linux-kernel - Re: [PATCH 3/3] Input: applespi: Add trace

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Message-ID: <YC2FUwOdIoKKg1Ew@google.com>
Date:   Wed, 17 Feb 2021 13:06:27 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     "Life is hard, and then you die" <ronald@...ovation.ch>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Sergiu Cuciurean <sergiu.cuciurean@...log.com>,
        Lee Jones <lee.jones@...aro.org>, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] Input: applespi: Add trace_event module param for
 early tracing.

On Wed, Feb 17, 2021 at 12:52:57PM -0800, Life is hard, and then you die wrote:
> 
>   Hi Dmitry,
> 
> On Wed, Feb 17, 2021 at 12:26:18PM -0800, Dmitry Torokhov wrote:
> > 
> > On Wed, Feb 17, 2021 at 11:07:18AM -0800, Ronald Tschalär wrote:
> > > The problem is that tracing can't be set via sysfs until the module is
> > > loaded, at which point the keyboard and trackpad initialization commands
> > > have already been run and hence tracing can't be used to debug problems
> > > here.
> > > 
> > > Adding this param allows tracing to be enabled for messages sent and
> > > received during module probing. It takes comma-separated list of events,
> > > e.g.
> > > 
> > >   trace_event=applespi_tp_ini_cmd,applespi_bad_crc
> > 
> > You can unbind and rebind a device to a driver via sysfs as many times
> > as needed (see bind and unbind driver sysfs attributes), so I believe
> 
> Hmm, I'm going to have to play with that a bit, but one place it still
> won't help I think is something we ran into in practise: init was
> failing during boot, but was successfull later on.

Maybe compiling module as a built-in and then using kernel command line
option to initiate the trace would work?

If this facility is really needed, it would be beneficial for other
modules as well, and thus better implemented in the module loading code
to activate desired tracing after loading the module but before invoking
module init code.

Thanks.

-- 
Dmitry