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

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Date:   Fri, 17 Feb 2023 23:02:46 +1100
From:   Orlando Chamberlain <orlandoch.dev@...il.com>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Mark Gross <markgross@...nel.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lukas Wunner <lukas@...ner.de>,
        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

On Fri, 17 Feb 2023 11:05:31 +1100
Orlando Chamberlain <orlandoch.dev@...il.com> wrote:
> > 
> > Question are we not worried about MacBooks with an "APP000B"
> > ACPI device (with a value IORSOURCE_MEM entry) but which do not
> > actually have a gmux, because they are iGPU only ?  
> 
> 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. I'll see if I can
> get people with those models to test if it's a real gmux. There does
> seem to be a pattern in that those three all have AMD GPU's.

Kerem Karabay managed to find the acpi tables and macOS's ioreg from and
iMacPro1,1:

https://github.com/khronokernel/DarwinDumped/blob/master/iMacPro/iMacPro1%2C1/Darwin%20Dumper/DarwinDumper_3.0.4_30.12_15.30.40_iMacPro1%2C1_Apple_X64_High%20Sierra_17C2120_apple/ACPI%20Tables/DSL/DSDT.dsl#L10423
https://github.com/khronokernel/DarwinDumped/blob/master/iMacPro/iMacPro1%2C1/Darwin%20Dumper/DarwinDumper_3.0.4_30.12_15.30.40_iMacPro1%2C1_Apple_X64_High%20Sierra_17C2120_apple/IORegistry/IOReg.txt#L5096

The DSDT table has the same APP000B device as MacBooks with actual gmux,
while the ioreg has no mention of Apple's driver AppleMuxControl2 being
used for that device.

I think that confirms Apple has not fixed the issue of putting
APP000B's where they don't need to.

Solutions to this I can think of are:

- Use DMI matching to ignore product_names "iMacPro1,1" "iMac20,1",
  "iMac20,2"
- Maybe check if the MMIO region for gmux is filled with 0xff*

*I don't know if this would work or not as I don't have a machine to
check with. On my machine everything surrounding the 16 bytes used for
gmux is 0xff:

# hexdump -n48 -C -s 0xfe0b01f0 /dev/mem
fe0b01f0  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
fe0b0200  00 00 3e 4f 00 00 00 00  00 00 00 00 00 00 14 00  |..>O............|
fe0b0210  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|

so maybe on the iMacPro and iMac's, this would all be 0xff.

> 
> I've looked at dmesg or at least lsmod on all the models with the T2
> chip and there wasn't evidence of any other models having that error
> or having apple-gmux loaded on any models that shouldn't have a gmux,
> other than the three mentioned above. Of course I don't know if its
> possible for there to be firmware versions where this isn't the case.
> 
> > 
> > I have learned the hard way (through backlight control regressions
> > in 6.1) that at least some older model MacBooks with an IO resource
> > have an APP000B ACPI device without them actually having a gmux,
> > these get caught by the version check and then do not pass the
> > indexed check so that apple_gmux_detect() properly returns false.
> > 
> > Maybe make gmux_mmio_read32() a static inline inside
> > include/linux/apple-gmux.h and try to read the version here ?  
> 
> For that would we need to ioremap() and iounmap()?
> > 
> > Has this been tested on iGPU only T2 Macs?  
> 
> I don't think so. 
> 
> > 
> > Regards,
> > 
> > Hans
> > 
> >   
>