The fixes are:
1. Don't copy PCI::DeviceIdentifier during construction. This is a heavy
structure to copy so we definitely don't want to do that. Instead, use
a const reference to it like what happens in other parts in the Kernel.
2. Declare the constructor as explicit to avoid construction errors.
Previously we `VERIFY()`ed that the device supports variable-rate audio
(VRA). Now, we query the VRA bit and if VRA is not supported, we do not
enable double-rate audio and disallow setting any sample rate except
the fixed 48kHz rate as defined by the AC'97 specification. This should
allow the driver to function on a wider array of hardware.
Note that in the AC'97 specification, DRA without VRA is allowed when
supported: this effectively doubles the sample rate to 96kHZ. For now,
we ignore that possibility and let it default to 48kHZ.
Executing `asctl set r 96000` no longer results in weird sample rates
being set on the audio devices. SB16 checks for a sample rate between 1
and 44100 Hz, while AC97 implements double-rate support which allows
sample rates between 8kHz and 96kHZ.
This factors out some hardcoded PCMOut registers into a new private
class called AC97Channel, which wraps around a channel's registers and
provides some shared functionality.
No functional changes.
