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

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Date:	Wed, 27 May 2015 07:53:29 +0200
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Michael Marineau <michael.marineau@...eos.com>
Cc:	Darren Hart <dvhart@...radead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	platform-driver-x86@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Petri Hodju <petrihodju@...oo.com>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [Patch v3] apple-gmux: lock iGP IO to protect from vgaarb
 changes

Hi Michael,

On Tue, 26 May 2015 21:47:49 -0700 Darren Hart wrote:
> 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?

A full kernel log up to including post-initrd loading of gmux would be
useful.

As far as I have seen nouveau should not be doing unneeded vgaarb
operations by itself (though userspace might be) as opposed to closed
nvidia driver.

If your systems allows, try booting to init=/bin/bash, then check for
backlight, load nouveau, check for backlight and finally load gmux and
check backlight (putting i915 in the mix where initrd/userspace puts
it would be nice).

Thanks,
Bruno
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