linux-kernel - Re: [PATCH v3 2/4] platform/chrome: cros_ec_typec: Use Thunderbolt 3 cable discover mode VDO in USB4 mode

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Message-ID: <CACeCKafuh57RjjKJBxh96nvMDinbJ-QXho_8Kfzbe_-vPROmRg@mail.gmail.com>
Date:   Thu, 19 Nov 2020 19:13:51 -0800
From:   Prashant Malani <pmalani@...omium.org>
To:     "Patel, Utkarsh H" <utkarsh.h.patel@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "heikki.krogerus@...ux.intel.com" <heikki.krogerus@...ux.intel.com>,
        "enric.balletbo@...labora.com" <enric.balletbo@...labora.com>,
        "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        "Shaikh, Azhar" <azhar.shaikh@...el.com>
Subject: Re: [PATCH v3 2/4] platform/chrome: cros_ec_typec: Use Thunderbolt 3
 cable discover mode VDO in USB4 mode

Hi Utkarsh,

On Thu, Nov 19, 2020 at 6:32 PM Patel, Utkarsh H
<utkarsh.h.patel@...el.com> wrote:
>
> Hi Prashant,
>
> > -----Original Message-----
> > From: Prashant Malani <pmalani@...omium.org>
> > Sent: Thursday, November 19, 2020 12:09 AM
> > To: Patel, Utkarsh H <utkarsh.h.patel@...el.com>
> > Cc: linux-kernel@...r.kernel.org; linux-usb@...r.kernel.org;
> > heikki.krogerus@...ux.intel.com; enric.balletbo@...labora.com; Mani,
> > Rajmohan <rajmohan.mani@...el.com>; Shaikh, Azhar
> > <azhar.shaikh@...el.com>
> > Subject: Re: [PATCH v3 2/4] platform/chrome: cros_ec_typec: Use Thunderbolt
> > 3 cable discover mode VDO in USB4 mode
> >
> > Hi Utkarsh,
> >
> > On Wed, Nov 18, 2020 at 10:32:09PM -0800, Utkarsh Patel wrote:
> > > Configure Thunderbolt 3 cable generation value by filling Thunderbolt
> > > 3 cable discover mode VDO to support rounded Thunderbolt 3 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 v3:
> > > - Added a check for cable's TBT support before filling TBT3 discover mode
> > >   VDO.
> > >
> > > 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..68b17ee1d1ae 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);
> > > +   /*
> > > +    * Filling TBT3 Cable VDO when TBT3 cable is being used to establish
> > > +    * USB4 connection.
> > > +    */
> > > +   if (pd_ctrl->cable_gen) {
> > > +           data.tbt_cable_vdo = TBT_MODE;
> > > +
> > > +           if (pd_ctrl->control_flags &
> > USB_PD_CTRL_ACTIVE_LINK_UNIDIR)
> > > +                   data.tbt_cable_vdo |= TBT_CABLE_LINK_TRAINING;
> > > +
> > > +           data.tbt_cable_vdo |= TBT_SET_CABLE_ROUNDED(pd_ctrl-
> > >cable_gen);
> > > +   }
> >
> > I think the following would decouple Rounded Support and Active Cable Link
> > Training?:
>
> Any reason you would want to decouple them?

Is there anything in the spec that says Active Cable Link Training
needs Rounded Cable support (or vice versa)?
If yes, could you kindly point me to the relevant portion in the spec
that states this?

If no, then the two should be set independently based on the response
from the Chrome EC.

FWIW, Table F-11 ( TBT3 Cable Discover Mode VDO Responses) from the
USB Type-C Cable & Connector Spec (Rel 2.0) suggests
the two are independent bits although I don't have access to the TBT3
spec to confirm.

BR,

-Prashant