From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mailout2.w1.samsung.com ([210.118.77.12]:24942 "EHLO mailout2.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758775Ab3GRNeY (ORCPT ); Thu, 18 Jul 2013 09:34:24 -0400 Message-id: <51E7EEDC.9080003@samsung.com> Date: Thu, 18 Jul 2013 15:34:20 +0200 From: Marek Szyprowski MIME-version: 1.0 To: Ricardo Ribalda Delgado Cc: Pawel Osciak , Kyungmin Park , Mauro Carvalho Chehab , linux-media@vger.kernel.org, open list Subject: Re: [PATCH] videobuf2-dma-sg: Minimize the number of dma segments References: <1373880874-9270-1-git-send-email-ricardo.ribalda@gmail.com> <51E65577.7010403@samsung.com> <51E69F49.10500@samsung.com> <51E7960C.8050707@samsung.com> In-reply-to: Content-type: text/plain; charset=UTF-8; format=flowed Content-transfer-encoding: 7bit Sender: linux-media-owner@vger.kernel.org List-ID: Hello, On 7/18/2013 9:39 AM, Ricardo Ribalda Delgado wrote: > Hello again: > > I have started to implemt it, but I think there is more hidden work in > this task as it seems. In order to call dma_map_sg and > max_dma_segment_size I need acess to the struct device, but (correct > me if I am wrong), vb2 is device agnostic. Adding the above > functionality will mean not only updating marvell-ccic and solo6x10, > but updating all the vb2 buffers. For getting device pointer, vb2-dma-sg need to be extended with so called 'allocator context'. Please check how it is done in vb2-dma-contig (vb2_dma_contig_init_ctx() function). > Also after some readings, maybe the sg compactation should not be done > here, but in dma_map_sg. According to the doc: > > """ > The implementation is free to merge several consecutive sglist entries > into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any > consecutive sglist entries can be merged into one provided the first one > ends and the second one starts on a page boundary - in fact this is a huge > advantage for cards which either cannot do scatter-gather or have very > limited number of scatter-gather entries) and returns the actual number > of sg entries it mapped them to. On failure 0 is returned. > """ > > So, my proposal would be to alloc with alloc_pages to try to get > memory as coherent as possible, then split the page, set the sg in > PAGE_SIZE lenghts, and then let the dma_map_sg do its magic. if it > doesnt do compactation, fix dma_map_sg, so more driver could take > advantage of it. Right, this approach is probably the best one, but this way you would need to do the compaction in every dma-mapping implementation for every supported architecture. IMHO vb2-dma-sg can help dma-mapping by at least by allocating memory in larger chunks and constructing shorter scatter list. Updating dma-mapping functions across all architectures is a lot of work and testing, so for initial version we should focus on vb2-dma-sg. Memory allocators already do some work to ease mapping a buffer to dma space. > I could also of course fix marvell-ccic and solo6x10 to use sg_table. > > Does anything of this make sense? I would also like to help you as much as possible, but for the next 10 days I will be not available for both personal reasons and holidays. If you have any questions, feel free to leave them on my mail, I will reply asap I get back. Best regards -- Marek Szyprowski Samsung R&D Institute Poland