Re: [PATCH v4] mmc: Add ioctl to let userspace apps send ACMDs

[John Calixto <john.calixto@modsystems.com>]

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On Wed, 13 Apr 2011, Arnd Bergmann wrote:
> On Tuesday 12 April 2011, Michał Mirosław wrote:
> > > +       unsigned int cmd_timeout_ms;
> > > +       __u64 data_ptr;  /* DAT buffer */
> > 
> > This will be more natural if you have an anonymous union here:
> > union {
> >   __u64 data_ptr_
> >  void *data_ptr;
> > };
> 
> No, that really does not work. It's important for all members of the ioctl data
> structure to have a fixed size, independent of the size of long or pointer.
> 
> If you do a union, the pointer ends up in the first 32 bits of the 64 bit member,
> which does not work on big-endian architectures. It also doesn't work on 31
> bit architectures, although that is a minor worry here.
> 

Gah!  OK, so much for the v5 I just sent then.  What do you think about
the compat_ioctl that I sent in v2?  It means having the extra 32-bit
compat structure, but at least all the compat overhead is conditional
upon CONFIG_COMPAT.  If you're not using CONFIG_COMPAT, you don't get
any compat cruft.

John