libhtc_ril.so and segfaults

If you try to get a new Android version, in this case CyanogenMod9, working on your old phone you have to deal with binary blobs. One of these blobs is the library talking to the radio, libhtc_ril.so.

I wanted to document what I learned about libhtc_ril.so. I’ve wanted to get the library version matching my baseband version working with cm9. This resulted it several segfaults. So I’ve started to strace the rild process to find what’s going wrong, which permissions are missing etc. The library doesn’t check return values so it segfaults. One of these segfaults was a missing kernel interface called usb_function_switch. The file should be in /sys/devices/platform/msm_hsusb/usb_function_switch. I’ve implemented that function in the kernel and it still segfaulted and I had no idea what to do now. Today I analyzed the RADIO logs and stumpled upon:

D/RILJ    (  328): [0100]> SCREEN_STATE: false
D/HTC_RIL ( 1360): ril_func_screen_state_notified():called
D/HTC_RIL ( 1360): ril_func_screen_state_notified():Not found 'ether:' in USB_STATE_PATH

As it segfaulted directly after closing /sys/devices/platform/msm_hsusb/usb_function_switch it smelled like it expeced to have something like:

ether:disable

I’ve dived into the code and found out that in my kernel tree it was called rndis and in the htc kernel tree it was called ether. So I’ve fixed that and added the other values of /sys/devices/platform/msm_hsusb/usb_function_switch it started to work just fine. I hope this post will help other developers with similar problems.

This is the full set of the usb_function_switch:

ether:disable
accessory:disable
usb_mass_storage:enable
adb:enable
cdc_ethernet:disable
diag:disable
modem:disable
serial:disable