On Mon, Dec 20, 2021 at 04:34:13PM +0200, Mathias Nyman wrote:

On 17.12.2021 15.55, Peter Zijlstra wrote:

quoted

On Fri, Dec 17, 2021 at 01:01:43PM +0200, Mathias Nyman wrote:

quoted

I can reproduce this.
Looks like problems started when driver converted to readl_poll_timeout_atomic() in:

796eed4b2342 usb: early: convert to readl_poll_timeout_atomic()

I can confirm, reverting that solves the boot hang, things aren't quite
working for me though.

quoted

Seems to hang when read_poll_timeout_atomic() calls ktime_* functions.
Maybe  it's too early for ktime.

It certainly is, using ktime for delay loops sounds daft to me anyhow.

quoted

After reverting that patch it works again for me.

[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.16.0-rc3+ root=UUID=a652986c-fbc6-4341-85c3-b4ad4402f130 ro debug ignore_loglevel sysrq_always_enabled usbcore.autosuspend=-1 earlyprintk=xdbc force_early_printk sched_verbose ftrace=nop mitigations=off nokaslr
...
[    0.000000] xhci_dbc:early_xdbc_parse_parameter: dbgp_num: 0
...
[    3.161367] xhci_dbc:early_xdbc_setup_hardware: failed to setup the connection to host

Ok, this is some other issue. I got the boot messages over USB
(running minicom at the other end, listening to ttyUSB0)

I have a regular A->A USB3 cable, not the special one with a wire
missing. Given the dbc thing works, I feel this ought to work too.

quoted

The machine does boot.. but I *am* getting tons of:

[  485.546898] usb usb4-port4: Cannot enable. Maybe the USB cable is bad?
[  485.546963] usb usb4-port4: config error

This is expected when xhci driver takes over after the early dbc driver,
xhci driver resets xHC controller, and all ports turn to normal host ports again.

Because of the special cable you now have two hosts connected to each other,
both trying to enumerate a device.

This whole transition from earlyprintk xdbc to normal xhci driver is 
not very userfriendly.

Uhhmm... but but that shouldn't be. What if I want to keep using xdbc as
console after that point? specifically, I'll probably end up having:

 earlyprintk=xdbc,keep

once all this starts working. The whole point of early consoles is that
they're more reliable than regular consoles, not that they're 'early'.

quoted

However, when I do:

$ echo enable > /sys/bus/pci/devices/0000:00:14.0/dbc

Yes, this works as it turns on the DbC feature on in xHC hardware,
which turns the first USB port into a usb device.

But if this works, why isn't earlyprintk working for me?