(In reply to Julian from comment #65)
> (In reply to Alex Deucher from comment #64)
> > (In reply to Julian from comment #63)
> > > At first I thought it was an issue with the writeback feature that caches
> > > certain register values because it also caches an rptr value that is used in
> > > the driver's gpu_lockup_check and is, to my knowledge, never actually
> > > written to.
> > 
> > More likely it never gets written because the GPU has hung due to something
> > else.
> 
> Sorry, I meant that I've searched the radeon source for the lines of code
> where the value was accessed from. I've only found instances of it being
> read, no instances of it being written to. But I won't put too much stock
> into that since I hardly know the code base at all. I plan to mess with the
> code a little now that I know how to compile single modules. If I find
> something substantial that this might be a bug, I'll open a new issue about
> it.
> 

The gpu writes to it.  The driver only reads from it.  The GPU shadows the
value in system memory so that the driver doesn't have to read the register
directly.