Re: [PATCH 4/5] drm/radeon: Don't register Thunderbolt eGPU with vga_switcheroo

From: Lukas Wunner <lukas@wunner.de>
To: Alex Deucher <alexdeucher@gmail.com>
Cc: Michael Jamet <michael.jamet@intel.com>,
	amd-gfx list <amd-gfx@lists.freedesktop.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Maling list - DRI developers <dri-devel@lists.freedesktop.org>,
	Alex Deucher <alexander.deucher@amd.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Christian Koenig <christian.koenig@amd.com>,
	Peter Wu <peter@lekensteyn.nl>
Subject: Re: [PATCH 4/5] drm/radeon: Don't register Thunderbolt eGPU with vga_switcheroo
Date: Wed, 8 Mar 2017 06:01:54 +0100	[thread overview]
Message-ID: <20170308050154.GA4250@wunner.de> (raw)
In-Reply-To: <CADnq5_MgKOOaxTGx3=bs=ZopSKkZqxoOnxd=+hv-ik5PzRi4Gw@mail.gmail.com>

On Tue, Mar 07, 2017 at 03:30:30PM -0500, Alex Deucher wrote:
> On Fri, Feb 24, 2017 at 2:19 PM, Lukas Wunner <lukas@wunner.de> wrote:
> > An external Thunderbolt GPU can neither drive the laptop's panel nor be
> > powered off by the platform, so there's no point in registering it with
> > vga_switcheroo.  In fact, when the external GPU is runtime suspended,
> > vga_switcheroo will cut power to the internal discrete GPU, resulting in
> > a lockup.
> 
> I'm not necessarily opposed to this, but I'd prefer something more
> generic.  E.g., what happens if someone uses another dGPU in a docking
> station or some other sort of PCIe bridge?

Such a dGPU is only relevant to vga_switcheroo if it can either drive
the panel or be powered off by the platform.  Does such a product exist?

I've never heard of one, and think that's because such a product doesn't
make sense:  A docking staton is not power-constrained, it's stationary
and connected to a wall outlet, so there's no need to power the dGPU off
when it's not in use.

And at a docking station you're usually connected to a larger monitor,
so having the dGPU drive the laptop's smaller panel isn't necessary either.
In the rare cases where there's no larger monitor, you just use the dGPU
for render offloading, just as you would for contemporary ATPX laptops.

OTOH, dGPUs in Thunderbolt enclosures *do* exist and connecting them
to an ATPX machine causes failure, as explained in the commit message.

> I think on AMD platforms
> at least we should be able to determine what devices are the
> switcheroo devices based on information in the ATIF and ATPX ACPI
> methods.  In that case, we can be explicit in which devices we
> register with vga_switcheroo.

Is there public documentation on these methods somewhere?

Thanks,

Lukas
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