i'm not sure this answers the problems with the previous patch.  i noticed some
changes in netif.c not directly related to the patch with netgetl changed to returning a ulong.
i left that alone, since those changes still make sense to me.  perhaps the return value would be
better u32int.  i don't know.

previous comments:

	proposal to allow access to a device's mtu limits.  mintu <= mtu <= maxtu.
	mintu and maxtu are supposed to be "hardware" limits.  in the case of igbe,
	it's difficult to shuffle the pool so no reconfiguration is required, but in the
	general case, Netif->mtu() is called (if it exists) to modify mtu.
	
	the file /net/ether$i/mtu contains the three 11-digit numbers: mintu, mtu, maxtu.
	
	this is part of a script i use to set up interfaces for my aoe initiator
	
		mtu = 8192
	
		for(n in 1 2 3){
			bind -a '#l'^$n /net
			ether = /net/ether^$n
			<>$ether/clone{
				i=`{read}
				echo mtu $mtu>$ether/$i/ctl
			}
		}
	
	this does not yet play nicely in an ethernet configuration or with ip/ipconfig
	because they don't know about it and the ipifc mtu is not directly connected to
	the hardware mtu.  perhaps a new option could force setting of the hardware's
	mtu.  and in all cases the hardware's mtu setting should be a ceiling to the ipifc
	mtu.
	
	in any event, i think that's to tackle later after we have access to hw mtu settings.
	
	- erik

Notes:
Wed Sep 19 19:14:17 EDT 2007 geoff
    deferred.


Reference: /n/sources/patch/saved/netif-mtu2
Date: Mon Jun 18 17:48:00 CES 2007
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/9/port/netif.c	Mon Jun 18 17:42:55 2007
+++ /sys/src/9/port/netif.c	Mon Jun 18 17:42:49 2007
@@ -66,7 +66,7 @@
 
 	/* second level contains clone plus all the conversations */
 	t = NETTYPE(c->qid.path);
-	if(t == N2ndqid || t == Ncloneqid || t == Naddrqid){
+	if(t == N2ndqid || t == Ncloneqid || t == Naddrqid || t == Nmtuqid){
 		switch(i) {
 		case DEVDOTDOT:
 			q.type = QTDIR;
@@ -89,8 +89,12 @@
 			q.path = Nifstatqid;
 			devdir(c, q, "ifstats", 0, eve, 0444, dp);
 			break;
+		case 4:
+			q.path = Nmtuqid;
+			devdir(c, q, "mtu", 0, eve, 0444, dp);
+			break;
 		default:
-			i -= 4;
+			i -= 5;
 			if(i >= nif->nfile)
 				return -1;
 			if(nif->f[i] == 0)
@@ -244,6 +248,9 @@
 		return readnum(offset, a, n, f->type, NUMSIZE);
 	case Nifstatqid:
 		return 0;
+	case Nmtuqid:
+		snprint(up->genbuf, sizeof up->genbuf, "%11.ud %11.ud %11.ud\n", nif->minmtu, nif->mtu, nif->maxmtu);
+		return readstr(offset, a, n, up->genbuf);
 	}
 	error(Ebadarg);
 	return -1;	/* not reached */
@@ -287,7 +294,7 @@
 netifwrite(Netif *nif, Chan *c, void *a, long n)
 {
 	Netfile *f;
-	int type;
+	int type, mtu;
 	char *p, buf[64];
 	uchar binaddr[Nmaxaddr];
 
@@ -331,6 +338,16 @@
 			f->scan = type;
 			nif->scan++;
 		}
+	} else if((p = matchtoken(buf, "mtu")) != 0){
+		mtu = atoi(p);
+		// zero resets default.
+		if(mtu != 0)
+		if(mtu < nif->minmtu || mtu > nif->maxmtu)
+			error(Ebadarg);
+		if(nif->hwmtu)
+			nif->mtu = nif->hwmtu(nif->arg, mtu);
+		else
+			nif->mtu = mtu;
 	} else if(matchtoken(buf, "bridge")){
 		f->bridge = 1;
 	} else if(matchtoken(buf, "headersonly")){
@@ -558,7 +575,7 @@
 }
 
 void
-hnputv(void *p, uvlong v)
+hnputv(void *p, vlong v)
 {
 	uchar *a;
 
