[PATCH 2/3] early_printk: Add force_early_printk kernel parameter

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
---
 kernel/printk/printk.c |   74 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 49 insertions(+), 25 deletions(-)

--- 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
@@ -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
+
+#ifdef CONFIG_EARLY_PRINTK
+	if (force_early_printk && early_console)
+		return early_vprintk(fmt, args);
 #endif
 
 	if (level == LOGLEVEL_SCHED) {
@@ -1970,7 +2009,12 @@ asmlinkage __visible int printk(const ch
 	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;
@@ -2020,26 +2064,6 @@ DEFINE_PER_CPU(printk_func_t, printk_fun
 
 #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)
 {