linux-kernel - Re: Driver: PCIe: 'pci_map_sg' returning invalid bus address?

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Message-ID: <20100804145154.GC23544@phenom.dumpdata.com>
Date:	Wed, 4 Aug 2010 10:51:54 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	sa212+lkml@...onix.com, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: Driver: PCIe: 'pci_map_sg' returning invalid bus address?

On Wed, Aug 04, 2010 at 09:03:53PM +0900, FUJITA Tomonori wrote:
> On Wed, 04 Aug 2010 12:22:32 +0100
> Evan Lavelle <sa212+lkml@...onix.com> wrote:
> 
> > FUJITA Tomonori wrote:
> > >> Made some progress here. The problem is that this is 32-bit PAE kernel, 
> > >> so 'dma_addr_t' is 64-bit. However, I have a 32-bit PCIe card, so I need 
> > >> a 32-bit dma_addr_t. How do I do this? In other words, how do I handle 
> > >> 32-bit PCI cards on PAE or 64-bit systems? My code sets the DMA mask to 
> > >> 32 bits but this is *not* sufficient:
> > >>
> > >> pci_set_dma_mask(my_dev, DMA_32BIT_MASK)
> > > 
> > > It doesn't work on x86_32 kernel if your driver doesn't work with the
> > > block layer or the network subsystem.
> > 
> > Sorry, not sure that I understand this. Are you saying that I can't set 
> > a DMA mask on x86_32 unless I have a block or network driver?
> 
> Yeah, the mask is ignored. As I wrote in the previous mail, x86_32
> doesn't have a bounce mechanism so dma_map_{single|sg} can't do
> anything for a buffer above 32bit even if the mask is 32bit.
> 
> 
> > > If your driver can't handle 64bit DMA, you need bounce buffer. 
> > 
> > The problem is not that I can't handle 64-bit DMA in the driver, but 
> > that the PCI card can't do 64-bit DMA. I tell the kernel this by calling 
> > 'pci_set_dma_mask' with a 32-bit mask, but it appears to be ignoring my 
> > request and then giving me a 64-bit dma_addr_t for the 32-bit PCI card.
> 
> If your card can't do 64-bit DMA, you need a bounce buffer mechanism.
> 
> Options are:
> 
> - your driver implements its own bounce buffer mechanism (as some
>   driver do).
> 
> - add swiotlb support to x86_32 (I don't think that it's difficult but
>   I might miss something).

I think the highmem support might be a bit tricky. The PowerPC folks
did some work in there, so it _ought_ to work.

Evan, you could edit arch/x86/Kconfig and change:
 config SWIOTLB
         def_bool y if X86_64

to say
	def_bool y if X86

and see how it works? FYI, it might wreak havoc on your machine thought,
so be sure you have a fail-safe kernel and backup your root/home
directory.

(FYI, I made Xen-SWIOTLB be capable of running under X86_32 and so far
no trouble.. but that is not baremetal obviously).
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