Comment 20 for bug 21574

I checked the hardware on my HP nw8240 laptop and the problem occurs because after suspend to RAM, the 5V power supply to the audio power amp IC is turned off. This is *not* controlled by the amplifier power down pin of the AC97 codec as would usually be the case, so nothing you can do with the AC97 will fix the problem. Indeed, all the AC97 registers are the same before and after suspend. The audio controller PCI configuration space registers are also the same.

Clearly, the nw8240 (and maybe other HP laptops) control the audio power amp power supply from somewhere else. I couldn't figure out physically where the signal comes from on the motherboard. I guess it has to do with the ACPI subsystem and the embedded controller-- but as I don't know much about this I am not able to debug it. I am not even sure whether the embedded controller is part of the Intel chipset, or a separate chip. I did play around with an adapted version of the kernel ibm_acpi module's ecdump utility to read and modify the embedded controller registers--but the best I could do by randomly tweaking the registers was crash my machine. Maybe someone who knows a bit about ACPI could suggest some productive ways of mapping out the hardware? Presumably other special features of the laptop, like the shock sensor, could also be accessed through ACPI if this could be figured out.

It is important to note that this issue is almost certainly HP-specific and not related to the numerous other "no sound after resume" issues that people have, which are fixable by simply setting the AC97 codec registers correctly.

It would be great if someone who is interested in this issue and knows something about ACPI and/or HP laptops could contact me so we could diagnose it. You can contact me on <email address hidden>