Re: PATCH: sundance #5 (variable per-interface MTU support)
From: Donald Becker <hidden>
Date: 2002-09-19 22:23:23
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On Thu, 19 Sep 2002, Jason Lunz wrote:
This is a straightforward merge of variable mtu from donald's driver.
- np->rx_buf_sz = (dev->mtu <= 1500 ? PKT_BUF_SZ : dev->mtu + 32); + np->rx_buf_sz = (dev->mtu <= 1500 ? PKT_BUF_SZ : dev->mtu + 36); Errrmm, not quite right. Try np->rx_buf_sz = (dev->mtu <= 1520 ? PKT_BUF_SZ : dev->mtu + 16); The idea is that all ethernet-like drivers always allocate skbuffs of the same size, PKT_BUF_SZ (1536=3*512), unless a jumbo MTU forces a larger size. Specificially, using VLANs (+4 bytes on the frame size) on some interfaces should not result in a mix of allocation sizes. Most VLAN-like encapsulation should add fewer than (1536-1518 = 18) 18 extra bytes. BTW, always leave a few extra bytes at the end of the data buffer. You never know when some chip might decide to dribble an extra word or two, or include the CRC because someone frobbed the driver. -- Donald Becker becker@scyld.com Scyld Computing Corporation http://www.scyld.com 410 Severn Ave. Suite 210 Second Generation Beowulf Clusters Annapolis MD 21403 410-990-9993