linux-kernel - Re: [PATCH v2 4/5] apple-gmux: support MMIO gmux on T2 Macs

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Date:   Mon, 20 Feb 2023 09:44:54 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Lukas Wunner <lukas@...ner.de>,
        Orlando Chamberlain <orlandoch.dev@...il.com>
Cc:     Mark Gross <markgross@...nel.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Seth Forshee <sforshee@...nel.org>,
        Aditya Garg <gargaditya08@...e.com>,
        Aun-Ali Zaidi <admin@...eit.net>,
        Kerem Karabay <kekrby@...il.com>
Subject: Re: [PATCH v2 4/5] apple-gmux: support MMIO gmux on T2 Macs

Hi,

On 2/19/23 14:39, Lukas Wunner wrote:
> On Fri, Feb 17, 2023 at 11:05:31AM +1100, Orlando Chamberlain wrote:
>> On Thu, 16 Feb 2023 14:27:13 +0100 Hans de Goede <hdegoede@...hat.com> wrote:
>> It looks like iMac20,1, iMac20,2, and iMacPro1,1 have APP000B:
>>
>> apple_gmux: Failed to find gmux I/O resource
>>
>> iMac20,2: https://linux-hardware.org/?probe=ec2af584b3&log=dmesg
>> iMac20,1: https://linux-hardware.org/?probe=fee7644b9c&log=dmesg
>> iMacPro1,1: https://linux-hardware.org/?probe=6c26c9ff8c&log=dmesg
>>
>> But I'm not sure if they actually have it or not.
> 
> A number of iMacs support Target Display Mode, i.e. you can plug in
> an external computer to the iMac's DisplayPort and use the iMac as
> its screen.  Those iMac models do contain a gmux to mux the display
> between internal GPU and external DisplayPort connection.  Linux
> does not have support for this, sadly.  It would require generalizing
> vga_switcheroo for use cases beyond dual GPU laptops.  Florian Echtler
> has been looking into Target Display Mode but I'm not sure he got it
> working:
> 
> https://lore.kernel.org/all/e6b44662-8b05-fb8d-660e-77153680d11b@butterbrot.org/
> https://lore.kernel.org/all/71a48bf5-54b5-6d7b-d574-e0aa6c1acb80@butterbrot.org/
> https://lore.kernel.org/all/1497603751-22360-1-git-send-email-floe@butterbrot.org/
> 
> I believe the Mac Pro (the trashcan one) also contains a gmux and
> an APP000B device in DSDT.  I recall seeing a bug report due to a
> splat in the gmux driver on that machine.  Back then I confirmed
> in the schematics that it does contain a gmux, though I think it's
> only used for brightness, not GPU switching.

Erm, the Mac Pro (the trashcan one) does not have an internal LCD
panel, right?  So I don't think the gmux will be used for brightness
control there ...

Regards,

Hans