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9183c6f1a2
Logitech C922 internal SOF does not increases at a stable rate of 1kHz. 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> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org> Reviewed-by: Ricardo Ribalda <ribalda@chromium.org> Signed-off-by: Oleksandr Natalenko <oleksandr@natalenko.name> Reviewed-by: Tomasz Figa <tfiga@chromium.org> Link: https://lore.kernel.org/r/20240323-resend-hwtimestamp-v10-3-b08e590d97c7@chromium.org Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.