Re: [Patch v3] apple-gmux: lock iGP IO to protect from vgaarb changes

[Darren Hart <dvhart@infradead.org>]

On Tue, May 26, 2015 at 12:10:48PM -0700, Michael Marineau wrote:
> FYI, this actually broke backlight controls on my MBP11,3 because the
> assumption the patch makes that gmux is always loaded before graphics
> drivers didn't hold true. At least for me dracut included the nouveau
> module in the initrd but not gmux, ensuring the ordering was wrong. No
> errors were reporting, and gmux still offered the backlight device, it
> just became inoperable. I worked around this for my kernel by building
> gmux into vmlinuz instead of as a module but that isn't going to in
> more general configs because there is an apple backlight driver which
> cannot be built at all in that configuration.
> 

Thank you for reporting this Michael,

That is tough as nouveau doesn't have an explicit dependency on gmux, so we
could do something like a passive request_module(), but if it isn't in the
initrd image, it would still fail as you describe.

> Is there a way to make the ordering between nouveau and gmux more
> explicit/reliable? Can gmux complain loudly if the ordering is ever
> wrong?

It should print an error if the probe fails due to the IO already being in use
or if it can't be allocated. The disabled IO case is only info level though,
perhaps that should be higher priority. Printing something when failing to probe
seems like a reasonable thing to do.

Michael, which message do you get if you boot with "debug" or "loglevel=6" when
apple-gmux is not built-in?

-- 
Darren Hart
Intel Open Source Technology Center