Skip to content

Latest commit

 

History

History
84 lines (75 loc) · 4.23 KB

README.md

File metadata and controls

84 lines (75 loc) · 4.23 KB

uvc_patch for multiple usb cameras

Quick Start for Kernel Version 5.15.136-tegra

If you are using the 5.15.136-tegra kernel, you can directly replace the uvcvideo.ko module without rebuilding the kernel:

  1. Locate the current uvcvideo.ko module: 
    modinfo uvcvideo | grep filename
  2. Backup the existing module: 
    sudo cp /lib/modules/5.15.136-tegra/kernel/drivers/media/usb/uvc/uvcvideo.ko /lib/modules/5.15.136-tegra/kernel/drivers/media/usb/uvc/uvcvideo.ko.bak
  3. Replace it with the patched version: 
    sudo rmmod uvcvideo
sudo cp path/to/new/uvcvideo.ko /lib/modules/$(uname -r)/kernel/drivers/media/usb/uvc/
sudo depmod -a
sudo modprobe uvcvideo

Troubleshooting Guide: Running Multiple USB Cameras on Linux and Overcoming Bandwidth Limitations

1. Problem Statement

Users often encounter errors like Not enough bandwidth for new device state or failures to initialize devices when running multiple USB cameras simultaneously on Linux. This issue persists even with compressed formats like MJPEG or low resolutions, preventing scaling up applications requiring multiple video streams.

2. Root Cause Analysis: USB Bandwidth Reservation

  • Shared Bandwidth: USB controllers have finite bandwidth shared among connected devices.
  • Isochronous Transfers: Video streaming uses USB isochronous transfers, requiring guaranteed bandwidth.
  • Peak Bandwidth Reservation: USB controllers reserve bandwidth based on the peak rate declared by the camera firmware, even if the actual data rate is lower.
  • Problematic Firmware: Some cameras declare excessively high peak bandwidth requirements, saturating the USB controller's capacity.

3. Standard Troubleshooting & Solutions

  • Check USB Topology: Use lsusb -t to identify how cameras are connected.
  • Separate Devices: Connect cameras to different USB root hubs or controllers.
  • Use Compression: Switch to compressed formats like MJPEG or H.264.
  • Lower Resolution/Framerate: Request lower settings to reduce bandwidth.
  • Use uvcvideo Quirks: Apply kernel module parameters like quirks=....

4. Kernel Patch for uvcvideo (If the above not work or suit for you)

To address the issue of excessive bandwidth reservation, a kernel patch was applied to modify the uvcvideo driver:

  • Goal: Override the dwMaxPayloadTransferSize during negotiation for compressed formats.
  • Patch Code: 
    // in the last part of function uvc_fixup_video_ctrl()
if (format->flags & UVC_FMT_FLAG_COMPRESSED) {
    u32 bandwidth = 512 * 8;
    if (stream->dev->udev->speed == USB_SPEED_HIGH)
        bandwidth /= 8;
    ctrl->dwMaxPayloadTransferSize = bandwidth;
}
  • Build Process:
    1. Obtain kernel source matching the running kernel (uname -r).
    2. Modify drivers/media/usb/uvc/uvc_video.c as shown above.
    3. Update the Makefile to ensure uvcvideo is built as a module: 
      obj-m += uvcvideo.o
uvcvideo-objs := uvc_driver.o uvc_queue.o uvc_v4l2.o uvc_video.o uvc_ctrl.o \
                 uvc_status.o uvc_isight.o uvc_debugfs.o uvc_metadata.o
ifeq ($(CONFIG_MEDIA_CONTROLLER),y)
uvcvideo-objs += uvc_entity.o
endif
obj-$(CONFIG_USB_VIDEO_CLASS) += uvcvideo.o
all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
    4. Build the module: 
      make
    5. Install the module as described in the "Quick Start" section.

5. Outcome

The patch bypasses the excessive bandwidth reservation, allowing multiple cameras to stream simultaneously. For example, 8 cameras (4 per USB controller) streaming 1280x720 MJPEG @ 30fps were successfully launched using GStreamer pipelines.

6. Warnings and Considerations

  • This patch is specific to buggy camera firmware and affects all UVC devices using compressed formats.
  • The fixed bandwidth value might be too low for other cameras or higher-quality streams.
  • Kernel modifications require rebuilding/reinstalling the module after updates.
  • This workaround bypasses standard USB bandwidth management and may cause instability if actual data rates exceed the forced low payload size.