Faster resume probing

[Dylan Van Assche]

Hi,


> I've also been working with USB QMI modems (Quectel and Simcom) and
> getting suspend-to-ram/resume working smoothly, so I figured perhaps
> I
> can share some of the things I've learnt along the way. But do note
> that I'm not by any means an expert here - this is my first foray
> into
> usb & modems, so do take what I say with a grain of salt.
> 
> Firstly, I agree with Aleksander's opinion above - I think the
> underlying problem is the fact that the USB connection is being reset
> upon resume. I'd try to solve that first - disable MM & NM altogether
> for now, and focus on the qmi & usb layers. Use qmicli to set up a
> connection or at least check basic info with commands like
> qmicli --device=/dev/cdc-wdm0 --device-open-proxy --dms-get-model
> Then start looking at the USB side of things. The kernel docs
>  
> https://www.kernel.org/doc/html/v4.14/driver-api/usb/power-management.html
> provide some good background info on this area.
> 
> If I had a pinephone or similar hardware to test with I might be able
> to provide some more guidance, however I can perhaps shed some light
> by showing what was required to get my particular device working.
> My device is an IMX6ULL, and the modem is connected directly to the
> USB host pins on the chip itself. No USB hub ICs involved. Power to
> the modem is controlled externally (in my case it's always on).

Sounds like the PinePhone :) 
The EG25-G modem is also continously powered on and directly connected
to the A64 USB. Audio is routed over I2S directly in the hardware.
I will have a look at your suggestion on the PinePhone :) 

> USB info:
>     root at imx6ul-var-dart:~# lsusb -tv
>     /:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ci_hdrc/1p, 480M
>         ID 1d6b:0002 Linux Foundation 2.0 root hub
>     /:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ci_hdrc/1p, 480M
>         ID 1d6b:0002 Linux Foundation 2.0 root hub
>         |__ Port 1: Dev 2, If 0, Class=Vendor Specific Class,
> Driver=option, 480M
>             ID 2c7c:0125 Quectel Wireless Solutions Co., Ltd. EC25
> LTE modem
>         |__ Port 1: Dev 2, If 1, Class=Vendor Specific Class,
> Driver=option, 480M
>             ID 2c7c:0125 Quectel Wireless Solutions Co., Ltd. EC25
> LTE modem
>         |__ Port 1: Dev 2, If 2, Class=Vendor Specific Class,
> Driver=option, 480M
>             ID 2c7c:0125 Quectel Wireless Solutions Co., Ltd. EC25
> LTE modem
>         |__ Port 1: Dev 2, If 3, Class=Vendor Specific Class,
> Driver=option, 480M
>             ID 2c7c:0125 Quectel Wireless Solutions Co., Ltd. EC25
> LTE modem
>         |__ Port 1: Dev 2, If 4, Class=Vendor Specific Class,
> Driver=qmi_wwan, 480M
>             ID 2c7c:0125 Quectel Wireless Solutions Co., Ltd. EC25
> LTE modem
> 
> The following 3 tweaks are needed to get the modem persisting across
> a
> suspend/resume:
>    echo auto > /sys/bus/usb/devices/1-1/power/control
>    echo enabled > /sys/bus/platform/devices/2184000.usb/power/wakeup
>    echo enabled > /sys/bus/platform/devices/ci_hdrc.0/power/wakeup
> 

So with these your modem always stays connected over USB?
How does ModemManager react to this when resuming/suspending?
ModemManager still reprobes the device right?

> Obviously your USB setup will be different so the above won't work,
> but perhaps it might provide some direction. The first one - setting
> control to auto - makes sense to me, but I cannot see why the other
> two are needed. Having a good poke around the /sys/ tree is a good
> start anyway - set control=auto and wakeup=enabled on all the usb
> devices on/above the modem. Finding the equivalent of my 2nd line
> might be trickier and need some datasheet or device-tree reading to
> correctly identify it.
> If you have USB hub devices in the chain then things will be more
> complex, but I'd be surprised if they've set up the hardware that
> way.
> 
> Finally, to allow the modem to wakeup the host, the following two
> tweaks were needed:
>     echo enabled > /sys/bus/usb/devices/usb1/power/wakeup
>     echo enabled > /sys/bus/usb/devices/1-1/power/wakeup
> 

PinePhone uses the RING indicator of the EG25-G modem which gets
triggered when an URC or something is available. This RING indicator is
connected to a interrupt capable GPIO pin of the A64 host CPU.
As soon as the RING indicator is triggered, the host CPU will wake up,
even without any USB connection.

> Also, note I'm suspending via the very raw
>    echo mem > /sys/power/state
> so I can be sure that no setup/teardown is occuring. But obviously
> pm-suspend or systemctl suspend is the right way to do things for
> production units.
> 

We use indeed systemctl suspend of elogind to suspend the system.

Kind regards,
Dylan Van Assche