Re: [RFC] V4L2_PIX_FMT_MJPEG vs V4L2_PIX_FMT_JPEG

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hello,

On Monday, 1 October 2018 15:42:56 EEST Nicolas Dufresne wrote:
> Le lundi 01 octobre 2018 à 10:43 +0200, Hans Verkuil a écrit :
> > It turns out that we have both JPEG and Motion-JPEG pixel formats defined.
> > 
> > Furthermore, some drivers support one, some the other and some both.
> > 
> > These pixelformats both mean the same.
> > 
> > I propose that we settle on JPEG (since it seems to be used most often)
> > and add JPEG support to those drivers that currently only use MJPEG.
> 
> Thanks for looking into this. As per GStreamer code, I see 3 alias for
> JPEG. V4L2_PIX_FMT_MJPEG/JPEG/PJPG. I don't know the context, this code
> was written before I knew GStreamer existed. It's possible there is a
> subtle difference, I have never looked at it, but clearly all our JPEG
> decoder handle these as being the same.
> 
> https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/sys/v4l2/gstv4l
> 2object.c#n956

Some old code to give a bit of context:

https://github.com/TimSC/mjpeg/

It should be feasible to implement a decoder that inserts the right Huffman 
table when none exists, but it has to be explicitly handled. An out-of-band 
mechanism to convey the information seems potentially useful to me.

> > We also need to update the V4L2_PIX_FMT_JPEG documentation since it just
> > says TBD:
> > 
> > https://www.linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/pixfmt-compres
> > sed.html
> > 
> > $ git grep -l V4L2_PIX_FMT_MJPEG
> > drivers/media/pci/meye/meye.c
> > drivers/media/pci/solo6x10/solo6x10-v4l2-enc.c
> > drivers/media/platform/sti/delta/delta-cfg.h
> > drivers/media/platform/sti/delta/delta-mjpeg-dec.c
> > drivers/media/usb/cpia2/cpia2_v4l.c
> > drivers/media/usb/go7007/go7007-driver.c
> > drivers/media/usb/go7007/go7007-fw.c
> > drivers/media/usb/go7007/go7007-v4l2.c
> > drivers/media/usb/s2255/s2255drv.c
> > drivers/media/usb/uvc/uvc_driver.c
> > drivers/staging/media/zoran/zoran_driver.c
> > drivers/staging/vc04_services/bcm2835-camera/bcm2835-camera.c
> > drivers/usb/gadget/function/uvc_v4l2.c
> > 
> > It looks like s2255 and cpia2 support both already, so that would leave
> > 8 drivers that need to be modified, uvc being the most important of the
> > lot.
> > 
> > Any comments?

-- 
Regards,

Laurent Pinchart