Re: [PATCH v1] usb: typec: tcpm: Add kernel config to wrap around tcpm logs

[Badhri Jagan Sridharan]

On Mon, Apr 10, 2023 at 10:00 AM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On Mon, Apr 10, 2023 at 02:00:08AM -0700, Badhri Jagan Sridharan wrote:
> > On Mon, Apr 10, 2023 at 1:27 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Apr 10, 2023 at 01:08:55AM -0700, Badhri Jagan Sridharan wrote:
> > > > On Mon, Apr 10, 2023 at 12:45 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Apr 10, 2023 at 07:31:34AM +0000, Badhri Jagan Sridharan wrote:
> > > > > > This change adds CONFIG_TCPM_LOG_WRAPAROUND which when set allows the
> > > > > > logs to be wrapped around. Additionally, when set, does not clear
> > > > > > the TCPM logs when dumped.
> > > > > >
> > > > > > Signed-off-by: Badhri Jagan Sridharan <badhri@xxxxxxxxxx>
> > > > > > ---
> > > > > >  drivers/usb/typec/tcpm/Kconfig | 6 ++++++
> > > > > >  drivers/usb/typec/tcpm/tcpm.c  | 9 +++++++--
> > > > > >  2 files changed, 13 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/usb/typec/tcpm/Kconfig b/drivers/usb/typec/tcpm/Kconfig
> > > > > > index e6b88ca4a4b9..4dd2b594dfc9 100644
> > > > > > --- a/drivers/usb/typec/tcpm/Kconfig
> > > > > > +++ b/drivers/usb/typec/tcpm/Kconfig
> > > > > > @@ -18,6 +18,12 @@ config TYPEC_TCPCI
> > > > > >       help
> > > > > >         Type-C Port Controller driver for TCPCI-compliant controller.
> > > > > >
> > > > > > +config TCPM_LOG_WRAPAROUND
> > > > > > +     bool "Enable TCPM log wraparound"
> > > > > > +     help
> > > > > > +       When set, wraps around TCPM logs and does not clear the logs when dumped. TCPM logs by
> > > > > > +       default gets cleared when dumped and does not wraparound when full.
> > > > >
> > > > > Kconfig help text needs to be wrapped at the properly width.
> > > >
> > > > I assumed that the width is 100 characters, but it looks like it is
> > > > 80. Will fix it in the next version.
> > > > >
> > > > > And you do not provide any hint here as to why this is not the default
> > > > > option, or why someone would want this.
> > > >
> > > > "TCPM logs by default gets cleared when dumped and does not wraparound
> > > > when full." was intended
> > > > to convey why someone would want to set this. Perhaps it's not effective.
> > > >
> > > > Does the below look better:
> > > > "TCPM logs by default gets cleared when dumped and does not wraparound
> > > > when full. This can be overridden by setting this config.
> > > > When the config is set, TCPM wraps around logs and does not clear the
> > > > logs when dumped."
> > > >
> > > > Also, I could make this default if that's OK with Guenter.
> > > >
> > > > >
> > > > > So, why is this not just the default operation anyway?  Why would you
> > > > > ever want the logs cleared?
> > > >
> > > > I remember Guenter mentioning that he was finding it useful to not
> > > > wrap around the logs to fix problems
> > > > during tcpm_register_port (init sequence). IMHO wrapping around the
> > > > logs helps to triage interoperability
> > > > issues uncovered during testing. So both approaches have their own advantages.
> > >
> > > But as this is a build-time option, what would cause someone to choose
> > > one over the other, and then when the system is running, they can't
> > > change them?
> >
> > During initial phases of bringup, it makes sense to not wrap around
> > the logs, but, once bringup is done its most effective to wraparound
> > so that interop issues reported by the end users can be triaged where
> > TCPM logs are very effective.
>
> Not really, because the problem tends to be the remote device
> (or at least it used to be), not a driver under development.

Thanks for clarifying Guenter !
Right now if we DONT wrap around, once an issue is observed with a
remote device, the logs have to be cleared(if already full) and then
the issue has to be reproduced to collect the TCPM logbuffer
logsagain.

Having a log available _all_ the time, not just when explicitly
enabled is still very useful to catch hard to reproduce intertop
issues.

Given this would you be OK if I change the logic to wrap around always ?

IMHO based on what I have seen in the last couple of years, this would
also cover the boot with accessory connected as if the link gets into
a reset loop, the sequence after the reset reveals what had gone
wrong.

>
> > Also, without wrapping around, the logbuffer logs are completely stale
> > after the user goes through a few USB connect and disconnect cycles
> > till the system is rebooted.
>
> Unless they are cleared.

Ah yes. I forgot about that. Wrapping around would still make TCPM
logbuffer logs  more effective to debug issues with remote device.

Thanks,
Badhri





>
> Again, the premise is wrong here. The idea was to ensure that the
> beginning of a problem is caught and available in the log, not its tail.
> This includes "beginning" as the behavior immediately after boot regarding
> an already connected device, and the behavior observed when inserting
> a device into the running system. Again, in both cases it was most
> important to catch the beginning of a problem, not its tail.
>
> > If we don't want to pursue the Kconfig option, we should atleast make
> > TCPM default to wrapping the logs around when full so we could
> > maximise the use of the logbuffer contents.
> >
>
> I don't really agree, but then I am not in a position to argue either.
> Maybe the premise and reasons have changed since I wrote the driver.
>
> Guenter
>
> > >
> > > That does not seem good at all.
> > >
> > > Why not just use tracing instead of this odd custom log buffer?  That's
> > > a better solution overall, right?
> >
> > Tracing is not enabled by default in most systems. End users don't
> > want to retry and reproduce the failure to collect traces even if they
> > could reproduce it consistently.
> > So tracing was not proving handy here.
> >
> > Thanks,
> > Badhri
> >
> > >
> > > thanks,
> > >
> > > greg k-h