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Date: Mon, 5 Feb 2001 08:56:34 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>
To: Manfred Spraul <manfred@colorfullife.com>
cc: "Stephen C. Tweedie" <sct@redhat.com>, Christoph Hellwig <hch@caldera.de>,
        Steve Lord <lord@sgi.com>, linux-kernel@vger.kernel.org,
        kiobuf-io-devel@lists.sourceforge.net,
        Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
 /notify  + callback chains
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On Mon, 5 Feb 2001, Manfred Spraul wrote:
> "Stephen C. Tweedie" wrote:
> > 
> > You simply cannot do physical disk IO on
> > non-sector-aligned memory or in chunks which aren't a multiple of
> > sector size.
> 
> Why not?
> 
> Obviously the disk access itself must be sector aligned and the total
> length must be a multiple of the sector length, but there shouldn't be
> any restrictions on the data buffers.

In fact, regular IDE DMA allows arbitrary scatter-gather at least in
theory. Linux has never used it, so I don't know how well it works in
practice - I would not be surprised if it ends up causing no end of nasty 
corner-cases that have bugs. It's not as if IDE controllers always follow 
the documentation ;)

The _total_ length of the buffers have to be a multiple of the sector
size, and there are some alignment issues (each scatter-gather area has to
be at least 16-bit aligned both in physical memory and in length, and
apparently many controllers need 32-bit alignment). And I'd almost be
surprised if there wouldn't be hardware that wanted cache alignment
because they always expect to burst. 

But despite a lot of likely practical reasons why it won't work for
arbitrary sg lists on plain IDE DMA, there is no _theoretical_ reason it
wouldn't. And there are bound to be better controllers that could handle
it.

		Linus

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