Would prefer_shadow ever be 0?
Yes thats for the USB and vmware type users, where the buffer is just a main RAM buffer not in VRAM or uncached memory.
Isn't preferred_depth just a static (non-driver-specific) preference list like "24, 30, 16, 15, 8" or something? Do we not expose the list of supported depths somewhere else?
No we don't as we have userspace drivers that know as much as our kernel drivers about the hw, I was tempted to go for a list, but it gets a bit messy to enumerate, and really the generic driver doesn't care, it just wants to pick one value for autoconfiguration, if someone sets a value in xorg.conf and it blows up well thats their issue and they can pick another value.
Dave.