Re: [PATCH 03/12] dt-bindings: display: mediatek: add EXDMA yaml for MT8196

[Krzysztof Kozlowski <krzk@kernel.org>]

On 17/01/2025 11:36, Paul-pl Chen (陳柏霖) wrote:
>>> b/Documentation/devicetree/bindings/display/mediatek/mediatek,exdma
>>> .yaml
>>
>> Filename matching compatible.
>>
>> Why is this in display? DMA goes to dma.
> 
> Hi Krzysztof ,
> 
> Regarding the issue of the EXDMA driver, we have conducted an internal
> survey of drivers under the DMA subsystem. We found that EXDMA operates


I did not talk about driver. I talked about this patch. Look at patch
title - it starts with dt-bindings. Is here anything about driver? No.
Why do we talk about driver?

> differently from typical DMA drivers, and therefore we believe that the
> EXDMA driver may not be suitable to be placed under the
> driver/mediatek/drm directory. The main reasons are as follows:
> 
> (1)No Memory Allocation within EXDMA Engine:
> The EXDMA engine does not perform memory allocation operations itself.
> Instead, it relies on GEM (Graphics Execution Manager) to allocate
> memory.Traditional DMA drivers often handle their own memory
> allocations, but in the case of EXDMA, memory management is delegated
> to GEM.
> 
> (2)Primary Task of EXDMA:
> The main function of EXDMA is to transfer buffers allocated by GEM to
> the subsequent display pipeline.
> EXDMA serves as a bridge between memory allocated by GEM and the
> display components, rather than acting as a general-purpose DMA engine.
> Based on the points above, we have decided to place the EXDMA driver
> under the DRM display subsystem rather than under the DMA subsystem.


I don't care if it uses GEM or kernel allocator or even 3rd party
allocator. The question is: what is this device? If it is performing
DMA, then it should be placed in "dma" directory. The rdma was placed
differently but as you can easily check: it was never acked/reviewed, so
don't use it as an example.

Of course if it does not perform DMA, then it should not be in dma, but
then I don't agree on using dma-cells here and anything like that in the
driver.

Best regards,
Krzysztof