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Re: [Xen-devel] dma_map_single returns 0 on ARM

On 27/10/14 15:44, Stefano Stabellini wrote:
On Mon, 27 Oct 2014, Zoltan Kiss wrote:
On 25/10/14 15:11, Stefano Stabellini wrote:
On Fri, 24 Oct 2014, Zoltan Kiss wrote:

I'm trying to fire up the Ethernet port of a Hisilicon D01 board, and it
when it wants to map the buffers to the device:

                phys = dma_map_single(&ndev->dev, priv->rx_buf[i],
                                RX_BUF_SIZE, DMA_FROM_DEVICE);

This returns 0, while running on bare metal it works. Does anyone has an
advice where should I look?

dma_map_single should just be a wrapper around dma_ops->map_page that in
dom0 is implemented by xen_swiotlb_map_page.
  From looking at the code the issue appears to be that dma_capable
returns false for your device.

In any case even if dma_map_single returned a valid address, keep in
mind that at the moment if the device is not dma coherent Linux needs
to be compiled with CONFIG_ARM_LPAE to be able to cache flush the
buffers correctly. But the failure in that case is a memory corruption
after the dma request is completed.

Thanks for the advices. I've managed to figure out that dma_map_single fails
because dev->dma_mask was a NULL pointer. I've set it during probe with this:

dma_coerce_mask_and_coherent(&ndev->dev, DMA_BIT_MASK(32));

And the mappings seems to be set up properly (a bit below 512M), but the
returned buffers are filled up with 0xAA, which I guess is probably some
default value. Turning on and off CONFIG_ARM_LPAE did not helped.
I'll keep looking why is that, if you have an idea let me know.

Are you trying to do regular DMA in dom0 or are you running other guests?
If you are just doing DMA in dom0, then I don't have any suggestions:
that should work.
I have only Dom0 running. And one important thing I forgot to mention: due to other restrictions I'm using a 3.14 kernel right now!


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