Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752037AbdGERdb (ORCPT ); Wed, 5 Jul 2017 13:33:31 -0400 Received: from verein.lst.de ([213.95.11.211]:55654 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751716AbdGERd3 (ORCPT ); Wed, 5 Jul 2017 13:33:29 -0400 Date: Wed, 5 Jul 2017 19:33:27 +0200 From: Christoph Hellwig To: Marek Szyprowski Cc: Thierry Escande , Mauro Carvalho Chehab , Sakari Ailus , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, Pawel Osciak , Kyungmin Park , Christoph Hellwig , Hans Verkuil , Shuah Khan , Russell King Subject: Re: [PATCH v3 0/2] [media] videobuf2-dc: Add support for cacheable MMAP Message-ID: <20170705173327.GD5417@lst.de> References: <1477471926-15796-1-git-send-email-thierry.escande@collabora.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1501 Lines: 29 On Mon, Jul 03, 2017 at 11:27:32AM +0200, Marek Szyprowski wrote: > The main question here if we want to merge incomplete solution or not. As > for now, there is no support in ARM/ARM64 for NON_CONSISTENT attribute. > Also none of the v4l2 drivers use it. Sadly support for NON_CONSISTENT > attribute is not fully implemented nor even defined in mainline. > DMA_ATTR_NON_CONSISTENT is the way to get the dma_alloc_noncoherent semantics through the dma_alloc_attr API, and as such I think it is pretty well defined, although the documentation in Documentation/DMA-attributes.txt is really bad and we need to improve it, by merging it with the dma_alloc_noncoherent description in Documentation/DMA-API.txt. My series to remove dma_alloc_noncoherent updates the latter to mention DMA_ATTR_NON_CONSISTENT, but we should probably merge Documentation/DMA-API.txt, Documentation/DMA-attributes.txt and Documentation/DMA-API-HOWTO.txt into a single coherent document. > I know that it works fine for some vendor kernel trees, but supporting it in > mainline was a bit controversial. There is no proper way to sync cache for > such > buffers. Calling dma_sync_sg worked so far, but it has to be first agreed as > a proper DMA API. As documented in Documentation/DMA-API.txt the proper way to sync noncoherent/nonconsistent regions is to call dma_cache_sync. It seems like it generally is the same as dma_sync_range/sg so if we could eventually merge these APIs that should reduce the confusion further.