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

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Date:   Thu, 10 Dec 2020 11:10:27 -0800
From:   Badhri Jagan Sridharan <badhri@...gle.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kyle Tso <kyletso@...gle.com>,
        Will McVicker <willmcvicker@...gle.com>
Subject: Re: [PATCH 1/5] USB: typec: tcpm: Prevent log overflow by removing
 old entries

Hi Guenter,

While I agree with what you are saying, since the logbuffer does not
have the intelligence to drop older entries where no issues were seen,
logbuffer gets full pretty quickly with good instances and there is no
space left to log the bad instance. Should wrapping this in a config
option be a better way to go about this  ? When the config optioin is
set, old entries will be dropped.
Please let me know, I can update the patch and resend.

Thanks,
Badhri



On Thu, Dec 10, 2020 at 9:53 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On Thu, Dec 10, 2020 at 05:05:17PM +0100, Greg Kroah-Hartman wrote:
> > From: Badhri Jagan Sridharan <badhri@...gle.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@...ck-us.net>
> > Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > Cc: Kyle Tso <kyletso@...gle.com>
> > Signed-off-by: Badhri Jagan Sridharan <badhri@...gle.com>
> > Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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
> >