I've been playing around with the Treo's GPIOs and I've finally discovered how to prevent the GSM module from going to sleep! Setting GPIO 57 high seems to force the module to stay awake. I also discovered GPIO 11 is a wake signal from the GSM to the CPU. This GPIO has a rising edge whenever some message is sent (or is ready to be sent) .
Also, while CTS seems to be always set when the GSM is powered, it will queue data while RTS is deasserted. So while in sleep we should probably deassert RTS, wait for GPIO 11 to awaken us (on eg incoming call) and then assert RTS to receive our data.
Trying to put all this information into a driver is going to be interesting. I wonder how pluggable the PXA serial driver is. Maybe I'll have to resort to just keeping the modem constantly awake for now.
Update: On first glance set_txrx() may do the trick.
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