On Tue, 23 Jul 2019, Frank Bergmann wrote: > Hi! > > > I just want to make the case that this is quite an important issue. I > was very surprised to hear from the CIO of one of our customers that > they had the same issue. This kept them from offering Ubuntu and an > option for their Dell laptops. Like myself, these guys need to run > GoToMeeting/WebEx together with a Windows VM for business. > > > I understand this is probably a difficult bug and that it is difficult > to reproduce. However, this combination of WebCam + VM is required by > everybody except for a very few privileged "Linux only" developers. > > > > > It all depends on what that devices are asking for > > > The Logitech C922 has a H.264 compression on the chip. The "bandwidth > calculator" (https://www.google.es/search?q=H.264+bandwidth) says it > needs 1024kBps for 1080p @ 30fps. > > Not sure if this is true, and I'm not sure what the devices actually > does, though... > > > > Linux solved > > > Ok, so you have a clear idea what part of the driver causes the error, > that's a step forward. The situation is not as simple as you think. You're probably not aware of this, but with xHCI USB-3 controllers, bandwidth decisions are made by the hardware/firmware, not by the software. In particular, these decisions are not made by the kernel driver; they are made by the xHCI controller itself. Therefore changes to the kernel driver most likely cannot fix the problem. You would need to augment or replace your computer's xHCI USB-3 controller hardware. > But I can't imagine it's "real" bandwidth. USB 3.1 can handle two > full-HD screens plus a Gigabit Ethernet on the same 5GBit lanes... A few > megabit for compressed vido plus a few _kilo_ bits/s for audio... > > => Is there a way to debug these bandwidth requests for me? Maybe you > want to include bandwidth requests in the kernel logs in the future if > these issues are so wide spread? Here are some things you can do to provide more detailed debugging information. Unplug the webcam's USB cable, and then (as root) do: dmesg -C mount -t debugfs none /sys/kernel/debug echo 'module xhci_hcd =p' >/sys/kernel/debug/dynamic_debug/control echo 'module usbcore =p' >/sys/kernel/debug/dynamic_debug/control echo 81920 > /sys/kernel/debug/tracing/buffer_size_kb echo 1 > /sys/kernel/debug/tracing/events/xhci-hcd/enable Plug in and try (and fail!) to use the webcam. Send output of dmesg Send contents of /sys/kernel/debug/usb/devices Send contents of /sys/kernel/debug/tracing/trace I won't be able to interpret all the results for you, but Mathias (the xhci-hcd maintainer) should be able to. However, you ought to realize in advance that the results are likely to show only that the hardware refused to allocate the necessary bandwidth and did not provide any explanation for its refusal. > => Maybe you can recommend me a WebCam with tripod option that is known > to work well with Linux? The issue may depend more on the type of xHCI controller than on the type of webcam. Alan Stern > => From what I understand I should now test with VirtualBox and KVM/QEMU > and check if the same issue appears. I will do this and let you know > about the results. > > > > configuration of your devices and root hubs > > > I've got a brand new Dell XPX 15 9370 Laptop. No other USB hubs, > devices, no change to any configuration. I chose the model because it is > supposed to be the same that Linus Torvalds uses (well, XPS 13...) > > > > I can offer OpenVPN access to my laptop for reproducing the issues. I > also offer to buy a Logitech C922 WebCam if somebody wants to get > seriously down to this issue. > > > Best Regards, > Frank > > --- > Frank Bergmann > Dipl.-Ing., MBA > Founder ]project-open[ > Tel: +34 932 202 088 (Spain) > Cell: +34 609 953 751 > Fax: +34 932 890 729 > > mailto:frank.bergmann@xxxxxxxxxxxxxxxx > http://www.project-open.com > http://www.twitter.com/projop > http://www.facebook.com/projectopen.org > http://www.linkedin.com/in/fraber > http://www.xing.com/profile/Frank_Bergmann23
