[PATCH RFC v2 3/3] drm/dp: Add a drm_aux-dev module for reading/writing dpcd registers.

[Daniel Vetter]

On Tue, Sep 22, 2015 at 04:25:17PM +0300, Ville Syrjälä wrote:
> On Tue, Sep 22, 2015 at 02:59:24PM +0200, Daniel Vetter wrote:
> > On Tue, Sep 22, 2015 at 03:35:13PM +0300, Ville Syrjälä wrote:
> > > On Tue, Sep 22, 2015 at 02:17:51PM +0200, Daniel Vetter wrote:
> > > > Iirc short reads are ok in all cases, so we could even punt the restarting
> > > > to userspace by just doing short reads/writes (like sockets do).
> > > 
> > > Yeah, short writes due to -EFAULT sound more dangerous than short reads.
> > > But I'm not sure there's any point in allowing short reads either in
> > > this case, so just returning the error upfront if access_ok() complains
> > > seems like a sane option to me.
> > 
> > access_ok _only_ does static checks (on x86 it only checks that it's a
> > userspace address). Which means any kind of real faults will only happen
> > later on in the actual copy_to/from_user. I'd say we can go meh if that
> > happens - it's guaranteed to be userspace doing something silly since we
> > don't need to hold any of the mm locks ;-)
> 
> Hmm, true. So I guess on -EFAULT we should:
> 
> if (copy_{to,from}_user())
> 	return num_bytes_processed ? num_bytes_processed : -EFAULT;
> 
> Sound reasonable?

Yeah that's what I'd go with.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch