linux-kernel - Re: [PATCH 06/10] Revert "usb: typec: displayport: Receive DP Status Update NAK request exit dp altmode"

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Message-ID: <CAMMMRMctfOabMK3Rwfk_i+P0XsLECg5yOfhYO+9yqp-SFZrVEQ@mail.gmail.com>
Date: Tue, 17 Jun 2025 14:54:29 +0200
From: Andrei Kuchynski <akuchynski@...omium.org>
To: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc: Dmitry Baryshkov <lumag@...nel.org>, Abhishek Pandit-Subedi <abhishekpandit@...omium.org>, 
	Jameson Thies <jthies@...gle.com>, Benson Leung <bleung@...omium.org>, 
	Tzung-Bi Shih <tzungbi@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Guenter Roeck <groeck@...omium.org>, Pooja Katiyar <pooja.katiyar@...el.com>, 
	Badhri Jagan Sridharan <badhri@...gle.com>, RD Babiera <rdbabiera@...gle.com>, linux-usb@...r.kernel.org, 
	linux-kernel@...r.kernel.org, chrome-platform@...ts.linux.dev
Subject: Re: [PATCH 06/10] Revert "usb: typec: displayport: Receive DP Status
 Update NAK request exit dp altmode"

On Tue, Jun 17, 2025 at 10:54 AM Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
>
> Hi Andrei,
>
> On Mon, Jun 16, 2025 at 01:31:43PM +0000, Andrei Kuchynski wrote:
> > This reverts commit b4b38ffb38c91afd4dc387608db26f6fc34ed40b.
> >
> > The commit introduced a deadlock with the cros_ec_typec driver.
> > The deadlock occurs due to a recursive lock acquisition of
> > `cros_typec_altmode_work::mutex`.
> > The call chain is as follows:
> > 1. cros_typec_altmode_work() acquires the mutex
> > 2. typec_altmode_vdm() -> dp_altmode_vdm() ->
> > 3. typec_altmode_exit() -> cros_typec_altmode_exit()
> > 4. cros_typec_altmode_exit() attempts to acquire the mutex again
> >
> > This revert is considered safe as no other known driver sends back
> > DP_CMD_STATUS_UPDATE command with the NAK flag.
> >
> > Signed-off-by: Andrei Kuchynski <akuchynski@...omium.org>
> > ---
> >  drivers/usb/typec/altmodes/displayport.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
> > index b09b58d7311d..ac84a6d64c2f 100644
> > --- a/drivers/usb/typec/altmodes/displayport.c
> > +++ b/drivers/usb/typec/altmodes/displayport.c
> > @@ -393,10 +393,6 @@ static int dp_altmode_vdm(struct typec_altmode *alt,
> >               break;
> >       case CMDT_RSP_NAK:
> >               switch (cmd) {
> > -             case DP_CMD_STATUS_UPDATE:
> > -                     if (typec_altmode_exit(alt))
> > -                             dev_err(&dp->alt->dev, "Exit Mode Failed!\n");
> > -                     break;
>
> Commit b4b38ffb38c9 ("usb: typec: displayport: Receive DP Status
> Update NAK request exit dp altmode") addressed a very real problem
> with failure to execute data role swap. You are not really offering
> anything else for that issue here.

Thanks, I see the problem now. Reverting the patch is not feasible.

>
> Is it not an option to just schedule the mode exit here instead to
> solve the problem?

Of course, that's an option. Alternatively, maybe I could resolve the
deadlock within the `cros_ec_typec` driver. Regardless, this seems like
a separate patch.

>
> diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
> index b09b58d7311d..2abbe4de3216 100644
> --- a/drivers/usb/typec/altmodes/displayport.c
> +++ b/drivers/usb/typec/altmodes/displayport.c
> @@ -394,8 +394,7 @@ static int dp_altmode_vdm(struct typec_altmode *alt,
>         case CMDT_RSP_NAK:
>                 switch (cmd) {
>                 case DP_CMD_STATUS_UPDATE:
> -                       if (typec_altmode_exit(alt))
> -                               dev_err(&dp->alt->dev, "Exit Mode Failed!\n");
> +                       dp->state = DP_STATE_EXIT;
>                         break;
>                 case DP_CMD_CONFIGURE:
>                         dp->data.conf = 0;
>
>
> --
> heikki