Re: [PATCH v2] drivers/char: add timeout for XHCI console cable

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.10.2026 15:35, Marek Marczykowski-Górecki wrote:
> Do not wait indefinitely for XHCI to configure console connection. If
> the cable is not plugged in, timeout after about 1s and continue boot.
> If the cable is plugged in later, the console will be activated then
> (see final part of dbc_ensure_running()). When timeout occurs, print a
> warning message - it will be saved in the console ring buffer, to be
> later retrieved from dom0, or possibly later printed when the cable is
> finally connected. But, do not print the warning when waiting after
> controller reset, as that may have been called from from console
> dbc_putc() and printing here would result in (possibly infinite)
> recursion.
> 
> Try the timeout to be about 1s, which is way above time needed for
> configuring already connected cable. But, since this part runs very
> early in the Xen startup, time is not calibrated yet, and functions like
> NOW() do not work yet. Use raw TSC and estimate needed time to be at
> least 1s on a fast CPU (5GHz), and possibly longer on a slower one. This
> is not very accurate method, but fortunately it doesn't need to be.
> 
> This also helps with similar situation on resuming from suspend - if
> cable got unplugged in the meantime, system will not hang waiting for
> the console anymore. And finally, on some systems, when dom0 is also
> allowed to access the USB controller, it may put the device into low
> power mode that in some older controllers (for example Kaby Lake)
> disables also debug capability. In such a case, adding a timeout allows
> the system to resume, then dom0 can wake up the controller and Xen will
> resume XHCI console.
> 
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>