Re: [RFC PATCH 1/5] dt-bindings: dma: Add CMA Heap bindings

[Florent Tomasin <florent.tomasin@arm.com>]

Hi Rob

On 30/01/2025 23:20, Rob Herring wrote:
> 
> Why would panthor need CMA, it has an MMU.
> 
> In any case, I agree with Maxime that this is redundant.
> 

This is correct, the GPU has an MMU. The reason I introduced this custom
CMA DTB entry is to allow creation of a standalone DMA heap which can be
retrieved by Panthor using the API exposed by:
-
https://lore.kernel.org/lkml/20230911023038.30649-1-yong.wu@mediatek.com/#t

My understanding might be wrong, I am under the impression that current
CMA driver only has `dma_contiguous_default_area` as standalone
carved-out heap and we cannot have more than one. Please correct me if
this is invalid.

With the DMA Heap API I based the RFC on, Panthor kernel driver does not
manage the protected heap itself, it relies on an exporter to do it. On
some system the secure heap will communicate with a secure FW, on others
it will be a carved-out memory with restricted access. This is
integration specific. Panthor kernel driver will expect to import a DMA
buffer obtained from a heap.

For the development of the protected mode feature, I decided to modify
the CMA driver to create a standalone DMA heap and allocate a DMA buffer
from it. It helped me abstract the importing of a heap in Panthor kernel
driver. Someone may use a different heap driver to reproduce the setup.

* Additional information to help with the context:
Mali CSF GPU requires protected memory at the device level which does
not belong to a user space process in order to allow the FW to enter
protected mode. There is a single FW per GPU instance and the FW is
loaded a probe time.

Regards,
Florent