On Fri, 9 Feb 2007 17:36:27 +0900 Roman Mashak wrote:
> Hello,
>
> as I understood 'dev_base' points to the beginning of 'net_device'
> structure, not to the beginning of block allocated by alloc_netdev().
What 'dev_base' is that? The global dev_base is a linked list
of all struct net_device's in the system.
> So 'net_device' introduces a field named 'padded', which is used to get the
> start address of the whole memory block.
OK, back to each struct net_device.
> What I don't understand is:
> 1) how come that this structure needs any padding
Each struct net_device allocation is padded to 32 bytes. I guess
this is for cacheline alignment, but I don't see any comments
about that.
> 2) where is this padding physically done - at the head of structure or on
> the end?
Yes. It depends on where kzalloc() returned memory.
p = kzalloc(alloc_size, GFP_KERNEL);
if (!p) {
printk(KERN_ERR "alloc_netdev: Unable to allocate device.\n");
return NULL;
}
dev = (struct net_device *)
(((long)p + NETDEV_ALIGN_CONST) & ~NETDEV_ALIGN_CONST);
dev->padded = (char *)dev - (char *)p;
kzalloc() returns p, then p is adjusted to the next 32-byte
boundary (if it's already on a 32-byte boundardy, it won't be
changed). So dev could be 'p' increased by any value from 0 to
28.
---
~Randy
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