Via Velocity minimum MTU size

Via Velocity minimum MTU size

[Jay Cliburn]

Hello, 

I was advised by someone on the fedora-devel-list to post my question
here.

I'm trying to set the MTU size on my Via Velocity 6122 Gbit adapter from
the default of 1500 down to 1492, but I keep getting this error:

[root@osprey ~]# ifconfig eth0 mtu 1492
SIOCSIFMTU: Invalid argument

Turns out that via-velocity.h defines the minimum MTU at 1500 bytes.

#define VELOCITY_MIN_MTU    (1514-14)
#define VELOCITY_MAX_MTU    (9000)


And via-velocity.c enforces the limit.

    if ((new_mtu < VELOCITY_MIN_MTU) || new_mtu > (VELOCITY_MAX_MTU)) {
         VELOCITY_PRT(MSG_LEVEL_ERR, KERN_NOTICE "%s: Invalid MTU.\n",
                                vptr->dev->name);
         return -EINVAL;
    }

This is an Abit AV8 motherboard with an Athlon64 3000+ cpu running
Fedora Core 5, kernel 2.6.16-1.2111_FC5 x86_64.

Can someone please explain why the driver might constrain the NIC to a
minimum MTU size of 1500?

Thanks,
Jay

Re: Via Velocity minimum MTU size

[Francois Romieu]

Jay Cliburn <jacliburn@bellsouth.net> :
[...]
> Can someone please explain why the driver might constrain the NIC to a
> minimum MTU size of 1500?

None that I remember of. The driver suffered some breakage when changing
the mtu while the interface is up but it has been fixed since february
(i.e. way before 2.6.16-1.2111_FC5).

>From a cursory glance at the documentation, I see no specific low limit
for the mtu. So... Does it break when you lower VELOCITY_MIN_MTU ?

-- 
Ueimor