[PATCH v2] drm/panfrost: Document base field location constraint in panfrost_gem_object

[Boris Brezillon]

On Fri, 29 Nov 2019 19:40:38 +0100
Daniel Vetter <daniel at ffwll.ch> wrote:

> On Fri, Nov 29, 2019 at 03:19:36PM +0100, Boris Brezillon wrote:
> > On Fri, 29 Nov 2019 14:13:33 +0000
> > Steven Price <steven.price at arm.com> wrote:
> >   
> > > On 29/11/2019 13:56, Boris Brezillon wrote:  
> > > > I've spent hours chasing a memory corruption that was caused by
> > > > insertion of an extra field field before ->base. Let's document the
> > > > fact that base has to be the first field in panfrost_gem_object.
> > > > 
> > > > Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>    
> > > 
> > > This seems to be a limitation imposed by the gem_create_object()
> > > callback - e.g. it's assumed that kfree() can be directly called on the
> > > result. Useful to have the documentation though.  
> > 
> > Oh, you're right, I didn't catch that one.  
> 
> As a general rule of thumb, never insert anything before a struct member
> called base. Even more so if it's of the same kind of $thing, but less
> spezialed. This pattern is so common it's fairly often not documented
> anywhere.

I could argue that anything using container_of() in its to_xxx() helper
is misleading the user into thinking the position of the base field
doesn't matter (which is exactly what happened here), but I feel like I
already lost this battle, so I'll simply drop the patch.