Re: [PATCH v9 3/6] media: uvcvideo: Quirk for invalid dev_sof in Logitech C922

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

Hi Ricardo,

Thank you for the patch.

On Wed, Mar 15, 2023 at 02:30:14PM +0100, Ricardo Ribalda wrote:
> Logitech C922 internal SOF does not increases at a stable rate of 1kHz.

The next thing you know they will redefine to value of pi to be 3.

> This causes that the device_sof and the host_sof run at different rates,
> breaking the clock domain conversion algorithm. 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
> 
> Instead of disabling completely the hardware timestamping for such
> hardware we take the assumption that the packet handling jitter is
> under 2ms and use the host_sof as dev_sof.
> 
> We can think of the UVC hardware clock as a system with a coarse clock
> (the SOF) and a fine clock (the PTS). The coarse clock can be replaced
> with a clock on the same frequency, if the jitter of such clock is
> smaller than its sampling rate. That way we can save some of the
> precision of the fine clock.
> 
> To probe this point we have run three experiments on the Logitech C922.
> On that experiment we run the camera at 33fps and we analyse the
> difference in msec between a frame and its predecessor. If we display
> the histogram of that value, a thinner histogram will mean a better
> meassurement. The results for:
> - original hw timestamp: https://ibb.co/D1HJJ4x
> - pure software timestamp: https://ibb.co/QC9MgVK
> - modified hw timestamp: https://ibb.co/8s9dBdk
> 
> This bug in the camera firmware has been confirmed by the vendor.
> 
> lsusb -v
> 
> Bus 001 Device 044: ID 046d:085c Logitech, Inc. C922 Pro Stream Webcam
> Device Descriptor:
>   bLength                18
>   bDescriptorType         1
>   bcdUSB               2.00
>   bDeviceClass          239 Miscellaneous Device
>   bDeviceSubClass         2
>   bDeviceProtocol         1 Interface Association
>   bMaxPacketSize0        64
>   idVendor           0x046d Logitech, Inc.
>   idProduct          0x085c C922 Pro Stream Webcam
>   bcdDevice            0.16
>   iManufacturer           0
>   iProduct                2 C922 Pro Stream Webcam
>   iSerial                 1 80B912DF
>   bNumConfigurations      1
> 
> Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  drivers/media/usb/uvc/uvc_driver.c | 9 +++++++++
>  drivers/media/usb/uvc/uvc_video.c  | 8 ++++++++
>  drivers/media/usb/uvc/uvcvideo.h   | 1 +
>  3 files changed, 18 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 7aefa76a42b3..678a5736c9df 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -2549,6 +2549,15 @@ 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 1f416c494acc..4d566edb73e7 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -547,6 +547,14 @@ uvc_video_clock_decode(struct uvc_streaming *stream, struct uvc_buffer *buf,
>  	stream->clock.last_sof = dev_sof;
>  
>  	host_sof = usb_get_current_frame_number(stream->dev->udev);
> +
> +	/*
> +	 * On some devices, like the Logitech C922, the device SOF does not run
> +	 * at a stable rate of 1kHz. For those devices use the host SOF instead.

I'm still not very convinced, as without a formal proof I think there's
some luck involved, and it may break later. This being said, given that
this is gated by a quirk, it won't impact other devices, and we can deal
with regressions when they happen. Could we record it here:

	 * On some devices, like the Logitech C922, the device SOF does not run
	 * at a stable rate of 1kHz. For those devices use the host SOF instead.
	 * This improves the timestamp precision in the tests performed so far,
	 * even if the exact reason hasn't been fully determined.

or something along those lines ?

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> +	 */
> +	if (stream->dev->quirks & UVC_QUIRK_INVALID_DEVICE_SOF)
> +		dev_sof = host_sof;
> +
>  	time = uvc_video_get_time();
>  
>  	/*
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index 9a596c8d894a..07b2fdb80adf 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -73,6 +73,7 @@
>  #define UVC_QUIRK_FORCE_Y8		0x00000800
>  #define UVC_QUIRK_FORCE_BPP		0x00001000
>  #define UVC_QUIRK_WAKE_AUTOSUSPEND	0x00002000
> +#define UVC_QUIRK_INVALID_DEVICE_SOF	0x00004000
>  
>  /* Format flags */
>  #define UVC_FMT_FLAG_COMPRESSED		0x00000001
> 

-- 
Regards,

Laurent Pinchart