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

[Hans Verkuil <hverkuil@xs4all.nl>]

On 03/16/2015 09:36 AM, Florian Echtler wrote:
> Hello Hans,
> 
> On 15.03.2015 17:26, Hans Verkuil wrote:
>> On 03/12/2015 08:37 PM, Florian Echtler wrote:
>>> 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'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:
>>>
>>> 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 :-)
>>
> Thanks again for your help, new patch is submitted. Just for my
> understanding: is this a hardware limitation?

Yes. Apparently only USB 3 (XHCI) has no restriction on the segment length
in a SG list.

> And why does dma-sg select
> such a weird segment size like 4080?

It's not so much dma-sg as it is the USERPTR support. In USERPTR mode the
application malloc()s memory and when you map that virtual memory block to
a SG list for the underlying physical memory, then you see that malloc does
not align the start of the memory to a page, instead it starts somewhere in
the middle of the page and you end up with a SG descriptor with a 'weird'
length. You can't do anything about that, other than just disabling USERPTR
support.

Regards,

	Hans