Re: [PATCH 1/5] USB: typec: tcpm: Prevent log overflow by removing old entries

From: Guenter Roeck <linux@roeck-us.net>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: heikki.krogerus@linux.intel.com, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Badhri Jagan Sridharan <badhri@google.com>,
	Kyle Tso <kyletso@google.com>,
	Will McVicker <willmcvicker@google.com>
Subject: Re: [PATCH 1/5] USB: typec: tcpm: Prevent log overflow by removing old entries
Date: Thu, 10 Dec 2020 09:42:36 -0800	[thread overview]
Message-ID: <20201210174236.GB107395@roeck-us.net> (raw)
In-Reply-To: <20201210160521.3417426-2-gregkh@linuxfoundation.org>

On Thu, Dec 10, 2020 at 05:05:17PM +0100, Greg Kroah-Hartman wrote:
> From: Badhri Jagan Sridharan <badhri@google.com>
> 
> TCPM logs overflow once the logbuffer is full. Clear old entries and
> allow logging the newer ones as the newer would be more relevant to the
> issue being debugged.
> 
> Also, do not reset the logbuffer tail as end users might take back to
> back bugreports which would result in an empty buffer.
> 

Historically, the reason for not doing this was that, once a problem occurs,
the log would fill up quickly (typically with reconnect attempts), and the
actual reason for the problem would be overwritten. Maybe that reasoning
no longer applies; I just wanted to point out that there _was_ a reason for
not clearing old log entries.

Guenter

> Cc: Guenter Roeck <linux@roeck-us.net>
> Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Cc: Kyle Tso <kyletso@google.com>
> Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
> Signed-off-by: Will McVicker <willmcvicker@google.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  drivers/usb/typec/tcpm/tcpm.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index cedc6cf82d61..0ceeab50ed64 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -470,12 +470,6 @@ static bool tcpm_port_is_disconnected(struct tcpm_port *port)
>  
>  #ifdef CONFIG_DEBUG_FS
>  
> -static bool tcpm_log_full(struct tcpm_port *port)
> -{
> -	return port->logbuffer_tail ==
> -		(port->logbuffer_head + 1) % LOG_BUFFER_ENTRIES;
> -}
> -
>  __printf(2, 0)
>  static void _tcpm_log(struct tcpm_port *port, const char *fmt, va_list args)
>  {
> @@ -495,11 +489,6 @@ static void _tcpm_log(struct tcpm_port *port, const char *fmt, va_list args)
>  
>  	vsnprintf(tmpbuffer, sizeof(tmpbuffer), fmt, args);
>  
> -	if (tcpm_log_full(port)) {
> -		port->logbuffer_head = max(port->logbuffer_head - 1, 0);
> -		strcpy(tmpbuffer, "overflow");
> -	}
> -
>  	if (port->logbuffer_head < 0 ||
>  	    port->logbuffer_head >= LOG_BUFFER_ENTRIES) {
>  		dev_warn(port->dev,
> @@ -519,6 +508,9 @@ static void _tcpm_log(struct tcpm_port *port, const char *fmt, va_list args)
>  		  (unsigned long)ts_nsec, rem_nsec / 1000,
>  		  tmpbuffer);
>  	port->logbuffer_head = (port->logbuffer_head + 1) % LOG_BUFFER_ENTRIES;
> +	if (port->logbuffer_head == port->logbuffer_tail)
> +		port->logbuffer_tail =
> +			(port->logbuffer_tail + 1) % LOG_BUFFER_ENTRIES;
>  
>  abort:
>  	mutex_unlock(&port->logbuffer_lock);
> @@ -622,8 +614,6 @@ static int tcpm_debug_show(struct seq_file *s, void *v)
>  		seq_printf(s, "%s\n", port->logbuffer[tail]);
>  		tail = (tail + 1) % LOG_BUFFER_ENTRIES;
>  	}
> -	if (!seq_has_overflowed(s))
> -		port->logbuffer_tail = tail;
>  	mutex_unlock(&port->logbuffer_lock);
>  
>  	return 0;
> -- 
> 2.29.2
> 