Re: [PATCH v3 03/12] PCI: Move check for old Apple Thunderbolt controllers into a quirk

From: Lukas Wunner <lukas@wunner.de>
To: Mario Limonciello <mario.limonciello@amd.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	"open list:PCI SUBSYSTEM" <linux-pci@vger.kernel.org>,
	"open list:THUNDERBOLT DRIVER" <linux-usb@vger.kernel.org>,
	"open list:RADEON and AMDGPU DRM DRIVERS" 
	<amd-gfx@lists.freedesktop.org>,
	"open list:DRM DRIVERS" <dri-devel@lists.freedesktop.org>,
	"open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" 
	<nouveau@lists.freedesktop.org>,
	Andreas Noever <andreas.noever@gmail.com>,
	Michael Jamet <michael.jamet@intel.com>,
	Yehezkel Bernat <YehezkelShB@gmail.com>,
	Alexander.Deucher@amd.com, Hans de Goede <hdegoede@redhat.com>
Subject: Re: [PATCH v3 03/12] PCI: Move check for old Apple Thunderbolt controllers into a quirk
Date: Sun, 13 Feb 2022 10:19:20 +0100	[thread overview]
Message-ID: <20220213091920.GA15535@wunner.de> (raw)
In-Reply-To: <20220211193250.1904843-4-mario.limonciello@amd.com>

On Fri, Feb 11, 2022 at 01:32:41PM -0600, Mario Limonciello wrote:
> `pci_bridge_d3_possible` currently checks explicitly for a Thunderbolt
> controller to indicate that D3 is possible.  As this is used solely
> for older Apple systems, move it into a quirk that enumerates across
> all Intel TBT controllers.

I'm not so sure if it is only needed on Apple systems.

> @@ -2954,10 +2960,6 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
>  		if (pci_bridge_d3_force)
>  			return true;
>  
> -		/* Even the oldest 2010 Thunderbolt controller supports D3. */
> -		if (bridge->is_thunderbolt)
> -			return true;
> -
>  		/* Platform might know better if the bridge supports D3 */
>  		if (platform_pci_bridge_d3(bridge))
>  			return true;

The fact that Thunderbolt PCIe ports support D3 is a property of those
devices.  It's not a property of the platform or a quirk of a particular
vendor.

Hence in my view the current location of the check (pci_bridge_d3_possible())
makes sense wheras the location you're moving it to does not.

> +/* Apple machines as old as 2010 can do D3 with Thunderbolt controllers, but don't specify
> + * it in the ACPI tables
> + */

Apple started shipping Thunderbolt in 2011.
Intel brought the first chips to market in 2010.

The date is meaningful at the code's current location in
pci_bridge_d3_possible() because a few lines further down
there's a 2015 BIOS cut-off date.

Microsoft came up with an ACPI property that BIOS vendors may set
so that Windows knows it may put a Thunderbolt controller into D3cold.
I'm not even sure if that property was ever officially adopted by the
ACPI spec or if it's just a Microsoft-defined "standard".

Apple had been using its own scheme to put Thunderbolt controllers
into D3cold when nothing is plugged in, about a decade before Microsoft
defined the ACPI property.

I'm not sure if other vendors came up with their own schemes to
power-manage Thunderbolt.  We may regress those with the present
patch.

Thanks,

Lukas