linux-kernel - Re: [PATCH 2/3] early_printk: Add force_early

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Date:   Tue, 29 Nov 2016 15:02:40 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] early_printk: Add force_early_printk kernel parameter

On Tue 2016-10-18 19:08:32, Peter Zijlstra wrote:
> Add add the 'force_early_printk' kernel parameter to override printk()
> and force it into early_printk(). This bypasses all the cruft and fail
> from printk() and makes things work again.

IMHO, the patch makes perfect sense and helps with debugging
hard problems.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1751,10 +1787,13 @@ asmlinkage int vprintk_emit(int facility
>  	static unsigned int logbuf_cpu = UINT_MAX;
>  
>  #ifdef CONFIG_KGDB_KDB
> -	if (unlikely(kdb_trap_printk)) {
> -		r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> -		return r;
> -	}
> +	if (unlikely(kdb_trap_printk))
> +		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> +#endif

Please, find below an updated patch that has also my Reviewed-by.

In particular, the above change was moved to the first patch
to fix bisectability. The result after applying both patches
is still exactly the same.


>From e1c00ae67d07767ec8e5bddb1113c2badf31f4bd Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@...e.com>
Date: Tue, 29 Nov 2016 13:32:56 +0100
Subject: [PATCH 2/3] early_printk: Add force_early_printk kernel parameter

Add add the 'force_early_printk' kernel parameter to override printk()
and force it into early_printk(). This bypasses all the cruft and fail
from printk() and makes things work again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
 kernel/printk/printk.c | 68 ++++++++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 21 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 541ce7705353..bb612e5c2e00 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -344,6 +344,42 @@ __packed __aligned(4)
 #endif
 ;
 
+#ifdef CONFIG_EARLY_PRINTK
+struct console *early_console;
+
+static bool __read_mostly force_early_printk;
+
+static int __init force_early_printk_setup(char *str)
+{
+	force_early_printk = true;
+	return 0;
+}
+early_param("force_early_printk", force_early_printk_setup);
+
+static int early_vprintk(const char *fmt, va_list args)
+{
+	char buf[512];
+	int n;
+
+	n = vscnprintf(buf, sizeof(buf), fmt, args);
+	early_console->write(early_console, buf, n);
+
+	return n;
+}
+
+asmlinkage __visible void early_printk(const char *fmt, ...)
+{
+	va_list ap;
+
+	if (!early_console)
+		return;
+
+	va_start(ap, fmt);
+	early_vprintk(fmt, ap);
+	va_end(ap);
+}
+#endif
+
 /*
  * The logbuf_lock protects kmsg buffer, indices, counters.  This can be taken
  * within the scheduler's rq lock. It must be released before calling
@@ -1786,6 +1822,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
 #endif
 
+#ifdef CONFIG_EARLY_PRINTK
+	if (force_early_printk && early_console)
+		return early_vprintk(fmt, args);
+#endif
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -1959,7 +2000,12 @@ asmlinkage __visible int printk(const char *fmt, ...)
 	int r;
 
 	va_start(args, fmt);
-	r = vprintk_func(fmt, args);
+#ifdef CONFIG_EARLY_PRINTK
+	if (force_early_printk && early_console)
+		r = vprintk_default(fmt, args);
+	else
+#endif
+		r = vprintk_func(fmt, args);
 	va_end(args);
 
 	return r;
@@ -2009,26 +2055,6 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
 
 #endif /* CONFIG_PRINTK */
 
-#ifdef CONFIG_EARLY_PRINTK
-struct console *early_console;
-
-asmlinkage __visible void early_printk(const char *fmt, ...)
-{
-	va_list ap;
-	char buf[512];
-	int n;
-
-	if (!early_console)
-		return;
-
-	va_start(ap, fmt);
-	n = vscnprintf(buf, sizeof(buf), fmt, ap);
-	va_end(ap);
-
-	early_console->write(early_console, buf, n);
-}
-#endif
-
 static int __add_preferred_console(char *name, int idx, char *options,
 				   char *brl_options)
 {
-- 
1.8.5.6