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: <aFEtT5JAaGQIs-vG@kuha.fi.intel.com>
Date: Tue, 17 Jun 2025 11:54:39 +0300
From: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To: Andrei Kuchynski <akuchynski@...omium.org>
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"

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.

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

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