Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability

From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mathias Nyman <mathias.nyman@linux.intel.com>,
	tglx@linutronix.de, linux-usb@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org, Michal Kubecek <mkubecek@suse.cz>
Subject: Re: [PATCH v8 2/5] usb: early: add driver for xhci debug capability
Date: Thu, 1 Jun 2017 20:47:35 -0400	[thread overview]
Message-ID: <20170601204735.4e1399eb@vmware.local.home> (raw)
In-Reply-To: <20170601122551.3x6p3g2jofvwi7gk@hirez.programming.kicks-ass.net>

On Thu, 1 Jun 2017 14:25:51 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Jun 01, 2017 at 10:15:24AM +0200, Vlastimil Babka wrote:
> > Thanks. I didn't make it clear that the trace_printk() warning is
> > there even if the code using it doesn't actually execute (i.e. I
> > didn't specify any early_printk bootparam). There are some roastedy
> > tricks to detect the potential users, so that the buffers can be
> > allocated in advance to allow the first trace_printk() from any
> > context, I guess.
> > 
> > I'm not sure if there's a way to change it so that your driver
> > reports the trace_printk usage only in response to the bootparam
> > (which could also be a safe point to allocate ftrace buffers?).  
> 
> No, nor do we want to. There should not be a single caller to
> trace_printk() in normal kernels.

Correct. If you find a trace_printk() that would be useful in a
production environment, then make it a tracepoint.

That's why I have that nasty banner, to make sure trace_printk()s are
not the "quick way" to add tracepoint hacks.

-- Steve