Message-ID: <YR5nhmF3MXdjtCvs@pendragon.ideasonboard.com>
Date:   Thu, 19 Aug 2021 17:15:34 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media: uvcvideo: Quirk for hardware with invalid sof

Hi Ricardo,

On Thu, Aug 19, 2021 at 01:31:32PM +0200, Ricardo Ribalda wrote:
> On Thu, 19 Aug 2021 at 12:28, Laurent Pinchart wrote:
> > On Thu, Aug 19, 2021 at 08:27:00AM +0200, Ricardo Ribalda wrote:
> > > On Thu, 19 Aug 2021 at 00:22, Laurent Pinchart wrote:
> > > > On Wed, Aug 18, 2021 at 10:35:02PM +0200, Ricardo Ribalda wrote:
> > > > > The hardware timestamping code has the assumption than the device_sof
> > > > > and the host_sof run at the same frequency (1 KHz).
> > > > >
> > > > > Unfortunately, this is not the case for all the hardware. Add a quirk to
> > > > > support such hardware.
> > > > >
> > > > > Note on how to identify such hardware:
> > > > > When running with "yavta -c /dev/videoX" Look for periodic jumps of the
> > > > > fps. Eg:
> > > > >
> > > > > 30 (6) [-] none 30 614400 B 21.245557 21.395214 34.133 fps ts mono/SoE
> > > > > 31 (7) [-] none 31 614400 B 21.275327 21.427246 33.591 fps ts mono/SoE
> > > > > 32 (0) [-] none 32 614400 B 21.304739 21.459256 34.000 fps ts mono/SoE
> > > > > 33 (1) [-] none 33 614400 B 21.334324 21.495274 33.801 fps ts mono/SoE
> > > > > 34 (2) [-] none 34 614400 B 21.529237 21.527297 5.130 fps ts mono/SoE
> > > > > 35 (3) [-] none 35 614400 B 21.649416 21.559306 8.321 fps ts mono/SoE
> > > > > 36 (4) [-] none 36 614400 B 21.678789 21.595320 34.045 fps ts mono/SoE
> > > > > ...
> > > > > 99 (3) [-] none 99 614400 B 23.542226 23.696352 33.541 fps ts mono/SoE
> > > > > 100 (4) [-] none 100 614400 B 23.571578 23.728404 34.069 fps ts mono/SoE
> > > > > 101 (5) [-] none 101 614400 B 23.601425 23.760420 33.504 fps ts mono/SoE
> > > > > 102 (6) [-] none 102 614400 B 23.798324 23.796428 5.079 fps ts mono/SoE
> > > > > 103 (7) [-] none 103 614400 B 23.916271 23.828450 8.478 fps ts mono/SoE
> > > > > 104 (0) [-] none 104 614400 B 23.945720 23.860479 33.957 fps ts mono/SoE
> > > > >
> > > > > They happen because the delta_sof calculated at
> > > > > uvc_video_clock_host_sof(), wraps periodically, as both clocks drift.
> > > >
> > > > That looks plain wrong. First of all, the whole purpose of the SOF clock
> > > > is to have a shared clock between the host and the device. It makes no
> > > > sense for a device to have a free-running "SOF" clock. Given the log
> > > > above, the issue occurs so quickly that it doesn't seem to be a mere
> > > > drift of a free running clock. Could you investigate this more carefully
> > > > ?
> > >
> > > In my test the dev_sof runs at 887.91Hz and the dev_sof at 1000.35Hz.
> > > If I plot the difference of both clocks host_sof - (dev_sof % 2048), I
> > > get this nice graph https://imgur.com/a/5fQnKa7
> > >
> > >
> > > I agree that it makes not sense to have a free-running "SOF", but the
> > > manufacturer thinks otherwise :)
> >
> > In that case there's no common clock between the device and the host,
> > which means that clock recovery is impossible. The whole timestamp
> > computation should be bypassed, and the driver should use the system
> > timestamp instead.
> 
> Or said differently. The clock recovery is susceptible to the jitter
> in the frame acquisition.
> 
> If you have no jitter, the clock recovered will match the reality, and
> if you have bad jitter, it will be as bad as system timestamp.

The whole point of the clock recovery code is to convert a precise
timestamp, expressed using a device clock that the host has no access
to, to a system clock. This can only be done if the relationship between
the two clocks can be inferred, and the UVC specifies a mechanism to
allow this by using a common clock, in the form of the SOF counter. If
we don't have that, we're essentially screwed, and can't use the
algorithm implemented in the driver at all. I'd much rather skip is
completely in that case, instead of trying to hack the algorithm itself.

