From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758685Ab0AOXi3 (ORCPT ); Fri, 15 Jan 2010 18:38:29 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758674Ab0AOXiX (ORCPT ); Fri, 15 Jan 2010 18:38:23 -0500 Received: from ovro.ovro.caltech.edu ([192.100.16.2]:41022 "EHLO ovro.ovro.caltech.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758614Ab0AOXiV (ORCPT ); Fri, 15 Jan 2010 18:38:21 -0500 Date: Fri, 15 Jan 2010 15:38:20 -0800 From: "Ira W. Snyder" To: Stefani Seibold Cc: linux-kernel , Andrew Morton , Andi Kleen , Greg Kroah-Hartman , Alan Cox , Theodore Tso , Jens Axboe Subject: Re: [PATCH] enhanced reimplemented of the kfifo API Message-ID: <20100115233820.GF10591@ovro.caltech.edu> References: <1263546418.9732.11.camel@wall-e> <20100115192029.GC10591@ovro.caltech.edu> <1263591001.7531.1.camel@wall-e> <20100115220153.GE10591@ovro.caltech.edu> <1263593107.8935.1.camel@wall-e> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1263593107.8935.1.camel@wall-e> User-Agent: Mutt/1.5.20 (2009-06-14) X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.0 (ovro.ovro.caltech.edu); Fri, 15 Jan 2010 15:38:21 -0800 (PST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 15, 2010 at 11:05:07PM +0100, Stefani Seibold wrote: > Am Freitag, den 15.01.2010, 14:01 -0800 schrieb Ira W. Snyder: > > On Fri, Jan 15, 2010 at 10:30:01PM +0100, Stefani Seibold wrote: > > > > > > > I'm glad to see this example, it really illustrates how to use the new > > > > DMA functionality of the kfifo API. > > > > > > > > Is there any reason why a very large scatterlist could not be used? I > > > > have a driver that uses a large scatterlist (~4000 entries, allocated as > > > > a struct sg_table). I implemented my own copy_from_user() functionality > > > > into this scatterlist, but I'd love to use the kfifo DMA API instead. > > > > After filling the scatterlist, I use the usual DMA API's to transfer it > > > > to my device. > > > > > > > > > > No, but the API will only return max. two entries. > > > > > > > Ah, I see. __kfifo_alloc() uses kmalloc() internally. The kfifo API will > > not work for my purposes then, since I want to allocate a 16MB chunk of > > memory, and I'm very uncomfortable doing that with kmalloc(). I'll stick > > with my custom scatterlist code, which uses alloc_page() to fill in the > > scatterlist with order-0 allocations. > > > > I think you can use vmalloc() instead and assign this buffer with > kfifo_init(). > Yes, that might be an option. I presume I could still use dma_map_sg() on the resulting scatterlist. > > Another nitpick in the code: I've noticed that you use sgl++ in the > > setup_sgl() function. That should become "sgl = sg_next(sgl);" so that > > this code can work with an struct sg_table as well. > > No, its by definition an array. You cannot pass as struct sg_table to > the kfifo_dma_* functions. > A struct sg_table is a container for an underlying chain of struct scatterlists, allowing for easier allocation. See the definition of struct sg_table and the sgl member (include/linux/scatterlist.h line 11). IIRC, struct sg_table allocates arrays of struct scatterlist in PAGE_SIZE chunks, and chains them together. If you change from "sg++" to "sg_next()", you should be able to work with chained scatterlists. You can only have a single struct page * in one struct scatterlist. This would mean that if I tried to use kfifo_dma_out_prepare() to give me 16MB worth of data, I would need 4096 seperate struct scatterlist entries. One for each struct page * in that 16MB chunk. Jens, you added a lot of the scatterlist chaining code. Can you comment on this? I don't claim to be a scatterlist expert :) Ira