Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760855AbXJZOyV (ORCPT ); Fri, 26 Oct 2007 10:54:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751340AbXJZOyN (ORCPT ); Fri, 26 Oct 2007 10:54:13 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:53832 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750702AbXJZOyM (ORCPT ); Fri, 26 Oct 2007 10:54:12 -0400 Date: Fri, 26 Oct 2007 07:52:25 -0700 (PDT) From: Linus Torvalds To: Paul Mackerras cc: Rusty Russell , Boaz Harrosh , Jens Axboe , Alan Cox , Geert Uytterhoeven , Linux Kernel Development , mingo@elte.hu Subject: Re: [PATCH 09/10] Change table chaining layout In-Reply-To: <18209.29891.911465.715995@cargo.ozlabs.ibm.com> Message-ID: References: <1193076664-13652-10-git-send-email-jens.axboe@oracle.com> <471DCBB2.9020706@panasas.com> <200710251840.02157.rusty@rustcorp.com.au> <18209.29891.911465.715995@cargo.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1795 Lines: 47 On Fri, 26 Oct 2007, Paul Mackerras wrote: > > Linus Torvalds writes: > > > Nobody should *ever* walk the list to find the length. Does anybody really > > do that? Yes, we pass the thing down, but do people *need* it? > > Yes, I need it for devices that use the macintosh DBDMA > (descriptor-based DMA) hardware. The DBDMA hardware reads an array of > descriptors from system RAM, so I need to allocate an array and fill > it in with DBDMA command blocks (and then dma-map it and point the > device at it). Yes, for allocation purposes you'd need the size ahead of time, agreed. Otherwise you have to walk the list twice. > Maybe the drivers for devices that use DBDMA are now buggy. Certainly > filling in the array of DBDMA command blocks involves walking the > list, but it would extremely useful to know how much to allocate > before we start filling them in. So we at least need an upper bound > on the number of "real" entries, even if we don't have the exact > number. Hmm. Depending on where you do this, and if this is some block-layer specific driver/code (rather than necessarily a generic SG thing), you do have the req->nr_phys_segments thing which should be that for you (ie the SG list may have _fewer_ requests in it in case some of those entries got squashed together due to be contiguous). But yeah, I don't think it would be wrong at all to have a struct scatterlist_head { unsigned int entries; unsigned int flags; /* ? */ struct scatterlist *sg; }; which would be passed down at higher levels. Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/