[PATCH 2.6.21.1] nfs-root: added possibility to override default MTU (for UDP jumbo frames)

[manio@skyboo.net]

To use a NFS-root for UDP jumbo frames the kernel on the client need to bring
up interface with MTU set to 9000 bytes - otherwise it cannot contact server
with jumbo frames enabled (nfs server not responding, still trying) and cannot
boot. Added a kernel parameter named 'ipmtu' which can be used to specify
initial MTU size when booting via nfsroot.

Signed-off-by: Mariusz Bialonczyk <manio@skyboo.net>

diff -Nru linux-2.6.21.1-orig/Documentation/kernel-parameters.txt linux-2.6.21.1/Documentation/kernel-parameters.txt
--- linux-2.6.21.1-orig/Documentation/kernel-parameters.txt	2007-04-27 23:49:26.000000000 +0200
+++ linux-2.6.21.1/Documentation/kernel-parameters.txt	2007-07-01 18:47:11.000000000 +0200
@@ -720,6 +720,9 @@
 	ip2=		[HW] Set IO/IRQ pairs for up to 4 IntelliPort boards
 			See comment before ip2_setup() in drivers/char/ip2.c.
 
+	ipmtu=		[IP_PNP]
+			See Documentation/nfsroot.txt.
+
 	ips=		[HW,SCSI] Adaptec / IBM ServeRAID controller
 			See header of drivers/scsi/ips.c.
 
diff -Nru linux-2.6.21.1-orig/Documentation/nfsroot.txt linux-2.6.21.1/Documentation/nfsroot.txt
--- linux-2.6.21.1-orig/Documentation/nfsroot.txt	2007-04-27 23:49:26.000000000 +0200
+++ linux-2.6.21.1/Documentation/nfsroot.txt	2007-07-01 19:02:40.000000000 +0200
@@ -153,6 +153,16 @@
                 Default: any
 
 
+ipmtu=<mtu_value>
+
+  This parameter tells the kernel to override default MTU size to specified
+  <mtu_value>. Useful in cases where NFS server have jumbo frames enabled and
+  client can't connect via UDP because of default MTU value (in ethernet
+  usually 1500 bytes). With this option before bringing interface up, kernel
+  will set the passed MTU size. In case of NFS-root booting server and client
+  can use UDP jumbo frames (NFS's rsize and wsize set to 8192 for instance).
+
+
 
 
 3.) Boot Loader
diff -Nru linux-2.6.21.1-orig/net/ipv4/ipconfig.c linux-2.6.21.1/net/ipv4/ipconfig.c
--- linux-2.6.21.1-orig/net/ipv4/ipconfig.c	2007-04-27 23:49:26.000000000 +0200
+++ linux-2.6.21.1/net/ipv4/ipconfig.c	2007-07-01 15:44:44.000000000 +0200
@@ -113,6 +113,8 @@
  */
 int ic_set_manually __initdata = 0;		/* IPconfig parameters set manually */
 
+unsigned int ic_mtu __initdata = 0;		/* IPconfig MTU parameter: 0 - defaults, other - override */
+
 static int ic_enable __initdata = 0;		/* IP config enabled? */
 
 /* Protocol choice */
@@ -209,6 +211,11 @@
 			able &= ic_proto_enabled;
 			if (ic_proto_enabled && !able)
 				continue;
+			if (ic_mtu > 0)
+			{
+				printk(KERN_ERR "IP-Config: Overriding %s MTU to %d bytes\n", dev->name, ic_mtu);
+				dev->mtu = ic_mtu;
+			}
 			oflags = dev->flags;
 			if (dev_change_flags(dev, oflags | IFF_UP) < 0) {
 				printk(KERN_ERR "IP-Config: Failed to open %s\n", dev->name);
@@ -1506,5 +1513,14 @@
 	return ip_auto_config_setup(addrs);
 }
 
+static int __init mtu_config_setup(char *str)
+{
+	if (!str)
+		return 0;
+	ic_mtu = simple_strtoul(str, &str, 0);
+	return 1;
+}
+
 __setup("ip=", ip_auto_config_setup);
+__setup("ipmtu=", mtu_config_setup);
 __setup("nfsaddrs=", nfsaddrs_config_setup);