Re: [PATCH v3][RFC] add raw video stream support for Samsung SUR40

[Hans Verkuil <hverkuil@xs4all.nl>]

On 03/12/2015 08:37 PM, Florian Echtler wrote:
> Hello Hans,
> 
> On 09.03.2015 15:02, Hans Verkuil wrote:
>> On 03/09/2015 02:45 PM, Florian Echtler wrote:
>>> On 09.03.2015 11:09, Hans Verkuil wrote:
>>>> The error almost certainly comes from usb_submit_urb(). That function does some
>>>> checks on the sgl:
>>>>
>>>> I wonder it the code gets there. Perhaps a printk just before the return -EINVAL
>>>> might help here (also print the 'max' value).
>>>>
>>>> So you will have to debug a bit here, trying to figure out which test in the usb
>>>> code causes the usb_sg_wait error.
>>> I'll do my best to track this down. Do you think this is an error in my
>>> code, one in the USB subsystem, or some combination of both?
>>
>> If the USB core indeed requires scatter-gather segments of specific lengths
>> (modulo max), then that explains the problems.
>> So as suggested try to see if the usb core bails out in that check and what the
>> 'max' value is. It looks like only XHCI allows SG segments of any size, so I really
>> suspect that's the problem. But I also need to know the 'max' value to fully
>> understand the implications.
> Finally managed to confirm your suspicions on a kernel with a patched
> dev_err call at the location you mentioned:
> 
> Mar 12 20:33:51 sur40 kernel: [ 1159.509580]  (null): urb 0 length
> mismatch: length 4080, max 512
> Mar 12 20:33:51 sur40 kernel: [ 1159.509592] sur40 2-1:1.0: error -22 in
> usb_sg_wait
> 
> So the SG segments are expected in multiples of 512 bytes. I assume this
> is not something I can fix from within my driver?

No, you can't. I would use dma-sg, but disable the USERPTR support.
Also comment why USERPTR support is disabled.

This was interesting :-)

Regards,

	Hans