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

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

Best, Florian
-- 
SENT FROM MY DEC VT50 TERMINAL

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