@@ -568,7 +585,7 @@
 }
 
 void
-hnputl(void *p, uint v)
+hnputl(void *p, ulong v)
 {
 	uchar *a;
 
@@ -589,7 +606,7 @@
 	a[1] = v;
 }
 
-uvlong
+vlong
 nhgetv(void *p)
 {
 	uchar *a;
@@ -598,7 +615,7 @@
 	return ((vlong)nhgetl(a) << 32) | nhgetl(a+4);
 }
 
-uint
+ulong
 nhgetl(void *p)
 {
 	uchar *a;
--- /sys/src/9/port/netif.h	Mon Jun 18 17:43:05 2007
+++ /sys/src/9/port/netif.h	Mon Jun 18 17:43:02 2007
@@ -17,6 +17,7 @@
 	Nstatqid,
 	Ntypeqid,
 	Nifstatqid,
+	Nmtuqid,
 };
 
 /*
@@ -76,6 +77,9 @@
 	int	alen;			/* address length */
 	int	mbps;			/* megabits per sec */
 	int	link;			/* link status */
+	int	minmtu;
+	int 	maxmtu;
+	int	mtu;
 	uchar	addr[Nmaxaddr];
 	uchar	bcast[Nmaxaddr];
 	Netaddr	*maddr;			/* known multicast addresses */
@@ -100,6 +104,7 @@
 	void	*arg;
 	void	(*promiscuous)(void*, int);
 	void	(*multicast)(void*, uchar*, int);
+	int	(*hwmtu)(void*, int);	/* get/set mtu */
 	void	(*scanbs)(void*, uint);	/* scan for base stations */
 };
 
--- /sys/src/9/pc/devether.c	Mon Jun 18 17:43:20 2007
+++ /sys/src/9/pc/devether.c	Mon Jun 18 17:43:16 2007
@@ -4,7 +4,6 @@
 #include "dat.h"
 #include "fns.h"
 #include "io.h"
-#include "pool.h"
 #include "ureg.h"
 #include "../port/error.h"
 #include "../port/netif.h"
@@ -267,7 +266,7 @@
 		error(Ebadctl);
 	}
 
-	if(n > ether->maxmtu)
+	if(n > ether->mtu)
 		error(Etoobig);
 	if(n < ether->minmtu)
 		error(Etoosmall);
@@ -304,7 +303,7 @@
 	}
 	ether = etherxx[chan->dev];
 
-	if(n > ether->maxmtu){
+	if(n > ether->mtu){
 		freeb(bp);
 		error(Etoobig);
 	}
@@ -370,6 +369,7 @@
 	ether->tbdf = BUSUNKNOWN;
 	ether->mbps = 10;
 	ether->minmtu = ETHERMINTU;
+	ether->mtu = ETHERMAXTU;
 	ether->maxmtu = ETHERMAXTU;
 
 	if(cardno < 0){
@@ -415,8 +415,8 @@
 	if(ether->irq >= 0)
 		intrenable(ether->irq, ether->interrupt, ether, ether->tbdf, name);
 
-	i = sprint(buf, "#l%d: %s: %dMbps port 0x%luX irq %d",
-		ctlrno, cards[cardno].type, ether->mbps, ether->port, ether->irq);
+	i = sprint(buf, "#l%d: %s: %dMbps port 0x%luX irq %d tu %d",
+		ctlrno, cards[cardno].type, ether->mbps, ether->port, ether->irq, ether->mtu);
 	if(ether->mem)
 		i += sprint(buf+i, " addr 0x%luX", ether->mem);
 	if(ether->size)
--- /sys/src/9/pc/etherif.h	Mon Jun 18 17:43:33 2007
+++ /sys/src/9/pc/etherif.h	Mon Jun 18 17:43:31 2007
@@ -9,8 +9,6 @@
 
 	int	ctlrno;
 	int	tbdf;			/* type+busno+devno+funcno */
-	int	minmtu;
-	int 	maxmtu;
 	uchar	ea[Eaddrlen];
 
 	void	(*attach)(Ether*);	/* filled in by reset routine */