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

From: Lukas Wunner <lukas@wunner.de>
To: Peter Wu <peter@lekensteyn.nl>, Alex Deucher <alexander.deucher@amd.com>
Cc: Michael Jamet <michael.jamet@intel.com>,
	amd-gfx list <amd-gfx@lists.freedesktop.org>,
	dri-devel@lists.freedesktop.org,
	Bjorn Helgaas <bhelgaas@google.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Christian Koenig <christian.koenig@amd.com>
Subject: Re: [PATCH 4/5] drm/radeon: Don't register Thunderbolt eGPU with vga_switcheroo
Date: Wed, 8 Mar 2017 13:22:40 +0100	[thread overview]
Message-ID: <20170308122240.GB25042@h08.hostsharing.net> (raw)
In-Reply-To: <20170308104633.GA2448@al>

On Wed, Mar 08, 2017 at 11:46:33AM +0100, Peter Wu wrote:
> On Wed, Mar 08, 2017 at 06:01:54AM +0100, Lukas Wunner wrote:
> > 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 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?
> 
> The ACPI interface is documented in
> drivers/gpu/drm/amd/include/amd_acpi.h while
> drivers/gpu/drm/amd/amdgpu/amdgpu_atpx_handler.c contains some glue for
> ACPI and the amdgpu driver (similar code exists for radeon).

Ah, thanks Peter.

Unfortunately this method will not work on Macs.  So I guess on those
we're again dependent on deducing whether a dGPU is internal or external
by looking at the PCI hierarchy.

However on non-Macs it seems that ATIF_FUNCTION_GET_GRAPHICS_DEVICE_TYPES
should return the type for each built-in GPU.

@Alex:
How reliable is this, e.g. is it possible that vendors may have forgotten
to set these bits in the BIOS?  If we depend on ATIF to determine the
type of a dGPU and the information returned is incorrect, we risk not
registering a device when we actually should, thus causing a regression.

Also, could you explain which of these types should be registered with
vga_switcheroo and which shouldn't?  Likewise, which of these can be
powered down by the platform and should thus use the vga_switcheroo
power domain?

	ATIF_PX_REMOVABLE_GRAPHICS_DEVICE
	ATIF_XGP_PORT
	ATIF_VGA_ENABLED_GRAPHICS_DEVICE
	ATIF_XGP_PORT_IN_DOCK

Thanks,

Lukas
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