printk: Enable the use of more than one CON_BOOT (early console)

Today, register_console() assumes the following usage:

  - The first console to register with a flag set to CON_BOOT
    is the one and only bootconsole.

  - If another register_console() is called with an additional
    CON_BOOT, it is silently rejected.

  - As soon as a console without the CON_BOOT set calls
    registers the bootconsole is automatically unregistered.

  - Once there is a "real" console - register_console() will
    silently reject any consoles with it's CON_BOOT flag set.

In many systems (alpha, blackfin, microblaze, mips, powerpc,
sh, & x86), there are early_printk implementations, which use
the CON_BOOT which come out serial ports, vga, usb, & memory
buffers.

In many embedded systems, it would be nice to have two
bootconsoles - in case the primary fails, you always have
access to a backup memory buffer - but this requires at least
two CON_BOOT consoles...

This patch enables that functionality.

With the change applied, on boot you get (if you try to
re-enable a boot console after the "real" console has been
registered):

  root:/> dmesg | grep console
  bootconsole [early_shadow0] enabled
  bootconsole [early_BFuart0] enabled
  Kernel command line: root=/dev/mtdblock0 rw earlyprintk=serial,uart0,57600 console=ttyBF0,57600 nmi_debug=regs
  console handover:boot [early_BFuart0] boot [early_shadow0]  -> real [ttyBF0]
  Too late to register bootconsole early_shadow0

or:

  root:/> dmesg | grep console
  Kernel command line: root=/dev/mtdblock0 rw console=ttyBF0,57600
  console [ttyBF0] enabled

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>
Cc: "Mike Frysinger" <vapier.adi@gmail.com>
Cc: "Paul Mundt" <lethal@linux-sh.org>
LKML-Reference: <200907012108.38030.rgetz@blackfin.uclinux.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 92 insertions, 54 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index b4d97b54c1ec..41fe60995530 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -37,6 +37,12 @@
 #include <asm/uaccess.h>
 
 /*
+ * for_each_console() allows you to iterate on each console
+ */
+#define for_each_console(con) \
+        for (con = console_drivers; con != NULL; con = con->next)
+
+/*
  * Architectures can override it:
  */
 void asmlinkage __attribute__((weak)) early_printk(const char *fmt, ...)
@@ -412,7 +418,7 @@ static void __call_console_drivers(unsigned start, unsigned end)
 {
         struct console *con;
 
-        for (con = console_drivers; con; con = con->next) {
+        for_each_console(con) {
                 if ((con->flags & CON_ENABLED) && con->write &&
                                 (cpu_online(smp_processor_id()) ||
                                 (con->flags & CON_ANYTIME)))
@@ -544,7 +550,7 @@ static int have_callable_console(void)
 {
         struct console *con;
 
-        for (con = console_drivers; con; con = con->next)
+        for_each_console(con)
                 if (con->flags & CON_ANYTIME)
                         return 1;
 
@@ -1082,7 +1088,7 @@ void console_unblank(void)
 
         console_locked = 1;
         console_may_schedule = 0;
-        for (c = console_drivers; c != NULL; c = c->next)
+        for_each_console(c)
                 if ((c->flags & CON_ENABLED) && c->unblank)
                         c->unblank();
         release_console_sem();
@@ -1097,7 +1103,7 @@ struct tty_driver *console_device(int *index)
         struct tty_driver *driver = NULL;
 
         acquire_console_sem();
-        for (c = console_drivers; c != NULL; c = c->next) {
+        for_each_console(c) {
                 if (!c->device)
                         continue;
                 driver = c->device(c, index);
@@ -1134,25 +1140,49 @@ EXPORT_SYMBOL(console_start);
  * to register the console printing procedure with printk() and to
  * print any messages that were printed by the kernel before the
  * console driver was initialized.
+ *
+ * This can happen pretty early during the boot process (because of
+ * early_printk) - sometimes before setup_arch() completes - be careful
+ * of what kernel features are used - they may not be initialised yet.
+ *
+ * There are two types of consoles - bootconsoles (early_printk) and
+ * "real" consoles (everything which is not a bootconsole) which are
+ * handled differently.
+ *  - Any number of bootconsoles can be registered at any time.
+ *  - As soon as a "real" console is registered, all bootconsoles
+ *    will be unregistered automatically.
+ *  - Once a "real" console is registered, any attempt to register a
+ *    bootconsoles will be rejected
  */
-void register_console(struct console *console)
+void register_console(struct console *newcon)
 {
         int i;
         unsigned long flags;
-        struct console *bootconsole = NULL;
+        struct console *bcon = NULL;
 
-        if (console_drivers) {
-                if (console->flags & CON_BOOT)
-                        return;
-                if (console_drivers->flags & CON_BOOT)
-                        bootconsole = console_drivers;
+        /*
+         * before we register a new CON_BOOT console, make sure we don't
+         * already have a valid console
+         */
+        if (console_drivers && newcon->flags & CON_BOOT) {
+                /* find the last or real console */
+                for_each_console(bcon) {
+                        if (!(bcon->flags & CON_BOOT)) {
+                                printk(KERN_INFO "Too late to register bootconsole %s%d\n",
+                                        newcon->name, newcon->index);
+                                return;
+                        }
+                }
         }
 
-        if (preferred_console < 0 || bootconsole || !console_drivers)
+        if (console_drivers && console_drivers->flags & CON_BOOT)
+                bcon = console_drivers;
+
+        if (preferred_console < 0 || bcon || !console_drivers)
                 preferred_console = selected_console;
 
