From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756685Ab1LWKil (ORCPT ); Fri, 23 Dec 2011 05:38:41 -0500 Received: from mailout2.w1.samsung.com ([210.118.77.12]:43994 "EHLO mailout2.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754212Ab1LWKik (ORCPT ); Fri, 23 Dec 2011 05:38:40 -0500 Date: Fri, 23 Dec 2011 11:38:34 +0100 From: Marek Szyprowski Subject: RE: [RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2_PAGE memops In-reply-to: To: "'Ming Lei'" 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'" Message-id: <015301ccc15f$053e61d0$0fbb2570$%szyprowski@samsung.com> Organization: SPRC MIME-version: 1.0 X-Mailer: Microsoft Office Outlook 12.0 Content-type: text/plain; charset=us-ascii Content-language: pl Content-transfer-encoding: 7BIT Thread-index: AczBWGo0Yg+hRe4ZRVmPUNEjrF7lqQABSfTg 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> <015201ccc156$033f73a0$09be5ae0$%szyprowski@samsung.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Friday, December 23, 2011 10:51 AM Ming Lei wrote: > On Fri, Dec 23, 2011 at 5:34 PM, Marek Szyprowski > wrote: > > >> 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. > > > > It can be easily increased for particular boards, there is no problem with this. > > It is not easily to increase it because there is very limited space reserved for > this purpose, see Documentation/arm/memory.txt. Also looks like it is > not configurable. It is really not a big issue to increase it by a few MBytes. > >> > 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] > > > > Your driver doesn't access video data inside kernel space, so this is also not an issue. > > Why not introduce it so that other drivers(include face detection) can > benefit with it? :-) We can get back into this once a driver which really benefits from comes. > >> > > >> > 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 *'. > > > > Then you MUST use cookie for it. vaddr method should return kernel virtual address > > to the buffer video data. Some parts of videobuf2 relies on this - it is used by file > > io emulator (read(), write() calls) and mmap equivalent for non-mmu systems. > > > > Manual casting in the driver is also a bad idea, that's why there are helper functions > > I don't see any casts are needed. The dma address can be got from vaddr with > dma_map_* easily in drivers, see the usage on patch 8/8(media: video: introduce > omap4 face detection module driver). Sorry, but I won't accept such driver/allocator which abuses the defined API. I've already pointed what vaddr method is used for. > > defined for both dma_contig and dma_sg allocators: vb2_dma_contig_plane_dma_addr() and > > vb2_dma_sg_plane_desc(). > > These two helpers are not needed and won't be provided by VIDEOBUF2_PAGE memops. I gave the example. Your allocator should have something similar. Best regards -- Marek Szyprowski Samsung Poland R&D Center