linux-kernel - Re: [PATCH v2 6/8] platform/chrome: cros_ec_typec: Use Thunderbolt 3 cable discover mode VDO in USB4 mode

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Date:   Tue, 17 Nov 2020 17:06:18 -0800
From:   Prashant Malani <pmalani@...omium.org>
To:     Utkarsh Patel <utkarsh.h.patel@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:USB NETWORKING DRIVERS" <linux-usb@...r.kernel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Rajmohan Mani <rajmohan.mani@...el.com>,
        Azhar Shaikh <azhar.shaikh@...el.com>
Subject: Re: [PATCH v2 6/8] platform/chrome: cros_ec_typec: Use Thunderbolt 3
 cable discover mode VDO in USB4 mode



On Tue, Nov 17, 2020 at 10:19 AM Prashant Malani <pmalani@...omium.org> wrote:
>
> Hi Utkarsh,
>
> On Fri, Nov 13, 2020 at 12:25:01PM -0800, Utkarsh Patel wrote:
> > Configure Thunderbolt3/USB4 cable generation value by filing Thunderbolt 3
> > cable discover mode VDO to support rounded and non-rounded Thunderbolt3/
> > USB4 cables.
> > While we are here use Thunderbolt 3 cable discover mode VDO to fill active
> > cable plug link training value.
> >
> > Suggested-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > Signed-off-by: Utkarsh Patel <utkarsh.h.patel@...el.com>
> >
> > --
> > Changes in v2:
> > - No change.
> > --
> > ---
> >  drivers/platform/chrome/cros_ec_typec.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/platform/chrome/cros_ec_typec.c b/drivers/platform/chrome/cros_ec_typec.c
> > index 8111ed1fc574..b7416e82c3b3 100644
> > --- a/drivers/platform/chrome/cros_ec_typec.c
> > +++ b/drivers/platform/chrome/cros_ec_typec.c
> > @@ -514,8 +514,18 @@ static int cros_typec_enable_usb4(struct cros_typec_data *typec,
> >       else if (pd_ctrl->control_flags & USB_PD_CTRL_ACTIVE_CABLE)
> >               data.eudo |= EUDO_CABLE_TYPE_RE_TIMER << EUDO_CABLE_TYPE_SHIFT;
> >
> > -     data.active_link_training = !!(pd_ctrl->control_flags &
> > -                                    USB_PD_CTRL_ACTIVE_LINK_UNIDIR);
> > +     /*
> > +      * This driver does not have access to the identity information or
> > +      * capabilities of the cable, so we don't know is it a real USB4 or
> > +      * TBT3 cable. Therefore pretending that it's always TBT3 cable by
> > +      * filling the TBT3 Cable VDO.
> > +      */
> > +     data.tbt_cable_vdo = TBT_MODE;
>
> Is it safe to be making this assumption unconditionally? It might work for
> Intel Mux agent but is it guaranteed to be safe for any other future
> mux implementation? In other words, what if a "true" USB4 cable is
> connected which doesn't have the Thunderbolt SVID alt mode?

I dug into this a bit more and can maybe articulate my concern better:

Is there a situation where both of the following are true ? :
- Cable type = EUDO_CABLE_TYPE_OPTICAL or EUDO_CABLE_TYPE_RE_TIMER
- No TBT_CABLE_LINK_TRAINING or TBT_CABLE_ROUNDED_SUPPORT defined (both
  these are 0).

If both the above are true, then in Patch 7/8, wouldn't we never hit the
else condition (labeled "Active USB cable") and therefore not set the
mode_data correctly?

>
> (Pre-fetching some alternatives in case the answer is no)
>
> You might want to check with the Cros EC team if you can repurpose a bit of
> the "reserved" field for specifying whether the cable is TBT or not.
>
> Either that or see if there is a way to determine from the pd_ctrl->cable_speed
> whether the cable is actually TBT or not.

It seems link cable_gen and USB_PD_CTRL_ACTIVE_LINK_UNIDIR are
reasonable proxies for whether the cable has TBT support, so perhaps
we should only set tbt_cable_vdo = TBT_MODE if either of those are
non-zero?

WDYT?

Best regards,

-Prashant