[PATCH 01/10] drm/etnaviv: add uapi for register read feature

[Russell King - ARM Linux]

On Mon, Jan 30, 2017 at 09:18:25PM +0100, Thierry Reding wrote:
> On Fri, Dec 09, 2016 at 12:21:22PM +0100, Christian Gmeiner wrote:
> > @@ -167,6 +174,9 @@ struct drm_etnaviv_gem_submit {
> >  	__u64 bos;            /* in, ptr to array of submit_bo's */
> >  	__u64 relocs;         /* in, ptr to array of submit_reloc's */
> >  	__u64 stream;         /* in, ptr to cmdstream */
> > +	__u64 readbacks;      /* in, ptr to array of submit_readback's */
> > +	__u32 nr_readbacks;   /* in, number of submit_readback's */
> > +	__u32 padding;
> >  };
> 
> What about ABI stability? How's this going to work when userspace uses
> old headers but runs against a new kernel? What about userspace using
> newer headers than the kernel?

+1, this is unacceptable.  We went through a period of making sure
that the ABI was going to be stable once we merged the driver into
the kernel, and the rule is that we don't ever break userspace.  This
does exactly that, so it's not permissible.

If this change is necessary, it needs to be a new ioctl number for the
call with the new structure, and the old ioctl has to keep working.

There are other users of etnaviv other than mesa that will break.

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