[Gta04-owner] Modem USB suspend (possible ~80mA reduction on active system?)

Sebastian Krzyszkowiak dos at dosowisko.net
Sun Jun 1 02:19:56 CEST 2014


I spent last days playing a lot with Cinterion PHS8 modem connected to
PC and operated by fsogsmd. And... one thing caught my attention.

PHS8 has a LCI - Low Current Indicator - which on my Neo900 prototype
board is connected to a LED. It shines when the modem is in SLEEP mode
with lowest power consumption rate.

Initially I thought that this LED is broken (and knowing that there
was some mistake in LEDs design of that proto it was easy to believe)
as I haven't seen it shining, but once, after playing with modem
settings that enable this indicator (without any results), I unplugged
the USB cable while leaving the modem on... and tada, LED shined

This meant that with USB cable attached, modem didn't go into SLEEP
state at all. However, quick look at modem spec told me that modem
should go into SLEEP state when either USB is disconnected or...

I recalled that PowerTOP had some options for auto-suspending USB, so
I launched it, went to "Tunables" and switched "Autosuspend for USB
device PH8 [Cinterion]" from "Bad" to "Good".

Tada! LED shined brighly (with some occasional blinks).

So it looks like Linux doesn't automatically enable power saving of
USB devices! I started to wonder if maybe something similar could have
place on GTA04 with its Option GTM601 modem. I've checked on SHR.

root at om-gta04:~# cat

"on" means "constantly on", so no suspend. Power consumption: about
250-260mA, stable

echo auto > /sys/devices/platform/usbhs_omap/ehci-omap.0/usb1/1-2/power/control

Power consumption: about 170-180mA, jumping back from time to time to
250-260mA and returning again to former ones. Jumps are probably
caused by modem resuming due to _QSIGQ reports (network signal
strength) which are sent pretty often - I guess configuring the modem
properly could reduce it and make the modem sleep more.

Communication with the modem appears to still work (or at least in no
worse way than before - modem support in SHR isn't perfect yet :P).

I have added following udev rule:

ACTION=="add", SUBSYSTEMS=="*", TEST=="power/control",

...and power saving mode is now enabled automatically on boot.

Is this something that was already known or a new finding? (it's for
sure something new for SHR it seems ;))

Sebastian Krzyszkowiak, dos

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