Reverse-engineer GPU fan control method on Toshiba Satellite L500?

Hello,

I have a Toshiba Satellite L500-164 (PSLJ3E) whose GPU fan (AMD Radeon HD 4650) keeps spinning loudly without slowing down when using the radeon driver. The propietary fglrx driver controls the fan correctly, though, so there must be a way to control it. However, I don't know how the needed information and/or operations are exposed by the laptop; lm-sensors detects no gpu-internal i2c thermal sensor (as would be supported by the radeon driver) and /sys/class/thermal shows no thermal zones. (Tested with today's mainline kernel.)

How can I find out how GPU fan control works on this laptop, e.g. if there is a Toshiba-specific ACPI interface for this? I am annoyed enough by this problem to do the needed reverse engineering research and at least help with the coding as much as I can, but I really don't know how to start. Any pointers would be welcome. (Please CC me when replying as I am not subscribed to this list.)

Regards,
Denis Washington