Re: [RFT PATCH 1/4] usb: dwc2: Simplify and fix DMA alignment code

Guenter Roeck
2020-03-01T16:24:23Z

Hi Antti,

On 3/1/20 7:51 AM, Antti Seppälä wrote:
> On Sat, 29 Feb 2020 at 18:33, Antti Seppälä <a.seppala@gmail.com> wrote:
>>
>> On Sat, 29 Feb 2020 at 17:25, Guenter Roeck <linux@roeck-us.net> wrote:
>>>
>>> Sigh. It would have been too simple. Too bad I can't test myself.
>>> I'd like to know if this is because URB_NO_TRANSFER_DMA_MAP is set on a
>>> transfer, or because the beginning of the buffer indeed needs to be aligned
>>> to the DMA cache line size on that system. In the latter case, the question
>>> is why the alignment to DWC2_USB_DMA_ALIGN (=4) works. In the former case,
>>> question would be why the realignment does any good in the first place.
>>>
>>> Any chance you can add some test code to help figuring out what exactly
>>> goes wrong ?
>>>
>>
>> Sure, I can try to help. Just let me know what code you would like to
>> insert and where and I'll see what I can do.
>>
>
> So I did some further research on this and it turns out that:
> - URB_NO_TRANSFER_DMA_MAP is not set on the offending transfers so
> the issue really is buffer alignment
> - DWC2_USB_DMA_ALIGN=4 "works" because in my limited testcase (usb
> 4g-dongle utilized via qmi-wwan) all transfers are unaligned. That is,
> every urb->transfer_buffer is misaligned by 2 bytes == unaligned
> - I can fix both issues and thus make the patch work on MIPS by
> modifying it like this:
>
> -#define DWC2_USB_DMA_ALIGN 4
> +#define DWC2_USB_DMA_ALIGN dma_get_cache_alignment()
>
> struct dma_aligned_buffer {
> void *old_xfer_buffer;
> - u8 data[0];
> + u8 data[0] ____cacheline_aligned;
> };
>

Thanks for the additional testing. That means that the existing code
is already broken, or am I missing something ?

Updating DWC2_USB_DMA_ALIGN to dma_get_cache_alignment() was part
of my initial fix, but then I abandoned it because I thought, well,
the existing alignment works, so that can't be the problem.

Anyway, using ____cacheline_aligned is an excellent idea. I'll make
the changes suggested above for the next version of my series.

Thanks,
Guenter