From: "Adam J. Richter" <adam@yggdrasil.com>
Date: Wed, 10 Jul 2002 23:09:53 -0700
Message-Id: <200207110609.XAA20931@adam.yggdrasil.com>
To: akpm@zip.com.au, dougg@torque.net, ingo.oeser@informatik.tu-chemnitz.de
Subject: Re: direct-to-BIO for O_DIRECT
Cc: axboe@suse.de, linux-kernel@vger.kernel.org, martin@dalecki.de
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2823
Lines: 69

Douglas Gilbert wrote:
>Ingo Oeser wrote:
[...]
>> It would be nice if we could just map a set of user pages to a scatterlist.
>
>After disabling kiobufs in sg I would like such a drop
>in replacement.
>
>> Developers of mass transfer devices (video grabbers, dsp devices, sg and
>> many others) would just LOVE you for this ;-)
>
>Agreed. Tape devices could be added to your list.
>Large page support will make for very efficient zero
>copy IO.
>
>> Block devices are the common case worth optimizing for, but character
>> devices just need to reimplement most of this, if they want the same 
>> optimizations. Some devices need mass transfers and are NOT blockdevices.
>
>> Please consider supporting them better for 2.5 in stuff similiar to BIOs
>> and DMA to/from user pages.
>
>CIOs?

	This is what I want to accomplish in my proposal to
pull most of the DMA transfer optimization code up from block
devices by generalizing DMA targets and turning struct scatterlist
into a linked list, discussed here:

	http://marc.theaimsgroup.com/?t=102487685000002&r=1&w=2

	I have not started coding this yet because:

	1. I'm tracking down a bug in the next revision of my proposed
	   bio_append patch (which eliminates {read,write}_full_page from
	   fs/buffers.c), and I want to hit that ball out of my court first.

	2. I want to look at aio to see if it has a better way or if it
	   could benefit from this.

	3. I want to accomodate Dave Miller's request for a non-PCI
	   generalization of pci_alloc_consistent, pci_map_single, etc.,
	   first, and that will depend on struct device, for which there are
	   some relevant changes working their way from Patrick Mochel
	   to Linus.

	4. After getting a general dma_alloc_consistent, etc. interface,
	   then I want to create a struct dma_target, to abstract
	   out the DMA capabilities currently maintained by the block
	   layer.  I hope that by doing this in stages, that it will be
	   more palatable to Jens, who expressed concern that the
	   my proposal to go to a linked list for struct scatterlist
	   was a bit too much change.

	Then, I think we'll be in a better position to go to a struct
scatterlist linked list or something similar that can be used by most
if not all big producers of IO.

	In the meantime, killing off kiobufs should be helpful.

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