[MIPS] DECstation: Optimised early printk()

 This is an optimised implementation of early printk() for the DECstation.  
After the recent conversion to a MIPS-specific generic routine using a 
character-by-character output the performance dropped significantly.  
This change reverts to the previous speed -- even at 9600 bps of the 
serial console the difference is visible with a naked eye; I presume for a 
framebuffer it is even worse (it may depend on exactly which one is used 
though).

 Additionally the change includes a fix for a problem that the old 
implementation had -- the format used would not actually limit the length 
of the string output.  This new implementation uses a local buffer to deal 
with it -- even with this additional copying it is much faster than the 
generic function.

 Plus this driver is registered much earlier than the generic one, 
allowing one to see critical messages, such as one about an incorrect CPU 
setting used, that are produced beforehand. :-)

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -64,7 +64,6 @@ config MACH_DECSTATION
 	select BOOT_ELF32
 	select DMA_NONCOHERENT
 	select NO_IOPORT
-	select SYS_HAS_EARLY_PRINTK
 	select IRQ_CPU
 	select SYS_HAS_CPU_R3000
 	select SYS_HAS_CPU_R4X00

arch/mips/dec/prom/console.c

@@ -3,7 +3,7 @@
  *
  *	DECstation PROM-based early console support.
  *
- *	Copyright (C) 2004  Maciej W. Rozycki
+ *	Copyright (C) 2004, 2007  Maciej W. Rozycki
  *
  *	This program is free software; you can redistribute it and/or
  *	modify it under the terms of the GNU General Public License
@@ -13,15 +13,35 @@
 #include <linux/console.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
+#include <linux/string.h>
 
 #include <asm/dec/prom.h>
 
-void prom_putchar(char c)
+static void __init prom_console_write(struct console *con, const char *s,
+				      unsigned int c)
 {
-	char s[2];
+	char buf[81];
+	unsigned int chunk = sizeof(buf) - 1;
 
-	s[0] = c;
-	s[1] = '\0';
+	while (c > 0) {
+		if (chunk > c)
+			chunk = c;
+		memcpy(buf, s, chunk);
+		buf[chunk] = '\0';
+		prom_printf("%s", buf);
+		s += chunk;
+		c -= chunk;
+	}
+}
+
+static struct console promcons __initdata = {
+	.name	= "prom",
+	.write	= prom_console_write,
+	.flags	= CON_BOOT | CON_PRINTBUFFER,
+	.index	= -1,
+};
 
-	prom_printf( s);
+void __init register_prom_console(void)
+{
+	register_console(&promcons);
 }

arch/mips/dec/prom/init.c

@@ -103,6 +103,9 @@ void __init prom_init(void)
 	if (prom_is_rex(magic))
 		rex_clear_cache();
 
+	/* Register the early console.  */
+	register_prom_console();
+
 	/* Were we compiled with the right CPU option? */
 #if defined(CONFIG_CPU_R3000)
 	if ((current_cpu_data.cputype == CPU_R4000SC) ||