On a side note, I think the whole clock recovery implementation should
move from the uvcvideo driver to userspace, where we'll have the ability
to perform floating point computation. The kernel implementation is
crude, it should be replaced with a linear regression.

> So this patch will still be better than nothing.
> 
> > I still find it hard to believe that a Logitech camera would get this
> > wrong.
> 
> I guess I can send you a device, or give you access to mine remotely
> if you do not believe me :)
> 
> > > > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > > > ---
> > > > > v2: Fix typo in frequency
> > > > >
> > > > >  drivers/media/usb/uvc/uvc_driver.c |  9 +++++++++
> > > > >  drivers/media/usb/uvc/uvc_video.c  | 11 +++++++++--
> > > > >  drivers/media/usb/uvc/uvcvideo.h   |  2 ++
> > > > >  3 files changed, 20 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> > > > > index 9a791d8ef200..d1e6cba10b15 100644
> > > > > --- a/drivers/media/usb/uvc/uvc_driver.c
> > > > > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > > > > @@ -2771,6 +2771,15 @@ static const struct usb_device_id uvc_ids[] = {
> > > > >         .bInterfaceSubClass   = 1,
> > > > >         .bInterfaceProtocol   = 0,
> > > > >         .driver_info          = UVC_INFO_QUIRK(UVC_QUIRK_RESTORE_CTRLS_ON_INIT) },
> > > > > +     /* Logitech HD Pro Webcam C922 */
> > > > > +     { .match_flags          = USB_DEVICE_ID_MATCH_DEVICE
> > > > > +                             | USB_DEVICE_ID_MATCH_INT_INFO,
> > > > > +       .idVendor             = 0x046d,
> > > > > +       .idProduct            = 0x085c,
> > > > > +       .bInterfaceClass      = USB_CLASS_VIDEO,
> > > > > +       .bInterfaceSubClass   = 1,
> > > > > +       .bInterfaceProtocol   = 0,
> > > > > +       .driver_info          = UVC_INFO_QUIRK(UVC_QUIRK_INVALID_DEVICE_SOF) },
> > > > >       /* Chicony CNF7129 (Asus EEE 100HE) */
> > > > >       { .match_flags          = USB_DEVICE_ID_MATCH_DEVICE
> > > > >                               | USB_DEVICE_ID_MATCH_INT_INFO,
> > > > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > > > index 6d0e474671a2..760ab015cf9c 100644
> > > > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > > > @@ -518,13 +518,20 @@ uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
> > > > >       /* To limit the amount of data, drop SCRs with an SOF identical to the
> > > > >        * previous one.
> > > > >        */
> > > > > -     dev_sof = get_unaligned_le16(&data[header_size - 2]);
> > > > > +     if (stream->dev->quirks & UVC_QUIRK_INVALID_DEVICE_SOF)
> > > > > +             dev_sof = usb_get_current_frame_number(stream->dev->udev);
> > > > > +     else
> > > > > +             dev_sof = get_unaligned_le16(&data[header_size - 2]);
> > > > > +
> > > > >       if (dev_sof == stream->clock.last_sof)
> > > > >               return;
> > > > >
> > > > >       stream->clock.last_sof = dev_sof;
> > > > >
> > > > > -     host_sof = usb_get_current_frame_number(stream->dev->udev);
> > > > > +     if (stream->dev->quirks & UVC_QUIRK_INVALID_DEVICE_SOF)
> > > > > +             host_sof = dev_sof;
> > > > > +     else
> > > > > +             host_sof = usb_get_current_frame_number(stream->dev->udev);
> > > > >       time = uvc_video_get_time();
> > > > >
> > > > >       /* The UVC specification allows device implementations that can't obtain
> > > > > diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> > > > > index cce5e38133cd..89d909661915 100644
> > > > > --- a/drivers/media/usb/uvc/uvcvideo.h
> > > > > +++ b/drivers/media/usb/uvc/uvcvideo.h
> > > > > @@ -209,6 +209,8 @@
> > > > >  #define UVC_QUIRK_RESTORE_CTRLS_ON_INIT      0x00000400
> > > > >  #define UVC_QUIRK_FORCE_Y8           0x00000800
> > > > >  #define UVC_QUIRK_FORCE_BPP          0x00001000
> > > > > +#define UVC_QUIRK_INVALID_DEVICE_SOF 0x00002000
> > > > > +
> > > > >
> > > > >  /* Format flags */
> > > > >  #define UVC_FMT_FLAG_COMPRESSED              0x00000001

-- 
Regards,

Laurent Pinchart

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