From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754671AbbHXU0d (ORCPT ); Mon, 24 Aug 2015 16:26:33 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:37321 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754274AbbHXU0b (ORCPT ); Mon, 24 Aug 2015 16:26:31 -0400 Date: Mon, 24 Aug 2015 13:26:30 -0700 From: Andrew Morton To: Jens Axboe Cc: Robert Jarzmik , Russell King - ARM Linux , Guennadi Liakhovetski , linux-kernel@vger.kernel.org, Mauro Carvalho Chehab , linux-media@vger.kernel.org Subject: Re: [PATCH] lib: scatterlist: add sg splitting function Message-Id: <20150824132630.814fe743f051d49531bb7a17@linux-foundation.org> In-Reply-To: <55DB7B4C.4010804@kernel.dk> References: <1439023450-2689-1-git-send-email-robert.jarzmik@free.fr> <55DB7B4C.4010804@kernel.dk> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 24 Aug 2015 14:15:08 -0600 Jens Axboe wrote: > On 08/08/2015 02:44 AM, Robert Jarzmik wrote: > > Sometimes a scatter-gather has to be split into several chunks, or sub > > scatter lists. This happens for example if a scatter list will be > > handled by multiple DMA channels, each one filling a part of it. > > > > A concrete example comes with the media V4L2 API, where the scatter list > > is allocated from userspace to hold an image, regardless of the > > knowledge of how many DMAs will fill it : > > - in a simple RGB565 case, one DMA will pump data from the camera ISP > > to memory > > - in the trickier YUV422 case, 3 DMAs will pump data from the camera > > ISP pipes, one for pipe Y, one for pipe U and one for pipe V > > > > For these cases, it is necessary to split the original scatter list into > > multiple scatter lists, which is the purpose of this patch. > > > > The guarantees that are required for this patch are : > > - the intersection of spans of any couple of resulting scatter lists is > > empty. > > - the union of spans of all resulting scatter lists is a subrange of > > the span of the original scatter list. > > - streaming DMA API operations (mapping, unmapping) should not happen > > both on both the resulting and the original scatter list. It's either > > the first or the later ones. > > - the caller is reponsible to call kfree() on the resulting > > scatterlists. > > > > Signed-off-by: Robert Jarzmik > > I think this looks fine. But do we really need the Kconfig option? It's > not a lot of code, and it seems silly to put the onus on the driver for > having to enable something that is a subset of the SG api. Blame me for that. It's so that all kernels don't need to carry a lump of code which only a small number of media drivers actually use. The tradeoff is a bit of once-off build-time effort versus a permanent runtime gain for many systems. That's a good tradeoff.