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

[Guenter Roeck]

On 4/10/23 13:57, Badhri Jagan Sridharan wrote:
> 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.

I have no strong opinion. As I said, maybe conditions have changed.
When I wrote the code, failure conditions happened all the time, and
reset loops overwrote the log repeatedly. I didn't implement the current
code because I was lazy or something, it was because wrap-around
did not work for me. Again, maybe the situation has changed.
And, to follow-up on Greg's question, maybe switching to tracing
instead of re-implementing it would make sense nowadays. I simply
don't know. I'll accept whatever solution you and Greg can agree on.

Thanks,
Guenter