s390/boot: Introduce ring buffer for boot messages

Collect all boot messages into a ring buffer independent of the current
log level. This allows to retain all boot-time messages, which is
particularly useful for analyzing early crashes.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/boot/printk.c

@@ -13,6 +13,20 @@
 
 int boot_console_loglevel = CONFIG_CONSOLE_LOGLEVEL_DEFAULT;
 bool boot_ignore_loglevel;
+char boot_rb[PAGE_SIZE * 2];
+size_t boot_rb_off;
+
+static void boot_rb_add(const char *str, size_t len)
+{
+	/* leave double '\0' in the end */
+	size_t avail = sizeof(boot_rb) - boot_rb_off - 1;
+
+	/* store strings separated by '\0' */
+	if (len + 1 > avail)
+		boot_rb_off = 0;
+	strcpy(boot_rb + boot_rb_off, str);
+	boot_rb_off += len + 1;
+}
 
 const char hex_asc[] = "0123456789abcdef";
 
@@ -229,5 +243,9 @@ void boot_printk(const char *fmt, ...)
 	}
 out:
 	va_end(args);
-	boot_console_earlyprintk(buf);
+	len = strlen(buf);
+	if (len) {
+		boot_rb_add(buf, len);
+		boot_console_earlyprintk(buf);
+	}
 }

arch/s390/include/asm/boot_data.h

@@ -15,4 +15,17 @@ extern unsigned long ipl_cert_list_size;
 extern unsigned long early_ipl_comp_list_addr;
 extern unsigned long early_ipl_comp_list_size;
 
+extern char boot_rb[PAGE_SIZE * 2];
+extern size_t boot_rb_off;
+
+#define boot_rb_foreach(cb)							\
+	do {									\
+		size_t off = boot_rb_off + strlen(boot_rb + boot_rb_off) + 1;	\
+		size_t len;							\
+		for (; off < sizeof(boot_rb) && (len = strlen(boot_rb + off)); off += len + 1) \
+			cb(boot_rb + off);					\
+		for (off = 0; off < boot_rb_off && (len = strlen(boot_rb + off)); off += len + 1) \
+			cb(boot_rb + off);					\
+	} while (0)
+
 #endif /* _ASM_S390_BOOT_DATA_H */