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SubjectRe: [PATCH] media: uvc: limit max bandwidth for HDMI capture
On Tue, Jun 22, 2021 at 09:50:37AM +0100, Dave Stevenson wrote:
> On Tue, 22 Jun 2021 at 09:29, Mauro Carvalho Chehab wrote:
> > Em Mon, 21 Jun 2021 21:22:26 +0300 Laurent Pinchart escreveu:
> >
> > > Hi Mauro,
> > >
> > > Thank you for the patch.
> >
> > Thanks for reviewing it!
> >
> > >
> > > On Mon, Feb 01, 2021 at 08:08:59PM +0100, Mauro Carvalho Chehab wrote:
> > > > This device:
> > > > 534d:2109 MacroSilicon
> > > >
> > > > Announces that it supports several frame intervals for
> > > > their resolutions for MJPEG compression:
> > > >
> > > > VideoStreaming Interface Descriptor:
> > > > bLength 46
> > > > bDescriptorType 36
> > > > bDescriptorSubtype 7 (FRAME_MJPEG)
> > > > bFrameIndex 1
> > > > bmCapabilities 0x00
> > > > Still image unsupported
> > > > wWidth 1920
> > > > wHeight 1080
> > > > dwMinBitRate 768000
> > > > dwMaxBitRate 196608000
> > > > dwMaxVideoFrameBufferSize 4147200
> > > > dwDefaultFrameInterval 166666
> > > > bFrameIntervalType 5
> > > > dwFrameInterval( 0) 166666
> > > > dwFrameInterval( 1) 333333
> > > > dwFrameInterval( 2) 400000
> > > > dwFrameInterval( 3) 500000
> > > > dwFrameInterval( 4) 1000000
> > > >
> > > > However, the highest frame interval (166666), which means 60 fps
> > > > is not supported. For such resolution, the maximum interval
> > > > is, instead 333333 (30 fps).
> > >
> > > What happens if you try to select it ?
> >
> > Basically, URBs get lost: they cause apps like qv4l2 to crash
> > sometimes, with:
> >
> > v4l-convert: libjpeg error: Corrupt JPEG data: premature end of data segment
> >
> > The image keeps blinking, and part of the image is replaced by
> > white noise.
> >
> > Clearly, it tries to send more data than the maximum available bandwidth
> > on this chipset.
>
> What platform are you running this on?
> I've previously encountered a USB3 camera module where the datastream
> was VERY bursty. The memcpy of the data from URB to V4L2 buffer took
> long enough that sometimes the module didn't have an URB to fill at
> the appropriate moment, and it dropped data. I seem to recall
> increasing UVC_URBS from the default of 5 to 10 to handle the peak
> data rate without loss, but it may have been higher still. This was on
> a ~1.5GHz Atom processor, so not lacking in performance.
>
> I wonder if the same is true in your case. If it's MJPEG compressed
> then the peak rate may again be high. Just a thought.

It's worth investigating indeed. How often are URBs dropped ? Does it
occur for every frame, or once in a while ?

> > Sent a v2 addressing the issues you pointed.

--
Regards,

Laurent Pinchart

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