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

Guenter Roeck
2020-03-01T17:13:38Z

On 3/1/20 8:51 AM, Antti Seppälä wrote:
> On Sun, 1 Mar 2020 at 18:24, Guenter Roeck <linux@roeck-us.net> wrote:
>>
>> 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 ?
>>
>
> Yes, I believe the existing code is broken if 4 byte aligned transfers
> occur. I seem to have no easy way to prove that though as none of my
> devices generate those.
>

I did see this a lot when connecting USB drives. I'll re-test next week
and provide more details. I do have some concern because I also saw
transfers with URB_NO_TRANSFER_DMA_MAP set but transfer_buffer was
not DMA aligned. However, it may well be that the provided DMA buffer
_was_ aligned in that situation. I'll re-test that and let you know.

Thanks,
Guenter