RE: [PATCH 0/2] Mark USB4 controllers as is_thunderbolt

From: "Deucher, Alexander" <Alexander.Deucher@amd.com>
To: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: "Limonciello, Mario" <Mario.Limonciello@amd.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Andreas Noever <andreas.noever@gmail.com>,
	"open list:PCI SUBSYSTEM" <linux-pci@vger.kernel.org>,
	"open list:THUNDERBOLT DRIVER" <linux-usb@vger.kernel.org>,
	Michael Jamet <michael.jamet@intel.com>,
	Yehezkel Bernat <YehezkelShB@gmail.com>
Subject: RE: [PATCH 0/2] Mark USB4 controllers as is_thunderbolt
Date: Mon, 7 Feb 2022 15:52:13 +0000	[thread overview]
Message-ID: <BL1PR12MB51442184B407A747E4C487F1F72C9@BL1PR12MB5144.namprd12.prod.outlook.com> (raw)
In-Reply-To: <YgE+g9JOJE6foThi@lahna>

[Public]

> -----Original Message-----
> From: Mika Westerberg <mika.westerberg@linux.intel.com>
> Sent: Monday, February 7, 2022 10:45 AM
> To: Deucher, Alexander <Alexander.Deucher@amd.com>
> Cc: Limonciello, Mario <Mario.Limonciello@amd.com>; Bjorn Helgaas
> <bhelgaas@google.com>; Andreas Noever <andreas.noever@gmail.com>;
> open list:PCI SUBSYSTEM <linux-pci@vger.kernel.org>; open
> list:THUNDERBOLT DRIVER <linux-usb@vger.kernel.org>; Michael Jamet
> <michael.jamet@intel.com>; Yehezkel Bernat <YehezkelShB@gmail.com>
> Subject: Re: [PATCH 0/2] Mark USB4 controllers as is_thunderbolt
> 
> Hi,
> 
> On Mon, Feb 07, 2022 at 03:00:19PM +0000, Deucher, Alexander wrote:
> > [Public]
> >
> > > -----Original Message-----
> > > From: Mika Westerberg <mika.westerberg@linux.intel.com>
> > > Sent: Monday, February 7, 2022 1:42 AM
> > > To: Limonciello, Mario <Mario.Limonciello@amd.com>
> > > Cc: Bjorn Helgaas <bhelgaas@google.com>; Andreas Noever
> > > <andreas.noever@gmail.com>; open list:PCI SUBSYSTEM <linux-
> > > pci@vger.kernel.org>; open list:THUNDERBOLT DRIVER <linux-
> > > usb@vger.kernel.org>; Michael Jamet <michael.jamet@intel.com>;
> > > Yehezkel Bernat <YehezkelShB@gmail.com>; Deucher, Alexander
> > > <Alexander.Deucher@amd.com>
> > > Subject: Re: [PATCH 0/2] Mark USB4 controllers as is_thunderbolt
> > >
> > > Hi Mario,
> > >
> > > On Fri, Feb 04, 2022 at 12:28:18PM -0600, Mario Limonciello wrote:
> > > > Various drivers in the kernel use `pci_is_thunderbolt_attached` to
> > > > designate behaving differently from a device that is internally in
> > > > the machine. This function relies upon the `is_thunderbolt`
> > > > designation which checks for a specific capability only set on Intel
> controllers.
> > > >
> > > > Non-Intel USB4 designs should also match this designation so that
> > > > they can be treated the same regardless of the host they're connected
> to.
> > >
> > > Not objecting this if really needed but since USB4 is supposed to be
> > > transparent to the native (tunneled) protocol, I would rather try to
> > > figure out if there is really need to change driver behaviour
> > > whether it is connected over USB4 or plugged natively on the PCIe slot.
> > >
> > > Can you elaborate a bit what kind of functionality needs this?
> > > Perhaps we can figure a better alternative?
> >
> > In the AMD GPU driver we use it to determine which dGPU is built into
> > a platform vs. externally connected since the internal one uses ACPI
> > for certain things and the external one does not.  There are probably
> > other ways to determine this, but it's not in place at the moment.
> 
> Can't you check if the device is behind a hotplug bridge? Then for certain it is
> "removable".

Unfortunately, most (all?) of the root ports on modern platforms are hotplug capable even if the hardware is soldered to the board.

> 
> The other option is to look for ACPI companion (ACPI_COMPANION()) of the
> device. AFAICT dGPUs don't have one (as the BIOS does not know in advance
> what will be connected to the hotplug ports) whereas internal does typically
> have one.

Yeah, this is probably the right way to do this.

Alex