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

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Date:	Tue, 26 May 2015 22:35:46 -0700
From:	Michael Marineau <michael.marineau@...eos.com>
To:	Darren Hart <dvhart@...radead.org>
Cc:	Bruno Prémont <bonbons@...ux-vserver.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

On Tue, May 26, 2015 at 9:47 PM, Darren Hart <dvhart@...radead.org> 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?

No error, gmux claims it worked:
[   13.693379] apple_gmux: Found gmux version 4.0.8 [indexed]
[   13.693400] vgaarb: device changed decodes:
PCI:0000:01:00.0,olddecodes=io+mem,decodes=io+mem:owns=none
[   13.693404] apple_gmux: locked IO for PCI:0000:01:00.0

Full dmesg: https://gist.github.com/marineam/0e5a23548e8b3b2e1d50
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