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Date: Thu, 5 Dec 2002 17:12:24 +1100
From: David Gibson <david@gibson.dropbear.id.au>
To: Miles Bader <miles@gnu.org>
Cc: "Adam J. Richter" <adam@yggdrasil.com>, jgarzik@pobox.com,
       davem@redhat.com, James.Bottomley@SteelEye.com,
       linux-kernel@vger.kernel.org
Subject: Re: [RFC] generic device DMA implementation
Message-ID: <20021205061224.GH1500@zax.zax>
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On Thu, Dec 05, 2002 at 11:49:52AM +0900, Miles Bader wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> > 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?
> 
> I'm not entirely sure what you mean by `using the cache flush macros,'
> but on one of my platforms, PCI consistent memory must be allocated from
> a special area.

Well, yes, you only need the cache flush macros on memory that *isn't*
consistent.

> It's also not clear what you mean by `for cases like this' -- do you
> mean, replace _all_ uses of xxx_alloc_consistent with kmalloc, or do you
> mean just those cases where pci_alloc_consistent currently returns 0?

I mean replace xxx_alloc_consistent() with kmalloc() and appropriate
calls to map_single() (or whatever) in those cases where we actually
care about reducing our usage of (genuinely) consistent memory and it
is possible to do so.

> If the former, it obviously doesn't work on my platform; if the latter,
> I guess this is what James' patch assumes the platform-specific
> dma_alloc_consistent function will do.

Well, with James approach you need a dma_sync() of some sort in pretty
much exactly the same places you need a map_single() or similar if you
used kmalloc() to start with.

-- 
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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