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

[Steven Price <steven.price@arm.com>]

On 02/05/2024 15:15, Boris Brezillon wrote:
> 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.

The heap handles returned to user space have the upper 16 bits encoding
the VM ID - so hopefully no one is doing anything crazy and splitting it
up to treat the lower part specially. And (unless I'm mistaken) the VM
IDs start from 1 so we'd still not have IDs of 0. So I don't think we
need the +- 1 part anywhere for tiler heaps.

I'd certainly consider it a user space bug to treat the handles as
anything other than opaque. Really user space shouldn't be treating 0 as
special either: the uAPI doesn't say it's not valid. But I'd be open to
updating the uAPI to say 0 is invalid if there's some desire for that.

Steve