[PATCH v2 6/7] drm/panfrost: Add support for GPU heap allocations

[Steven Price]

On 25/07/2019 17:13, Alyssa Rosenzweig wrote:
>> Either we should prevent mapping of HEAP objects
> 
> I'm good with that. AFAIK, HEAP objects shouldn't be (CPU) mmapped
> anyway in normal use; if you need them mapped (for debugging etc), it's
> no big deal to just.. not set the HEAP flag in debug builds.
> 
> Or do you mean GPU mapping?

Sorry, I was being sloppy again![1] I meant CPU mmapped. I agree there
isn't a strong use case for it.

I've been investigating/testing Panfrost kernel with the Arm Mali blob.
Apparently the blob in some cases creates a SAME_VA GROW_ON_GPF buffer -
since SAME_VA means permanently mapped on the CPU this translated to
mmapping a HEAP object. Why it does this I've no idea.

So I've got an interest in trying to maintain compatibility. kbase
places no restriction on mmapping buffers. The main use in the blob for
this is being able to dump buffers when debugging (i.e. dump buffers
before/after every GPU job). Ideally you also need a way of querying
which pages have been backed by faults (much easier with kbase where
that's always just the number of pages).

Steve

[1] kbase+the blob have different terminology here to Panfrost, I do
sometimes struggle with the idea of "not mapped on the GPU" - it's not
really a concept in kbase. All buffers are "mapped" - they just might be
"growable" and not yet full size... :) Hence "mapped" refers to the CPU.