Re: USB2 / USB3 compatibility problems: xhci_hcd 0000:00:06.0: WARN Wrong bounce buffer write length: 0 != 512

[Andreas Hartmann <andihartmann@01019freenet.de>]

On 25.01.21 at 21:06 Andreas Hartmann wrote:
> Hello Mathias,
> 
> On 25.01.21 at 19:28 Mathias Nyman wrote:
>> Hi
>>
>> On 25.1.2021 12.18, Andreas Hartmann wrote:
>>>
>>> Hello!
>>>
>>> Meanwhile I found the culprit:
>>>
>>> https://www.spinics.net/lists/linux-usb/msg141467.html
>>> and
>>> https://www.spinics.net/lists/linux-usb/msg141468.html
>>>
>>> Especially the last change breaks things here completely. After removing them
>>> by the attached patch, problems are gone and device works again as expected
>>> (I tested with the original 24 kB bulk size which was horribly broken w/o the
>>> attached patch). This means: the additional repair steps are not just breaking
>>> things but are even unnecessary (it's working perfectly without those changes)
>>> here.
>>
>> Unfortunately this isn't enough to remove the alignment code for those
>> controllers. This is just once specific usecase. We need to figure out what
>> really goes wrong.
>>
>> Looks like 0 bytes is copied from sg list to bounce buffer when we want 512
>> bytes copied. Just noticed the alignment code assumes sg lists are used without
>> checking it first.
>>
>> Could you add the below code and test again, it should print more debugging info.
> 
> See the attached file. That's the result of two times coping 1.3 GB via scp.
> 
> As the transfer mostly breaks after the wrong alignments and because
> there isn't any further alignment needed at all, I didn't do more tests.
> Hope this helps. Most importantly it should be investigated, why there
> isn't any additional alignment needed at all.

I'm not sure if it's important for you to know: The driver doesn't use struct 
scatterlist or num_mapped_sgs at all (if it's meant to be used by the sender at all).

But it sets URB_NO_TRANSFER_DMA_MAP (for data transfer among others).

Mlme packets are sent w/o bulk and w/o setting URB_NO_TRANSFER_DMA_MAP. All other 
packets are sent with URB_NO_TRANSFER_DMA_MAP turned on.



Thanks
Andreas