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

From: Laurent Pinchart
Date: Thu Mar 21 2024 - 19:44:49 EST

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@xxxxxxxxxxxx>
> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxxxx>

> + */
> + 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