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OK, then the normal jack detection doesn't work with this machine, as it seems. You can try the above to all pins for checking if any of other pins reacts to the headphone jack.

No luck I'm afraid - none of the other pins seem to react to the headphones. Is there anything else you can think of that I can try tomorrow?

Well, it might be notified via GPIO. Try to read GPIO pins except for GPIO0. That is, set the all unmasked, the direction to READ.

We have found that jack sense comes in on the headphone pin in a windows bootcamp install. However this bit isn't set in linux using either the default chip, bootloader, or windows pin configs. No other GPIO's are enabled in windows.

Ben