I have a simple hardware fix for the popping sound that happens on power
up, suspend, and resume. The net component impact is the removal of one
transistor. I have tested this fix and it works.
Details are in http://dev.laptop.org/ticket/977#comment:5
There is a software impact. With this change, in order to get sound
out, it is necessary to assert the EAPD pin on the CODEC. That pin,
deasserted by default, is nominally used to turn off the amp when
asserted. With the change, asserting that pin turns the amp on, not
off. Hardware engineers from Quanta and Analog Devices considered,
without success, several other hardware changes to eliminate that pop.
It boils down to the fact that we have either invert the sense of that
pin, or switch the amp control to an entirely separate GPIO pin on
another device (which are in short supply).
To assert that pin, write 0x8000 to codec register 0x26.
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