[GTALUG] USB-C/3.1 Video and Linux

Thu Mar 29 11:17:58 EDT 2018

On 24 March 2018 at 19:07, Russell <rreiter91 at gmail.com> wrote:

> On March 24, 2018 9:53:59 AM EDT, Russell Reiter <rreiter91 at gmail.com>
> wrote:
> >On 24 March 2018 at 08:08, Giles Orr via talk <talk at gtalug.org> wrote:
> >> On 23 March 2018 at 20:01, Scott Sullivan via talk <talk at gtalug.org>
> >wrote:
> >>>
> >>> On 2018-03-23 12:50 PM, Giles Orr via talk wrote:
> >>>>
> >>>> A couple days ago I got a Best Buy flyer, and they have this item:
> >>>>
> >>>>
> >>>>
> >https://www.bestbuy.ca/en-ca/product/asus-zenscreen-15-6-fh
> d-60hz-5ms-gtg-ips-lcd-monitor-mb16ac-dark-grey/10737845.aspx
> >>>>
> ><https://www.bestbuy.ca/en-ca/product/asus-zenscreen-15-6-f
> hd-60hz-5ms-gtg-ips-lcd-monitor-mb16ac-dark-grey/10737845.aspx>?
> >>>>
> >>>> It's a portable IPS LCD screen from Asus.  I have a similar item
> >from
> >>>> about three years ago that has two connectors, one for USB
> >power-only and
> >>>> the other for VGA/HDMI.  There were other models at the time that
> >used only
> >>>> one connector, USB for both data and power - but that required a
> >driver (and
> >>>> would have been painfully slow if it was USB2).
> >>>>
> >>>> But now here's this new model: it's one connector only, USB-C.  And
> >>>> presumably USB3.1.  And USB3.1 can carry video.  But at this point
> >I get
> >>>> lost among the standards and their implementations and which one
> >supports
> >>>> what.  I have an Asus Zenbook (it's Linux-only at this point) that
> >has a
> >>>> USB-C connector, but I suspect it's USB3.0.  So the first question
> >is: how
> >>>> do I check that?  Second, does Linux support video-over-USB3.1?  Do
> >I need a
> >>>> special cable?  Is this "Thunderbolt?"  How can I check if my
> >machine would
> >>>> support this?
> >>>
> >>>
> >>> The Spec you linked say it's DisplayLink, which is the software
> >based
> >>> video solution that requires a driver.
> >>>
> >>> https://en.wikipedia.org/wiki/DisplayLink
> >>>
> >>> USB-C is only a specification for a cable and connector, and
> >somewhat
> >>> agnostic about the data communications spec used across it. The
> >cables are
> >>> of high enough quality that they can support multiple data
> >communications
> >>> specs, generally referred to as alt-modes.
> >>>
> >>>
> >https://en.wikipedia.org/wiki/USB-C#Alternate_Mode_partner_specifications
> >>>
> >>> Thunderbolt 3 for example co-oped the USB-C cable spec for it's
> >cables,
> >>> much like in the previous generations it co-oped the
> >mini-displayport cable
> >>> spec.
> >>>
> >>> Intel makes a point that a port carrying Thunderbolt 3 should also
> >be
> >>> cross wired with USB 3.X and Display Port hardware to output those
> >signals
> >>> as well.
> >>>
> >>> So if you want a monitor that uses a Type-C connector, your looking
> >for a
> >>> display-port monitor. Which your not likely to find in the portable
> >market
> >>> yet.
> >>>
> >>
> >> Now that I knew what to look for, re-examination of the text and
> >pictures
> >> shows a couple interesting things: "USB Type-C connectivity with
> >hybrid
> >> signal solution for Type-C and Type-A source compatibility (requires
> >a
> >> DisplayLink driver for Type-A connection)."  So it's implying that it
> >> doesn't need a driver for Type-C.  And looking at the pictures,
> >there's a
> >> "DisplayPort" icon next to one of the buttons on the front.  So ...
> >>
> >> My other questions stand:
> >>
> >> - how can I figure out what USB protocol (3.0 or 3.1) the type-C
> >connector
> >> on my Linux computer supports?
> >
> >lspci -d ::0c03 -k  will display the vendors controller and kernel
> >driver in use.
> >
> >> - (clearer understanding leads to a clearer question here, although
> >I'm
> >> still not sure I'm saying it right) does Linux support DisplayPort
> >video
> >> over USB-C?
>
> Oops. No usb-c display context for this monitor. This test used the
> display link binary and a USB 2 adapter on Ubuntu LTS.
>
> https://www.phoronix.com/scan.php?page=article&item=asus-mb1
> 6ac-zenscreen&num=2
>
> >
> >I think the short answer is yes. If you can hard wire a DisplayPort to
> >HDMI adapter
> >cable you must surely be able to sink to the host connections logically
> >as well.
> >
> >Has somebody done this yet, I don't know.
> >
> >Since kernel 4.12 there has been development on a state machine for
> >hosting sinks.
> >
> >https://www.phoronix.com/scan.php?page=news_item&px=USB-Typ
> e-C-Promotion-Linux-4.15
> >
> >Power management and signalling are all represented, soft-jacking is
> >another issue.
> >
> >https://superuser.com/questions/1192638/whats-up-with-hdmi-
> and-displayport-over-usb-c
> >
> >*Alternate mode data*
> >
> >https://github.com/torvalds/linux/blob/master/drivers/usb/typec/tcpm.c
> >
> >HTH
>

Thanks Russell, Lennart.

I grabbed Fedora's very latest kernel this morning
(4.15.12-301.fc27.x86_64) given Russell's point that there's been a lot of
new feautures related to this.  After a reboot I ran 'lspci' - I have
USB3.0, not 3.1.  This is backed up by the Zenbook manual.  The manual
calls the port "USB Type-C/DisplayPort combo port" and says "Use a USB
Type-C adapter to connect your Notebook PC to an external display."  I
didn't find anything more specific than that, but it's promising.  It'll
probably be a long time before I test it though!

I haven't got my head around the spec enough to know: does the fact that
it's USB-C guarantee that it can deliver 20v and lots'o-watts to
accessories?  What about accepting that voltage from a charger?  I'd bet
that Asus hasn't bothered to set up that port to allow charging through it
as the computer also has a dedicated old-style charger/plug combo.

--
Giles
https://www.gilesorr.com/
gilesorr at gmail.com
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