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

[Florent Tomasin]

On 30/01/2025 13:28, Maxime Ripard wrote:
> Hi,
> 
> On Thu, Jan 30, 2025 at 01:08:57PM +0000, Florent Tomasin wrote:
>> Introduce a CMA Heap dt-binding allowing custom
>> CMA heap registrations.
>>
>> * Note to the reviewers:
>> The patch was used for the development of the protected mode
>> feature in Panthor CSF kernel driver and is not initially thought
>> to land in the Linux kernel. It is mostly relevant if someone
>> wants to reproduce the environment of testing. Please, raise
>> interest if you think the patch has value in the Linux kernel.
>>
>> Signed-off-by: Florent Tomasin <florent.tomasin@xxxxxxx>
>> ---
>>  .../devicetree/bindings/dma/linux,cma.yml     | 43 +++++++++++++++++++
>>  1 file changed, 43 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/dma/linux,cma.yml
>>
>> diff --git a/Documentation/devicetree/bindings/dma/linux,cma.yml b/Documentation/devicetree/bindings/dma/linux,cma.yml
>> new file mode 100644
>> index 000000000000..c532e016bbe5
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/dma/linux,cma.yml
>> @@ -0,0 +1,43 @@
>> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/dma/linux-cma.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Custom Linux CMA heap
>> +
>> +description:
>> +  The custom Linux CMA heap device tree node allows registering
>> +  of multiple CMA heaps.
>> +
>> +  The CMA heap name will match the node name of the "memory-region".
>> +
>> +properties:
>> +  compatible:
>> +    enum:
>> +      - linux,cma
>> +
>> +  memory-region:
>> +    maxItems: 1
>> +    description: |
>> +      Phandle to the reserved memory node associated with the CMA Heap.
>> +      The reserved memory node must follow this binding convention:
>> +       - Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>> +
>> +examples:
>> +  - |
>> +    reserved-memory {
>> +      #address-cells = <2>;
>> +      #size-cells = <2>;
>> +
>> +      custom_cma_heap: custom-cma-heap {
>> +        compatible = "shared-dma-pool";
>> +        reg = <0x0 0x90600000 0x0 0x1000000>;
>> +        reusable;
>> +      };
>> +    };
>> +
>> +    device_cma_heap: device-cma-heap {
>> +      compatible = "linux,cma";
>> +      memory-region = <&custom_cma_heap>;
>> +    };
> 
> Isn't it redundant with the linux,cma-default shared-dma-pool property
> already?
> 
> Maxime

Hi Maxime,

Please correct me if my understanding is wrong,

The existing properties: linux,cma-default and shared-dma-pool, do not
allow the creations of multiple standalone CMA heaps, those will create
a single CMA heap: `dma_contiguous_default_area`? Other CMA heaps will
be bound to a driver.

I introduced the "linux,cma" to allow creating multiple standalone CMA
heaps, with the intention of validating the protected mode support on
Mali CSG GPUs. It was included in the RFC in there are interests in
this approach.

Since the Panthor CSF kernel driver does not own or manage a heap,
I needed a way to create a standalone heap. The idea here is for the
kernel driver to be an importer. I relied on a patch series to retrieve
the heap and allocate a DMA buffer from it:
- dma_heap_find()
- dma_heap_buffer_alloc()
- dma_heap_put()

Ref:
https://lore.kernel.org/lkml/20230911023038.30649-1-yong.wu@xxxxxxxxxxxx/#t


Since the protected/secure memory management is integration specific,
I needed a generic way for Panthor to allocate from such heap.

In some scenarios it might be a carved-out memory, in others a FW will
reside in the system (TEE) and require a secure heap driver to allocate
memory (e.g: a similar approach is followd by MTK). Such driver would
implement the allocation and free logic.

Florent