Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> >> [ 44.059958] qmi_wwan 1-1:1.3 wwan0: unregister 'qmi_wwan' usb-ci_hdrc.1-1, We
..
>> That was always my thought until I tried kernel 5.1 under the same
>> platform (nothing changed except the kernel version), the kernel 5.1
>> can connect to the 4G modem, I could not tell the hardware engineer if
>> it was hardware problem where kernel 5.1 can connect, kernel 4.19
>> could not, how would you explain it? Seems some differences between
>> kernel 5.1 and kernel 4.19, what I could be missing?
>>
>> I cannot use kernel 5, we need kernel LTS on product, too late to wait
>> for 5.4 LTS.
>
> Can you use 'git bisect' to find the commit that fixes the issue? That
> way we can backport it to the 4.19.y tree for you.
Yes, please.
But if I were to guess based on the above info, then I'd start looking
at the chipidea changes. Commit 2c4593ecc920 ("usb: chipidea: host:
override ehci->hub_control") looks particularily interesting.
Bjørn
