linux-kernel - Re: [PATCH 3/4] platform/chrome: cros_ec_typec: Configure muxes at start of port update

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Message-ID: <YgIBv2SQdwXm7RLt@google.com>
Date:   Tue, 8 Feb 2022 13:38:07 +0800
From:   Tzung-Bi Shih <tzungbi@...gle.com>
To:     Prashant Malani <pmalani@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Benson Leung <bleung@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        "open list:CHROMEOS EC USB TYPE-C DRIVER" 
        <chrome-platform@...ts.linux.dev>
Subject: Re: [PATCH 3/4] platform/chrome: cros_ec_typec: Configure muxes at
 start of port update

On Mon, Feb 07, 2022 at 09:40:28PM +0000, Prashant Malani wrote:
> There are situations where the mux state reported by the Embedded
> Controller (EC), might lag the partner "connected" state. So, the mux
> state might still suggest that a partner is connected, while the PD
> "connected" state, being in Try.SNK (for example) suggests that the
> partner is disconnected.
> 
> In such a scenario, we will end up sending a disconnect command to the
> mux driver, followed by a connect command, since the mux is configured
> later. Avoid this by configuring the mux before
> registering/disconnecting a partner.

I failed to understand the description.  It looks like some protocol details.
Could you provide some brief explanation in the commit message?

On a related note, followed up the example scenario, which one of the
understanding is the most applicable:
1) The disconnect followed by a connect is suboptimal.  The patch cleans it.
2) The disconnect followed by a connect is a bug.  The patch fixes it.

> @@ -965,6 +965,11 @@ static int cros_typec_port_update(struct cros_typec_data *typec, int port_num)
>  	if (ret < 0)
>  		return ret;
>  
> +	/* Update the switches if they exist, according to requested state */
> +	ret = cros_typec_configure_mux(typec, port_num, &resp);
> +	if (ret)
> +		dev_warn(typec->dev, "Configure muxes failed, err = %d\n", ret);

It used the fact that the function returns `ret` at the end.  After the move,
the block is no longer the last thing before function returns.

Does it make more sense to return earlier if cros_typec_configure_mux() fails?
Does the rest of code need to be executed even if cros_typec_configure_mux()
fails?

> @@ -980,11 +985,6 @@ static int cros_typec_port_update(struct cros_typec_data *typec, int port_num)
>  	if (typec->typec_cmd_supported)
>  		cros_typec_handle_status(typec, port_num);
>  
> -	/* Update the switches if they exist, according to requested state */
> -	ret = cros_typec_configure_mux(typec, port_num, &resp);
> -	if (ret)
> -		dev_warn(typec->dev, "Configure muxes failed, err = %d\n", ret);
> -
>  	return ret;

If the function decides to return earlier, it can be `return 0;`.