linux-kernel - Re: ARM: 2.6.3[45] PCI regression (IXP4xx and PXA?)

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Message-Id: <20100815144122O.fujita.tomonori@lab.ntt.co.jp>
Date:	Sun, 15 Aug 2010 14:42:51 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	linux@....linux.org.uk
Cc:	fujita.tomonori@....ntt.co.jp, khc@...waw.pl,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arch@...r.kernel.org
Subject: Re: ARM: 2.6.3[45] PCI regression (IXP4xx and PXA?)

On Sat, 14 Aug 2010 19:46:05 +0100
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> On Sat, Aug 14, 2010 at 06:30:37PM +0900, FUJITA Tomonori wrote:
> > On Fri, 13 Aug 2010 22:54:13 +0100
> > Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> > >  This means that when dmabounce comes to allocate the replacement
> > > buffer, it gets a buffer which won't be accessible to the DMA
> > > controller
> > 
> > Really? looks like dmabounce does nothing for coherent memory that
> > dma_alloc_coherent() allocates.
> > 
> > The following very hacky patch works?
> 
> So what happens if you use a driver which uses dma_alloc_coherent()
> directly?  Should the driver really be passed memory which is
> inaccessible to the device because its outside the host bridge PCI
> window?

I'm not sure what you mean.

A driver which uses dma_alloc_coherent() directly should
work. dma_alloc_coherent() allocates memory with GFP_DMA with that
patch for dmabounce devices. So the driver gets the access-able
memory.

The memory that dma_alloc_coherent() returns should be always
consistent. We can't bounce it. All we can do is returning a memory
that a device (and its bus) can access to.

Krzysztof, can you try the patch?

> No, this is clearly wrong, so this patch doesn't fix anything.  It's
> a bodge, nothing more.  The real solution is to have _both_ masks
> both reduced down according to the host bridge, as we used to do.
> 
> So I suggest 6fee48c is reverted so that these platforms don't regress
> for -rc1.

Sorry, the original ARM code is wrong. dev->dma_mask and
dev->coherent_dma_mask represent the driver of dma restriction. ARM
tries to use them for both a driver and a bus so ARM needs a
workaround that doesn't set the driver of dma restriction to
dev->dma_mask and dev->coherent_dma_mask.

The proper solution is having a separate dma mask for a bus. I think
that POWERPC already does the similar. It has max_direct_dma_addr in
struct dev_archdata, which represents the dma restriction of a bus.

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