From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754755AbcDKMbg (ORCPT ); Mon, 11 Apr 2016 08:31:36 -0400 Received: from mail.kernel.org ([198.145.29.136]:36454 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753517AbcDKMbf (ORCPT ); Mon, 11 Apr 2016 08:31:35 -0400 Date: Mon, 11 Apr 2016 20:31:03 +0800 From: Shawn Guo To: Robin Murphy Cc: Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, Andrew Morton , linux-kernel@vger.kernel.org, Arnd Bergmann Subject: Re: [PATCH] scatterlist: use sg_dma_len() in sg_set_page() Message-ID: <20160411123103.GC13735@tiger> References: <1460342864-27891-1-git-send-email-shawnguo@kernel.org> <570B7CE8.3020307@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <570B7CE8.3020307@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 On Mon, Apr 11, 2016 at 11:31:04AM +0100, Robin Murphy wrote: > Hi Shawn, > > On 11/04/16 03:47, Shawn Guo wrote: > >The macro sg_dma_len(sg) is commonly used to retrieve length of sg, > >which could be 'dma_length' or 'length' field, depending on whether > >NEED_SG_DMA_LENGTH is enabled or not. On the other hand, many driver > >code use helper function sg_set_page() to set an sg entry pointing at > >a page, with offset and length set up in one call. But sg_set_page() > >does not consider NEED_SG_DMA_LENGTH case and only set up 'length' > >field. This causes problem on platforms like ARM64, where > >NEED_SG_DMA_LENGTH is enabled by default, i.e. sg_set_page() sets up > >'length' while sg_dma_len(sg) returns 'dma_length' field. > > > >The patch changes sg_set_page() to use sg_dma_len() for sg length setup > >as well, so that NEED_SG_DMA_LENGTH case can be handled. > > > >Signed-off-by: Shawn Guo > >--- > > include/linux/scatterlist.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > >diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h > >index 556ec1ea2574..b0e32ea594c3 100644 > >--- a/include/linux/scatterlist.h > >+++ b/include/linux/scatterlist.h > >@@ -114,7 +114,7 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page, > > { > > sg_assign_page(sg, page); > > sg->offset = offset; > >- sg->length = len; > >+ sg_dma_len(sg) = len; > > This looks wrong. If a driver is building a scatterlist, then it > needs to fill in the page, offset and length fields to describe the > physical layout - leaving sg->length uninitialised would be a recipe > for disaster - then pass it to the DMA API. Only the DMA API > implementation should be setting dma_addr and dma_len, and they may > not correspond to the physical layout at all (e.g. an IOMMU could > concatenate the entire list into a single much longer segment at > whatever arbitrary DMA address it chooses). Thanks for the hint, Robin. Yes, you're right, the proposed change is just wrong. I just found the correct fix to my problem, i.e. commit 70bc916b2c80 (staging: android: ion: Set the length of the DMA sg entries in buffer). It should be applied for stable kernel, IMO. Shawn