linux-kernel - Re: Another Obsolete Fix me in trace.h?

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Date:	Mon, 24 Nov 2014 22:49:40 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	nick <xerofoify@...il.com>, gleb@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Another Obsolete Fix me in trace.h?

2014-11-24 16:19-0500, Steven Rostedt:
> On Mon, 24 Nov 2014 22:00:01 +0100
> Radim Krčmář <rkrcmar@...hat.com> wrote:
> 
> > 2014-11-24 11:40+0100, Jan Kiszka:
> > The format string has to be a string literal[1]; we could change it to
> > allow expressions[2], but what we want is almost possible through a
> > direct call to trace_seq_printf()[3].
> > 
> > The raw result would look like
> > 
> >   #define __print(fmt, args...) ({ \
> >   	const char *buf_start = trace_seq_buffer_ptr(p); \
> >   	trace_seq_printf(p, fmt, args); \
> >   	trace_seq_putc(p, '\0'); \
> >   	buf_start; \
> >   	})
> > 
> >   TP_printk("%s%s", [...],
> >             __entry->has_error ? __print("(0x%x)", __entry->error_code) : "")
> > 
> > and would be acceptable if something __print-like made it into a ftrace
> > helper[4].  (Userspace won't be able to nicely print it otherwise.)
> 
> You mean if we add something like a __print_conditional(cond, fmt, ...);

The benefit of _conditional is cleaner code?

(_conditional would be possible as a #define on top of generic print,
 the ternary seems to be parsed correctly.)

> For this case you would have:
> 
>   TP_printk("%s%s", [...],
> 	__print_conditional(__entry->has_error, " (0x%x)", __entry->error_code));
> 
> Where __print_conditional() will return "" when "cond" is false, and
> will return the formatted string otherwise.

(This might introduce 'const char empty[] = ""'.)

> That wouldn't be too hard to implement.

I'll look at the patch tommorrow.

Thanks.
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