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

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Message-ID: <e5c43e32-98b9-df49-5208-66f5440185dc@roeck-us.net>
Date:   Sun, 1 Mar 2020 09:13:31 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Antti Seppälä <a.seppala@...il.com>
Cc:     Doug Anderson <dianders@...omium.org>,
        Minas Harutyunyan <hminas@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Boris ARZUR <boris@...bu.org>, linux-usb@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Martin Schiller <ms@....tdt.de>
Subject: Re: [RFT PATCH 1/4] usb: dwc2: Simplify and fix DMA alignment code

On 3/1/20 8:51 AM, Antti Seppälä wrote:
> On Sun, 1 Mar 2020 at 18:24, Guenter Roeck <linux@...ck-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@...il.com> wrote:
>>>>
>>>> On Sat, 29 Feb 2020 at 17:25, Guenter Roeck <linux@...ck-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