-        if (console->early_setup)
-                console->early_setup();
+        if (newcon->early_setup)
+                newcon->early_setup();
 
         /*
          *      See if we want to use this console driver. If we
@@ -1160,13 +1190,13 @@ void register_console(struct console *console)
          *      that registers here.
          */
         if (preferred_console < 0) {
-                if (console->index < 0)
-                        console->index = 0;
-                if (console->setup == NULL ||
-                    console->setup(console, NULL) == 0) {
-                        console->flags |= CON_ENABLED;
-                        if (console->device) {
-                                console->flags |= CON_CONSDEV;
+                if (newcon->index < 0)
+                        newcon->index = 0;
+                if (newcon->setup == NULL ||
+                    newcon->setup(newcon, NULL) == 0) {
+                        newcon->flags |= CON_ENABLED;
+                        if (newcon->device) {
+                                newcon->flags |= CON_CONSDEV;
                                 preferred_console = 0;
                         }
                 }
@@ -1178,47 +1208,53 @@ void register_console(struct console *console)
          */
         for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];
                         i++) {
-                if (strcmp(console_cmdline[i].name, console->name) != 0)
+                if (strcmp(console_cmdline[i].name, newcon->name) != 0)
                         continue;
-                if (console->index >= 0 &&
-                    console->index != console_cmdline[i].index)
+                if (newcon->index >= 0 &&
+                    newcon->index != console_cmdline[i].index)
                         continue;
-                if (console->index < 0)
-                        console->index = console_cmdline[i].index;
+                if (newcon->index < 0)
+                        newcon->index = console_cmdline[i].index;
 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE
                 if (console_cmdline[i].brl_options) {
-                        console->flags |= CON_BRL;
-                        braille_register_console(console,
+                        newcon->flags |= CON_BRL;
+                        braille_register_console(newcon,
                                         console_cmdline[i].index,
                                         console_cmdline[i].options,
                                         console_cmdline[i].brl_options);
                         return;
                 }
 #endif
-                if (console->setup &&
-                    console->setup(console, console_cmdline[i].options) != 0)
+                if (newcon->setup &&
+                    newcon->setup(newcon, console_cmdline[i].options) != 0)
                         break;
-                console->flags |= CON_ENABLED;
-                console->index = console_cmdline[i].index;
+                newcon->flags |= CON_ENABLED;
+                newcon->index = console_cmdline[i].index;
                 if (i == selected_console) {
-                        console->flags |= CON_CONSDEV;
+                        newcon->flags |= CON_CONSDEV;
                         preferred_console = selected_console;
                 }
                 break;
         }
 
-        if (!(console->flags & CON_ENABLED))
+        if (!(newcon->flags & CON_ENABLED))
                 return;
 
-        if (bootconsole && (console->flags & CON_CONSDEV)) {
-                printk(KERN_INFO "console handover: boot [%s%d] -> real [%s%d]\n",
-                       bootconsole->name, bootconsole->index,
-                       console->name, console->index);
-                unregister_console(bootconsole);
-                console->flags &= ~CON_PRINTBUFFER;
+        if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) {
+                /* we need to iterate through twice, to make sure we print
+                 * everything out, before we unregister the console(s)
+                 */
+                printk(KERN_INFO "console handover:");
+                for_each_console(bcon)
+                        printk("boot [%s%d] ", bcon->name, bcon->index);
+                printk(" -> real [%s%d]\n", newcon->name, newcon->index);
+                for_each_console(bcon)
+                        unregister_console(bcon);
+                newcon->flags &= ~CON_PRINTBUFFER;
         } else {
-                printk(KERN_INFO "console [%s%d] enabled\n",
-                       console->name, console->index);
+                printk(KERN_INFO "%sconsole [%s%d] enabled\n",
+                        (newcon->flags & CON_BOOT) ? "boot" : "" ,
+                        newcon->name, newcon->index);
         }
 
         /*
@@ -1226,16 +1262,16 @@ void register_console(struct console *console)
          *      preferred driver at the head of the list.
          */
         acquire_console_sem();
-        if ((console->flags & CON_CONSDEV) || console_drivers == NULL) {
-                console->next = console_drivers;
-                console_drivers = console;
-                if (console->next)
-                        console->next->flags &= ~CON_CONSDEV;
+        if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
+                newcon->next = console_drivers;
+                console_drivers = newcon;
+                if (newcon->next)
+                        newcon->next->flags &= ~CON_CONSDEV;
         } else {
-                console->next = console_drivers->next;
-                console_drivers->next = console;
+                newcon->next = console_drivers->next;
+                console_drivers->next = newcon;
         }
-        if (console->flags & CON_PRINTBUFFER) {
+        if (newcon->flags & CON_PRINTBUFFER) {
                 /*
                  * release_console_sem() will print out the buffered messages
                  * for us.
@@ -1287,11 +1323,13 @@ EXPORT_SYMBOL(unregister_console);
 
 static int __init disable_boot_consoles(void)
 {
-        if (console_drivers != NULL) {
-                if (console_drivers->flags & CON_BOOT) {
+        struct console *con;
+
+        for_each_console(con) {
+                if (con->flags & CON_BOOT) {
                         printk(KERN_INFO "turn off boot console %s%d\n",
-                                console_drivers->name, console_drivers->index);
-                        return unregister_console(console_drivers);
+                                con->name, con->index);
+                        return unregister_console(con);
                 }
         }
         return 0;