Re: dma_map_single returns 0 on ARM

[Zoltan Kiss <zoltan.kiss@linaro.org>]

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:
>>>> Hi,
>>>>
>>>> I'm trying to fire up the Ethernet port of a Hisilicon D01 board, and it
>>>> fails
>>>> 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!

Zoli