alloc_netdev() and 32 byte or 32 bit alignment???

[pradeep singh]

On 5/18/07, Eric Dumazet <dada1 at cosmosbay.com> wrote:
> On Fri, 18 May 2007 17:10:02 +0530
> "pradeep singh" <2500.pradeep at gmail.com> wrote:
>
> > Hi ,
> >
> > following code snippet is self evident in what it does -
> >
> > truct net_device *alloc_netdev(int sizeof_priv, const char *name,
> >               void (*setup)(struct net_device *))
> > {
> >       void *p;
> >       struct net_device *dev;
> >       int alloc_size;
> >
> >       /* ensure 32-byte alignment of both the device and private area */
> >       alloc_size = (sizeof(*dev) + NETDEV_ALIGN_CONST) & ~NETDEV_ALIGN_CONST;
> >       alloc_size += sizeof_priv + NETDEV_ALIGN_CONST;
> >
> >       p = kzalloc(alloc_size, GFP_KERNEL);
> >       if (!p) {
> >               printk(KERN_ERR "alloc_dev: Unable to allocate device.\n");
> >               return NULL;
> >       }
> > [snip]
> >
> > Is the comment correct?
>
> yes it seems correct
>
> > Why is it 'ensure 32 byte alignment', instead of 32 bit alignment?
>
> Its 32 bytes, not bits.
>
> #define NETDEV_ALIGN        32
> #define NETDEV_ALIGN_CONST  (NETDEV_ALIGN - 1)
>
> >
> > What am i missing here?
>
> I dont know :)
>
> This 32 bytes alignement maybe not up2date, because net_device contains several ____cacheline_aligned_in_smp

Thanks eric
got it.

--psr
>
> Would be nice to switch the alignement to max(L1_CACHE_SIZE, 32) nowadays, at least on SMP
>
>


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