Re: [PATCH v2 4/4] drm/panthor: Fix an off-by-one in the heap context retrieval logic

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 2 May 2024 15:03:51 +0100
Steven Price <steven.price@arm.com> wrote:

> On 30/04/2024 12:28, Boris Brezillon wrote:
> > ID 0 is reserved to encode 'no-tiler-heap', the heap ID range is
> > [1:MAX_HEAPS_PER_POOL], which we occasionally need to turn into an index
> > in the [0:MAX_HEAPS_PER_POOL-1] when we want to access the context object.  
> 
> This might be a silly question, but do we need ID 0 to be
> "no-tiler-heap"? Would it be easier to e.g. use a negative number for
> that situation and avoid all the off-by-one problems?
> 
> I'm struggling to find the code which needs the 0 value to be special -
> where is it exactly that we encode this "no-tiler-heap" value?

Hm, I thought we were passing the heap handle to the group creation
ioctl, but heap queue/heap association is actually done through a CS
instruction, so I guess you have a point. The only thing that makes a
bit hesitant is that handle=0 is reserved for all other kind of handles
we return, and I think I'd prefer to keep it the same for heap handles.

This being said, we could do the `+- 1` in
panthor_ioctl_tiler_heap_{create,destroy}() to keep things simple in
panthor_heap.c.