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Date: Thu, 5 Dec 2002 13:40:39 +1100
From: David Gibson <david@gibson.dropbear.id.au>
To: "Adam J. Richter" <adam@yggdrasil.com>
Cc: jgarzik@pobox.com, davem@redhat.com, James.Bottomley@SteelEye.com,
       linux-kernel@vger.kernel.org, miles@gnu.org
Subject: Re: [RFC] generic device DMA implementation
Message-ID: <20021205024039.GB1500@zax.zax>
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On Wed, Dec 04, 2002 at 05:21:04PM -0800, Adam J. Richter wrote:
> David Gibson wrote:
> >On Wed, Dec 04, 2002 at 11:47:14AM -0600, James Bottomley wrote:
> [...]
> >> The new DMA API allows a driver to advertise its level of consistent memory 
> >> compliance to dma_alloc_consistent.  There are essentially two levels:
> >> 
> >> - I only work with consistent memory, fail if I cannot get it, or
> >> - I can work with inconsistent memory, try consistent first but return 
> >> inconsistent if it's not available.
> >
> >Do you have an example of where the second option is useful?
> 
> 	From a previous discussion, I understand that there are some
> PCI bus parisc machines without consistent memory.

And there are PPCs without consistent memory, except by disabling
cache.

> >Off hand
> >the only places I can think of where you'd use a consistent_alloc()
> >rather than map_single() and friends is in cases where the hardware's
> >behaviour means you absolutely positively have to have consistent
> >memory.
> 
> 	That would result in big rarely used branches in device
> drivers or lots of ifdef's and the equivalent.  With James's approach,
> porting a driver to support those parisc machines (for example) would
> involve sprinkling in some calls to macros that would compile to
> nothing on the other machines.
> 
> 	Compare the code clutter involved in allowing those
> inconsistent parisc machines to run, say, the ten most popular
> ethernet controllers and the four most popular scsi controllers.  I
> think the difference in the resulting source code size would already
> be in the hundreds of lines.

For cases like this, I'm talking about replacing the
consistent_alloc() with a kmalloc(), then using the cache flush
macros.  Is there any machine for which this is not sufficient?

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson
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