Date: Mon, 1 Dec 2008 21:01:06 +0900
To: joro@8bytes.org
Cc: tsbogend@alpha.franken.de, linux-kernel@vger.kernel.org, mingo@elte.hu
Subject: Re: [PATCH v3] Fix dma_mapping_error for 32bit x86
From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
In-Reply-To: <20081201113451.GA24323@8bytes.org>
References: <20081129124627.F2ABDFA8C8@solo.franken.de>
	<20081201113451.GA24323@8bytes.org>
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On Mon, 1 Dec 2008 12:34:51 +0100
Joerg Roedel <joro@8bytes.org> wrote:

> On Sat, Nov 29, 2008 at 01:46:27PM +0100, Thomas Bogendoerfer wrote:
> > Devices like b44 ethernet can't dma from addresses above 1GB. The driver
> > handles this cases by falling back to GFP_DMA allocation. But for detecting
> > the problem it needs to get an indication from dma_mapping_error.
> > The bug is triggered by using a VMSPLIT option of 2G/2G.
> 
> Looks like your system uses swiotlb as the dma_ops backend. Its the only
> implementation providing the ops->mapping_error callback and does not
> use bad_dma_address as the error value.

I think that you misunderstand the problem.

He uses X86_32 so swiotlb should not be used (which is available on only
X86_64 and IA64 for now).

b44 needs an address under 1GB so it sets device->dma_mask to
DMA_30BIT_MASK. With VMSPLIT option of 2G/2G, I guess that b44 could
get addresses above 1GB from the networking subsystem. In such case,
nommu_map_single returns bad_dma_address properly, but on X86_32,
dma_mapping_error always returns 0 (success). So b44 wrongly thinks
that the address is under 1GB.

This patch fixes dma_mapping_error() to check a passed address
properly (compares it with bad_dma_address).

As I already wrote, the current git needs a patch modified slightly:

http://marc.info/?l=linux-kernel&m=122797163405377&w=2


> > Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> 
> Acked-by: Joerg Roedel <joro@8bytes.org>
> 
> > 
> > diff -ru orig/linux-2.6.27.4/include/asm-x86/dma-mapping.h linux-2.6.27.4/include/asm-x86/dma-mapping.h
> > --- orig/linux-2.6.27.4/include/asm-x86/dma-mapping.h	2008-10-26 00:05:07.000000000 +0200
> > +++ linux-2.6.27.4/include/asm-x86/dma-mapping.h	2008-10-26 11:06:14.000000000 +0100
> > @@ -74,15 +74,13 @@
> >  /* Make sure we keep the same behaviour */
> >  static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
> >  {
> > -#ifdef CONFIG_X86_32
> > -	return 0;
> > -#else
> > +#ifdef CONFIG_X86_64
> >  	struct dma_mapping_ops *ops = get_dma_ops(dev);
> >  	if (ops->mapping_error)
> >  		return ops->mapping_error(dev, dma_addr);
> >  
> > -	return (dma_addr == bad_dma_address);
> >  #endif
> > +	return (dma_addr == bad_dma_address);
> >  }
> >  
> >  #define dma_alloc_noncoherent(d, s, h, f) dma_alloc_coherent(d, s, h, f)
> > --
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