[uml-devel] RFC: lossless printk, version 2

[Werner Almesberger <werner@almesberger.net>]

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Second, cleaner version. The changes:

 - retry on EINTR
 - instead of trying to die, disable console permanently on any error
   but EINTR and EAGAIN
 - prints "nice" message on error
 - removed duplicate console_user.o in Makefile
 - moved lossless_stdio_console_write prototype from stdio_console.c
   to stdio_console.h

Thanks to all who have sent comments !

- Werner

-- 
  _________________________________________________________________________
 / Werner Almesberger, Buenos Aires, Argentina     werner@almesberger.net /
/_http://www.almesberger.net/____________________________________________/

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--- linux-2.6.4-orig/arch/um/Kconfig_char	Wed Mar 10 23:55:26 2004
+++ linux-2.6.4/arch/um/Kconfig_char	Wed Mar 31 22:18:21 2004
@@ -5,6 +5,14 @@
 	bool
 	default y
 
+config LOSSLESS_STDIO_CONSOLE
+	bool "Lossless standard output console"
+	depends on STDIO_CONSOLE
+	help
+	When console output goes to standard output, retry write operations
+	that would block. This is particularly useful when recording the
+	output of a UML system in a file.
+
 config SSL
 	bool "Virtual serial line"
 	help
--- linux-2.6.4-orig/arch/um/drivers/Makefile	Tue Mar 16 03:11:43 2004
+++ linux-2.6.4/arch/um/drivers/Makefile	Fri Apr  2 16:57:21 2004
@@ -41,6 +41,7 @@
 obj-$(CONFIG_UML_WATCHDOG) += harddog.o
 obj-$(CONFIG_BLK_DEV_COW) += cow_kern.o
 obj-$(CONFIG_BLK_DEV_COW_COMMON) += cow_user.o
+obj-$(CONFIG_LOSSLESS_STDIO_CONSOLE) += console_user.o
 
 obj-y += stdio_console.o $(CHAN_OBJS)
 
--- linux-2.6.4-orig/arch/um/drivers/stdio_console.h	Wed Mar 10 23:55:26 2004
+++ linux-2.6.4/arch/um/drivers/stdio_console.h	Fri Apr  2 17:09:00 2004
@@ -7,6 +7,9 @@
 #define __STDIO_CONSOLE_H
 
 extern void save_console_flags(void);
+
+void lossless_stdio_console_write(const char *string,unsigned len);
+
 #endif
 
 /*
--- linux-2.6.4-orig/arch/um/drivers/stdio_console.c	Tue Mar 16 03:11:43 2004
+++ linux-2.6.4/arch/um/drivers/stdio_console.c	Fri Apr  2 17:09:05 2004
@@ -194,6 +194,9 @@
 static void console_write(struct console *console, const char *string, 
 			  unsigned len)
 {
+#ifdef CONFIG_LOSSLESS_STDIO_CONSOLE
+	lossless_stdio_console_write(string,len);
+#else
 	struct line *line = &vts[console->index];
 
 	if(con_init_done)
@@ -201,6 +204,7 @@
 	console_write_chan(&line->chan_list, string, len);
 	if(con_init_done)
 		up(&line->sem);
+#endif
 }
 
 static struct tty_driver *console_device(struct console *c, int *index)
--- /dev/null	Fri Dec 12 23:38:58 2003
+++ linux-2.6.4/arch/um/drivers/console_user.c	Fri Apr  2 17:20:37 2004
@@ -0,0 +1,62 @@
+/*
+ * arch/um/drivers/console_user.c - Lossless console driver using write(2)
+ *
+ * Written 2003,2004 by Werner Almesberger
+ */
+
+
+#include <stdlib.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include "stdio_console.h"
+
+
+/*
+ * UML makes the console non-blocking. And indeed, even when stdout is
+ * redirected to a file, we may occasionally get EAGAIN, and lose a message or
+ * ten. This console write function just retries the write, without affecting
+ * other users of the console.
+ */
+
+
+void lossless_stdio_console_write(const char *string,unsigned len)
+{
+	static int console_broken = 0;
+	static char buf[100]; /* check with message size, below */
+	const char *p;
+	int wrote;
+
+	if (console_broken)
+		return;
+
+	for (p = string; len; p += wrote) {
+		while (1) {
+			wrote = write(1,p,len);
+			if (wrote >= 0)
+				break;
+
+			if (errno == EINTR)
+				continue;
+			if (errno == EAGAIN) {
+				usleep(100000); /* nap for 100ms */
+				continue;
+			}
+
+			/*
+			 * We're in trouble. Try to get the message out on
+			 * stderr, then turn off the console.
+			 */
+			sprintf(buf,"Console write failed with errno %d. "
+			    "Disabling console.\r\n",errno);
+			(void) write(2,buf,strlen(buf));
+
+			console_broken = 1;
+			return;
+		}
+		len -= wrote;
+	}
+}