linux-kernel - Re: [PATCH] usb: typec: tcpm: always rediscover when swapping DR

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Date:   Wed, 18 Aug 2021 16:02:24 +0800
From:   Kyle Tso <kyletso@...gle.com>
To:     Guenter Roeck <linux@...ck-us.net>, Icenowy Zheng <icenowy@...c.io>
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Badhri Jagan Sridharan <badhri@...gle.com>
Subject: Re: [PATCH] usb: typec: tcpm: always rediscover when swapping DR

On Tue, Aug 17, 2021 at 11:13 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On 8/17/21 2:36 AM, Heikki Krogerus wrote:
> > On Fri, Aug 13, 2021 at 12:31:31PM +0800, Icenowy Zheng wrote:
> >> Currently, TCPM code omits discover when swapping to gadget, and assume
> >> that no altmodes are available when swapping from gadget. However, we do
> >> send discover when we get attached as gadget -- this leads to modes to be
> >> discovered twice when attached as gadget and then swap to host.
> >>
> >> Always re-send discover when swapping DR, regardless of what change is
> >> being made; and because of this, the assumption that no altmodes are
> >> registered with gadget role is broken, and altmodes de-registeration is
> >> always needed now.
> >>
> >> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
> >> ---
> >>   drivers/usb/typec/tcpm/tcpm.c | 9 ++++-----
> >>   1 file changed, 4 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> >> index b9bb63d749ec..ab6d0d51ee1c 100644
> >> --- a/drivers/usb/typec/tcpm/tcpm.c
> >> +++ b/drivers/usb/typec/tcpm/tcpm.c
> >> @@ -4495,15 +4495,14 @@ static void run_state_machine(struct tcpm_port *port)
> >>              tcpm_set_state(port, ready_state(port), 0);
> >>              break;
> >>      case DR_SWAP_CHANGE_DR:
> >> -            if (port->data_role == TYPEC_HOST) {
> >> -                    tcpm_unregister_altmodes(port);
> >> +            tcpm_unregister_altmodes(port);
> >> +            if (port->data_role == TYPEC_HOST)
> >>                      tcpm_set_roles(port, true, port->pwr_role,
> >>                                     TYPEC_DEVICE);
> >> -            } else {
> >> +            else
> >>                      tcpm_set_roles(port, true, port->pwr_role,
> >>                                     TYPEC_HOST);
> >> -                    port->send_discover = true;
> >> -            }
> >> +            port->send_discover = true;
> >>              tcpm_ams_finish(port);
> >>              tcpm_set_state(port, ready_state(port), 0);
> >>              break;
> >
> > Why is it necessary to do discovery with data role swap in general?
> >
> > thanks,
> >
>
> Additional question: There are two patches pending related to DR_SWAP
> and discovery. Are they both needed, or do they both solve the same
> problem ?
>
> Thanks,
> Guenter

Hi, I just noticed this patch.

Part of this patch and part of my patch
https://lore.kernel.org/r/20210816075449.2236547-1-kyletso@google.com
are to solve the same problem that Discover_Identity is not sent in a
case where the port becomes UFP after DR_SWAP while in PD3.

The difference (for the DR_SWAP part) is that my patch does not set
the flag "send_discover" if the port becomes UFP after PD2 DR_SWAP.
That is because in PD2 Spec, UFP is not allowed to be the SVDM
Initiator.

This patch indeed solves another problem where
tcpm_unregister_altmodes should be called during PD3 DR_SWAP because
the port partner may return mode data in the latest Discover_Mode. For
the PD2 case, I don't think it needs to be called because PD2 DFP will
always return NAK for Discover_Mode. However it is fine because it is
safe to call tcpm_unregister_altmodes even if there is no mode data.

In fact, when I was tracing the code I found another bug. PD2 UFP is
not allowed to send Discover_Identity and Discover_Mode. I can send
another patch to address this problem.

thanks,
Kyle