linux-kernel - Re: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit

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Message-ID: <1242932497.25553.230.camel@localhost.localdomain>
Date:	Thu, 21 May 2009 20:01:37 +0100
From:	Ian Campbell <Ian.Campbell@...citrix.com>
To:	Becky Bruce <beckyb@...nel.crashing.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit

On Thu, 2009-05-21 at 14:27 -0400, Becky Bruce wrote:
> We have both in every case but one,  
> which is in swiotlb_map_page where we call address_needs_mapping()  
> without calling range_needs_mapping.

The reason it calls address_needs_mapping without range_needs_mapping is
that in the swiotlb_force=1 case it would trigger every time. If
address_needs_mapping and range_needs_mapping are merged as proposed and
they do not subsume the swiotlb_force check (and I don't think they
would) then I think this will work fine.

> It's not actually clear to me that we need that check, though.  Can 
> someone explain what case that was designed to catch?

If map_single fails and returns NULL then we try to use
io_tlb_overflow_buffer, if that is somehow not DMA-able (for the
particular device) then the check will trigger. I would have thought we
could arrange that the overflow buffer is always OK and really if
map_page is failing we must be close the edge already.

Ian.

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