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

[pradeep singh]

On 5/18/07, Sam Ravnborg <sam at ravnborg.org> wrote:
> On Fri, May 18, 2007 at 05:10:02PM +0530, pradeep singh 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?
> > Why is it 'ensure 32 byte alignment', instead of 32 bit alignment?
> >
> > What am i missing here?
>
> Google turned up following useful link:
> http://www.nabble.com/-RFC--changing-value-of-NETDEV_ALIGN-to-cacheline-size-t1620499.html
Thank you Sam.
This is really the answer i should have searched :-(.

A lesson to learn indeed.

Thank you once again.
--psr
>
> snip:
> The reason it's 32 is that old drivers depended on the
> struct being at least 32-byte aligned because they would
> embed structures DMA'd to/from the card in their private
> area and just assumed that would be aligned enough for
> the card's restrictions.
>
>         Sam
>


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