.de> <20190801162305.3m32chycsdjmdejk@willie-the-truck> <20190801163457.GB26588@lst.de> <20190801164411.kmsl4japtfkgvzxe@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190801164411.kmsl4japtfkgvzxe@willie-the-truck> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 01, 2019 at 05:44:12PM +0100, Will Deacon wrote: > > > Although arch_dma_mmap_pgprot() is a bit of a misnomer now that it only > > > gets involved in the non-coherent case. > > > > A better name is welcome. > > How about arch_dma_noncoherent_mmap_pgprot() ? Too long? Sounds a little long yes. And doesn't fix the additional problem that we don't just it for mmap but also for the in-kernel remapping these days. > > But my worry is how this interacts with architectures that have an > > uncached segment (mips, nios2, microblaze, extensa) where we'd have > > the kernel access DMA_ATTR_WRITE_COMBINE mappigns using the uncached > > segment, and userspace mmaps using pgprot_writecombine, which could > > lead to aliasing issues. But then again mips already supports > > DMA_ATTR_WRITE_COMBINE, so this must be ok somehow. I guess I'll > > need to field that question to the relevant parties. > > Or it's always been busted and happens to work out in practice... I've sent a ping to the mips folks. While we'are at it: arm64 and arm32 (optionally) map dma coherent allocations as write combine. I suspect this hasn't always just been busted but intentional (of course!), but is there any chance to get a quote from the arm architecture spec on why this is fine as it looks rather confusion? Also if we assume mips is buggy DMA_ATTR_WRITE_COMBINE really just seems to be there for old arm platforms, which makes the scope pretty limited.