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Message-ID: <20071026172838.GD8875@kernel.dk> Date: Fri, 26 Oct 2007 19:28:38 +0200 From: Jens Axboe <jens.axboe@...cle.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Paul Mackerras <paulus@...ba.org>, Rusty Russell <rusty@...tcorp.com.au>, Boaz Harrosh <bharrosh@...asas.com>, Alan Cox <alan@...rguk.ukuu.org.uk>, Geert Uytterhoeven <geert@...ux-m68k.org>, Linux Kernel Development <linux-kernel@...r.kernel.org>, mingo@...e.hu Subject: Re: [PATCH 09/10] Change table chaining layout On Fri, Oct 26 2007, Linus Torvalds wrote: > > > 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. Do you really allocate a fresh table for every command, or just a max sized one at init? > > 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. That'd be fine with me as well, but I really don't think that a lot of people really do need the sg count when you can just loop over the table until it returns NULL. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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