linux-kernel - Re: [PATCH v2] PM: fix dynamic debug within pm_pr

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Date:   Mon, 21 Mar 2022 14:51:36 +0000
From:   David Cohen <dacohen@...me>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] PM: fix dynamic debug within pm_pr_debug()

On Mon, Mar 21, 2022 at 03:20:20PM +0100, Rafael J. Wysocki wrote:
> On Sat, Mar 19, 2022 at 2:54 AM David Cohen <dacohen@...me> wrote:
> >
> > On Thu, Mar 17, 2022 at 02:45:11PM +0100, Rafael J. Wysocki wrote:
> > > On Sat, Mar 12, 2022 at 5:37 AM David Cohen <dacohen@...me> wrote:
> > > >
> > > > Currently, pm_pr_debug() and pm_deferred_pr_debug() use __pm_pr_debug()
> > > > to filter pm debug messages based on pm_debug_messages_on flag.
> > > > According to __pm_pr_debug() implementation, pm_deferred_pr_debug()
> > > > indirectly calls printk_deferred() within __pm_pr_debug() which doesn't
> > > > support dynamic debug, but pm_pr_debug() indirectly calls pr_debug()
> > >
> > > I'm not sure what you mean by pm_pr_debug().  There's no such thing in
> > > the kernel tree.
> > >
> > > Assuming that it means pm_pr_dbg(), it doesn't call pr_debug():
> >
> > Yeah, I apologize for the typo. I meant pm_pr_dbg(). I can fix that if
> > you're ok with the patch as per comments below.
> >
> > >
> > > #define pm_pr_dbg(fmt, ...) __pm_pr_dbg(false, fmt, ##__VA_ARGS__)
> > >
> > > and
> > >
> > > void __pm_pr_dbg(bool defer, const char *fmt, ...)
> > > {
> > > ...
> > >         if (defer)
> > >                printk_deferred(KERN_DEBUG "PM: %pV", &vaf);
> > >         else
> > >                printk(KERN_DEBUG "PM: %pV", &vaf);
> > >
> > > And as I said printk(KERN_DEBUG ...) is not equivalent to
> > > pr_debug(...), because it is not dynamic printk().
> >
> > The problem is not about __pm_pr_dbg() calling printk(). The problem is
> > the pm files that used to call pr_debug() were modified to call
> > pm_pr_dbg() in order to be behing the pm_debug_messages_on flag, as per
> > this commit:
> > 8d8b2441db96 PM / sleep: Do not print debug messages by default
>
> So what's the problem with setting pm_debug_messages_on in addition to
> enabling dynamic debug for a given file?

Let me be a bit more detailed:

Before "8d8b2441db96 PM / sleep: Do not print debug messages by default":
- pr_debug() was used
- The kernel pm files had dynamic debug support
- All the instances using pr_debug() are visible on
  /sys/kernel/debug/dynamic_debug/control
- pm_debug_messages_on flag was not supported

After "8d8b2441db96 PM / sleep: Do not print debug messages by default":
- pr_debug() was replaced with pm_pr_dbg()
- The kernel pm files where pm_pr_dbg() replaced pr_debug() *lost* dynamic
  debug support and they are no longer visible on
  /sys/kernel/debug/dynamic_debug/control
- pm_debug_messages_on flag was introduced

What my patch is doing:
- Reintroducing dynamic debug support to the same files who lost it
  after the patch mentioned above
- The instances using pr_pm_dbg() (which originally came from pr_debug())
  are reintroduced to /sys/kernel/debug/dynamic_debug/control
- pm_debug_messages_on flag is unaltered

Regards, David