From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754281Ab1LWJWa (ORCPT ); Fri, 23 Dec 2011 04:22:30 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:57836 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753917Ab1LWJW1 (ORCPT ); Fri, 23 Dec 2011 04:22:27 -0500 MIME-Version: 1.0 In-Reply-To: <010501ccc08c$1c7b7870$55726950$%szyprowski@samsung.com> References: <1323871214-25435-1-git-send-email-ming.lei@canonical.com> <1323871214-25435-5-git-send-email-ming.lei@canonical.com> <010501ccc08c$1c7b7870$55726950$%szyprowski@samsung.com> Date: Fri, 23 Dec 2011 17:22:23 +0800 Message-ID: Subject: Re: [RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2_PAGE memops From: Ming Lei To: Marek Szyprowski Cc: Mauro Carvalho Chehab , Tony Lindgren , Sylwester Nawrocki , Alan Cox , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, Pawel Osciak Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 22, 2011 at 5:28 PM, Marek Szyprowski wrote: > Hello, > > On Wednesday, December 14, 2011 3:00 PM Ming Lei wrote: > >> DMA contig memory resource is very limited and precious, also >> accessing to it from CPU is very slow on some platform. >> >> For some cases(such as the comming face detection driver), DMA Streaming >> buffer is enough, so introduce VIDEOBUF2_PAGE to allocate continuous >> physical memory but letting video device driver to handle DMA buffer mapping >> and unmapping things. >> >> Signed-off-by: Ming Lei > > Could you elaborate a bit why do you think that DMA contig memory resource > is so limited? If dma_alloc_coherent fails because of the memory fragmentation, > the alloc_pages() call with order > 0 will also fail. For example, on ARM, there is very limited kernel virtual address space reserved for DMA coherent buffer mapping, the default size is about 2M if I don't remember mistakenly. > > I understand that there might be some speed issues with coherent (uncached) > userspace mappings, but I would solve it in completely different way. The interface Also there is poor performance inside kernel space, see [1] > for both coherent/uncached and non-coherent/cached contig allocator should be the > same, so exchanging them is easy and will not require changes in the driver. > I'm planning to introduce some design changes in memory allocator api and introduce > prepare and finish callbacks in allocator ops. I hope to post the rfc after > Christmas. For your face detection driver using standard dma-contig allocator > shouldn't be a big issue. > > Your current implementation also abuses the design and api of videobuf2 memory > allocators. If the allocator needs to return a custom structure to the driver I think returning vaddr is enough. > you should use cookie method. vaddr is intended to provide only a pointer to > kernel virtual mapping, but you pass a struct page * there. No, __get_free_pages returns virtual address instead of 'struct page *'. thanks, -- Ming Lei [1], http://marc.info/?t=131198148500001&r=1&w=2