Re: [PATCH v5] media: uvcvideo: Fix bandwidth error for Alcor camera

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

Hi Ricardo,

On Mon, Jan 16, 2023 at 03:14:39PM +0100, Ricardo Ribalda wrote:
> On Sun, 15 Jan 2023 at 22:17, Laurent Pinchart wrote:
> >
> > From: Ai Chao <aichao@kylinos.cn>
> >
> > The Alcor Corp. Slave camera (1b17:6684 and 2017:0011) returns a wrong
> > dwMaxPayloadTransferSize value for compressed formats. Valid values are
> > typically up to 3072 bytes per interval (for high-speed, high-bandwidth
> > devices), and those faulty devices request 2752512 bytes per interval.
> > This is a firmware issue, but the manufacturer cannot provide a fixed
> > firmware.
> >
> > Fix this by checking the dwMaxPayloadTransferSize field, and hardcoding
> > a value of 1024 if it exceeds 3072 for compressed formats transferred
> > over isochronous endpoints. While at it, document the other quirk that
> > handles a bandwidth issue for uncompressed formats.
> >
> > Signed-off-by: Ai Chao <aichao@kylinos.cn>
> > ---
> > I have dropped the Reviewed-by tags as the patch has changed
> > significantly.
> >
> > Ricardo, do you know if the 3072 bytes limit is fine with super-speed
> > devices, or does it need to be increased ?
> 
> We have enough documentation to let ChatGPT make the code for us :)

It will still need some improvements :-)

> I am going to try tonight on two superspeed cameras. Will let you know tomorow.
> 
> > ---
> >  drivers/media/usb/uvc/uvc_video.c | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > index d4b023d4de7c..c6351d3b24cf 100644
> > --- a/drivers/media/usb/uvc/uvc_video.c
> > +++ b/drivers/media/usb/uvc/uvc_video.c
> > @@ -200,6 +200,20 @@ static void uvc_fixup_video_ctrl(struct uvc_streaming *stream,
> >         if ((ctrl->dwMaxPayloadTransferSize & 0xffff0000) == 0xffff0000)
> >                 ctrl->dwMaxPayloadTransferSize &= ~0xffff0000;
> >
> > +       /*
> > +        * Many devices report an incorrect dwMaxPayloadTransferSize value. The
> > +        * most common issue is devices requesting the maximum possible USB
> > +        * bandwidth (3072 bytes per interval for high-speed, high-bandwidth
> > +        * isochronous endpoints) while they actually require less, preventing
> > +        * multiple cameras from being used at the same time due to bandwidth
> > +        * overallocation.
> > +        *
> > +        * For those devices, replace the dwMaxPayloadTransferSize value based
> > +        * on an estimation calculated from the frame format and size. This is
> > +        * only possible for uncompressed formats, as not enough information is
> > +        * available to reliably estimate the bandwidth requirements for
> > +        * compressed formats.
> > +        */
> >         if (!(format->flags & UVC_FMT_FLAG_COMPRESSED) &&
> >             stream->dev->quirks & UVC_QUIRK_FIX_BANDWIDTH &&
> >             stream->intf->num_altsetting > 1) {
> > @@ -236,6 +250,23 @@ static void uvc_fixup_video_ctrl(struct uvc_streaming *stream,
> >
> >                 ctrl->dwMaxPayloadTransferSize = bandwidth;
> >         }
> > +
> > +       /*
> > +        * Another issue is with devices that report a transfer size that
> > +        * greatly exceeds the maximum supported by any existing USB version
> > +        * for isochronous transfers.  For instance, the "Slave camera" devices
> > +        * from Alcor Corp. (2017:0011 and 1b17:66B8) request 2752512 bytes per
> > +        * interval.
> > +        *
> > +        * For uncompressed formats, this can be addressed by the FIX_BANDWIDTH
> > +        * quirk, but for compressed format we can't meaningfully estimate the
> > +        * required bandwidth. Just hardcode it to 1024 bytes per interval,
> > +        * which should be large enough for compressed formats.
> > +        */
> > +       if ((format->flags & UVC_FMT_FLAG_COMPRESSED) &&
> > +           ctrl->dwMaxPayloadTransferSize > 3072 &&
> > +           stream->intf->num_altsetting > 1)
> > +               ctrl->dwMaxPayloadTransferSize = 1024;
> 
> - Maybe we should add a debug message if we are doing this?

I can do that.

> - Also I do not like that the value that we use as trigger (3072) is
> different than the quirked value (1024)
> 
> Something like:
> 
> value = min(3072, value)
> 
> makes more sense

That would prevent two such devices from working in parallel, when they
likely could as 1024 bytes should be enough. That's the reason for the
UVC_QUIRK_FIX_BANDWIDTH for uncompressed formats.

> >  }
> >
> >  static size_t uvc_video_ctrl_size(struct uvc_streaming *stream)

-- 
Regards,

Laurent Pinchart