When should gpio-restart be used/avoided?

[Lech Perczak]

Hi,

Yes, I can confirm, this is used to reset the modem on rebooting the device.

At least when used in conjunction with uqmi, the modem would misbehave 
otherwise,
and connection attempts would fail after rebooting the device. The same 
is true for its ath79-based predecessors.
I think this issue deserves investigation on its own.

Issue with this pin is that if we'd just like to reset the modem, it's 
not possible without resetting the device,
as this is basically a "suicide switch" rather than just USB power cutoff.

Kind regards,
Lech

W dniu 2022-08-13 o 22:48, Robert Marko pisze:
> I had just that question today when reviewing a similar ZTE modem.
> It appears that its to reset the modem as well:
> https://github.com/openwrt/openwrt/pull/10418/files#r945125301
>
> Regards,
> Robert
>
> On Sat, 13 Aug 2022 at 19:13, Bjørn Mork <bjorn at mork.no> wrote:
>> I just got myself a ZTE286D.  But after installing OpenWrt I had an
>> unexpected problem on reboot. The Openwrt DTS includes a gpio-restart
>> device:
>>          gpio-restart {
>>                  compatible = "gpio-restart";
>>                  gpios = <&tlmm 8 GPIO_ACTIVE_HIGH>;
>>          };
>>
>>
>> The gpio-restart driver is therefore used to reboot the system due to
>> default priorities.
>>
>> I have two concerns about this:
>>
>>   1) it's different from OEM, and
>>   2) it cuts power to the 3.3V VCC console line on reboot
>>
>> A quick test indicates that reboot works just fine without using
>> gpio-restart in OpenWrt too.  And a similar node seems to be rare in
>> this target.  AFAICS, this is the only device having one.
>>
>> So I wonder if we could/should remove that gpio-restart device.  What do
>> you think?
>>
>> The reason I ask is because the VCC toggling causes unnecessary problems
>> for me.  I realize that I have an unusual setup, but I have connected a
>> bluetooth module to the console port.  Toggling the console VCC on
>> reboot makes it hard/imposible to enter the bootloader, or to capture
>> the whole boot log.  It takes some time before the console server
>> notices that the bluetooth device disappeared and tries to reconnect. I
>> believe this is an unwanted side effect.  And it wasn't like that with
>> OEM.
>>
>> This is of course only a minor problem. In addition to just pathcing the
>> DTS myself, I could also put something like
>>
>>    test -w /sys/bus/platform/drivers/restart-gpio/unbind && echo gpio-restart >/sys/bus/platform/drivers/restart-gpio/unbind
>>
>> into /etc/rc.local.  That's a perfectly fine workaround.  But I wanted
>> to ask becasuse of the different behaviour in OEM.
>>
>>
>> Bjørn
>>
>>
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-- 
Pozdrawiam,
Lech Perczak
lech.perczak at gmail.com
Mobile: +48 694 309 185