Re: [RFC PATCH 17/22] thunderbolt: Add initial support for USB4

[Mika Westerberg]

On Tue, Oct 01, 2019 at 02:59:06PM +0000, Mario.Limonciello@xxxxxxxx wrote:
> 
> 
> > -----Original Message-----
> > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Sent: Tuesday, October 1, 2019 9:54 AM
> > To: Mika Westerberg
> > Cc: linux-usb@xxxxxxxxxxxxxxx; Andreas Noever; Michael Jamet; Yehezkel Bernat;
> > Rajmohan Mani; Nicholas Johnson; Lukas Wunner; Alan Stern; Limonciello, Mario;
> > Anthony Wong; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [RFC PATCH 17/22] thunderbolt: Add initial support for USB4
> > 
> > 
> > [EXTERNAL EMAIL]
> > 
> > On Tue, Oct 01, 2019 at 04:09:05PM +0300, Mika Westerberg wrote:
> > > On Tue, Oct 01, 2019 at 02:47:48PM +0200, Greg Kroah-Hartman wrote:
> > > > > -	  Thunderbolt Controller driver. This driver is required if you
> > > > > -	  want to hotplug Thunderbolt devices on Apple hardware or on PCs
> > > > > -	  with Intel Falcon Ridge or newer.
> > > > > +	  USB4 (Thunderbolt) driver. USB4 is the public spec based on
> > > > > +	  Thunderbolt 3 protocol. This driver is required if you want to
> > > > > +	  hotplug Thunderbolt and USB4 compliant devices on Apple
> > > > > +	  hardware or on PCs with Intel Falcon Ridge or newer.
> > > >
> > > > Wait, did "old" thunderbolt just get re-branded as USB4?
> > >
> > > Not but the driver started supporting USB4 as well :)
> > >
> > > USB4 is pretty much public spec of Thunderbolt 3 but with some
> > > differences in register layouts (this is because Thunderbolt uses some
> > > vendor specific capabilities which are now moved to more "standard"
> > > places).
> > 
> > Ok, then we need to rename the Kconfig option as well, otherwise no one
> > will "know" that this changed, so they will not be prompted for it.
> > 
> > > > Because if I have an "old" laptop that needs Thunderbolt support, how am
> > > > I going to know it is now called USB4 instead?
> > >
> > > Well the Kconfig option tries to have both names there:
> > >
> > >   tristate "USB4 (Thunderbolt) support"
> > >
> > > and then
> > >
> > >   USB4 (Thunderbolt) driver. USB4 is the public spec based on
> > >   Thunderbolt 3 protocol. This driver is required if you want to hotplug
> > >   Thunderbolt and USB4 compliant devices on Apple hardware or on PCs
> > >   with Intel Falcon Ridge or newer.
> > >
> > > and the Kconfig option is still CONFIG_THUNDERBOLT. I know this is
> > > confusing but I don't have better ideas how we can advertise both. I
> > > borrowed this "format" from firewire.
> > 
> > CONFIG_USB4 instead?
> 
> How about CONFIG_USB4_PCIE?
> 
> I think that will help align that certain aspects of USB4 can be built optionally.

But this is not about PCIe - it is just one type of a tunnel that is
optional with USB4.

> > > > Shouldn't there just be a new USB4 option that only enables/builds the
> > > > USB4 stuff if selected?  Why would I want all of this additional code on
> > > > my old system if it's not going to do anything at all?
> > >
> > > USB4 devices are backward compatible with Thunderbolt 3 so you should be
> > > able to plug in USB4 device to your old Thunderbolt 3 laptop for
> > > example. It goes the other way as well. Some things are optional but for
> > > example USB4 hubs must support also Thunderbolt 3.
> > >
> 
> If PCIe tunnels are an optional feature in USB4, how can it be mandatory to support
> Thunderbolt 3?

It is mandatory for USB4 hubs. For peripheral devices and hosts
Thunderbolt 3 support is optional. So for example you could have USB4
host that does not enter Thunderbolt 3 alternate mode at all so it only
supports USB4 devices.