Re: Some questions about the UVC gadget

Re: Some questions about the UVC gadget

[Felipe Balbi]

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Hi,

Kelly Huang <kinghuangdk17@gmail.com> writes:
>  Dear Mr.Balbi,
>
> I am a college student from China. Recently, I am doing some research on
> the UVC gadget. After reading the source code, I found that the uvc gadget
> framework only supports two types of video streaming format, the
> UNCOMPRESSED and the MJPEG.
>
> Now, I am trying to add H.264 support. I wonder if the Linux kernel has
> already support it or not. It will be appreciated if you can give me some
> advice.
>
> Thank you for your time.

It's a good idea to add mailing lists and other relevant people to the
loop.

-- 
balbi

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Re: Some questions about the UVC gadget

[Laurent Pinchart]

Hello Kelly,

(CC'ing Paul Elder)

On Thursday, 22 February 2018 11:03:42 EET Felipe Balbi wrote:
> Kelly Huang writes:
> >  Dear Mr.Balbi,
> > 
> > I am a college student from China. Recently, I am doing some research on
> > the UVC gadget. After reading the source code, I found that the uvc gadget
> > framework only supports two types of video streaming format, the
> > UNCOMPRESSED and the MJPEG.
> > 
> > Now, I am trying to add H.264 support. I wonder if the Linux kernel has
> > already support it or not. It will be appreciated if you can give me some
> > advice.
> > 
> > Thank you for your time.
> 
> It's a good idea to add mailing lists and other relevant people to the
> loop.

I'm afraid the Linux UVC gadget driver doesn't support H.264. While H.264 
support could be implemented using UVC 1.1, I wouldn't recommend this as the 
UVC 1.1 H.264 specification is a hack that is not and will not be supported in 
the Linux UVC host driver. UVC 1.5 is the way to go for H.264. This shouldn't 
be too difficult to implement on the gadget side, but the host UVC driver also 
misses UVC 1.5 support.

Paul has recently started working on the UVC gadget driver to revive it along 
with the userspace helper application. Further down on his to-do list he told 
me he would like to implement UVC 1.5 support on the host side, but that won't 
be for the near future (no pressure Paul :-)).

-- 
Regards,

Laurent Pinchart

Re: Some questions about the UVC gadget

[Laurent Pinchart]

Hello Kelly,

Sorry for the late reply, your e-mail got buried in my inbox :-/

On Friday, 23 February 2018 05:36:55 EEST Kelly Huang wrote:
> Dear Mr.Pinchart,
> 
> > I'm afraid the Linux UVC gadget driver doesn't support H.264. While H.264
> > support could be implemented using UVC 1.1, I wouldn't recommend this as
> > the UVC 1.1 H.264 specification is a hack that is not and will not be
> > supported in the Linux UVC host driver. UVC 1.5 is the way to go for
> > H.264.
> 
> I have a  Logitech C920 usb camera which claims H.264 support. When I used
> it under my usb protocol analyzer, I found that one of the CS_INTERFACE
> descriptor had a VS_FORMAT_FRAME_BASED subtype, and the guidFormat is
> '48323634-1000-800000AA-389B71', including the 'H264' symbols.
> 
> I don't know if that is the way you talked about implementing H.264 using
> UVC 1.1? It seems that I need to rename some descriptors of the UVC gadget
> driver and write a userspace application to fill /dev/videoX with H.264
> streams. If so, can it work correctly?

I spoke a bit too fast in my previous e-mail. H.264 support with UVC 1.1 
should be OK, as long as you don't use the H.264 UVC 1.1 stream multiplexing 
method that allows transmitting multiple video streams over a single endpoint.

The support H.264 with UVC 1.1 you will need to create the corresponding 
descriptors, and to implement support in the userspace helper application for 
the H.264 extension unit (XU) defined in the UVC 1.1 specification.

-- 
Regards,

Laurent Pinchart