Re: Driver: PCIe: 'pci_map_sg' returning invalid bus address?

From: FUJITA Tomonori
Date: Wed Aug 04 2010 - 08:04:12 EST

On Wed, 04 Aug 2010 12:22:32 +0100
Evan Lavelle <sa212+lkml@xxxxxxxxxxx> 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).
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