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

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Message-ID: <cc043dcd-b46e-4b4b-9b80-0450efde86eb@kernel.org>
Date: Sat, 18 Jan 2025 09:37:17 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Paul-pl Chen (陳柏霖) <Paul-pl.Chen@...iatek.com>,
 "robh@...nel.org" <robh@...nel.org>, "krzk+dt@...nel.org"
 <krzk+dt@...nel.org>, "conor+dt@...nel.org" <conor+dt@...nel.org>,
 "chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: Sunny Shen (沈姍姍) <Sunny.Shen@...iatek.com>,
 Sirius Wang (王皓昱) <Sirius.Wang@...iatek.com>,
 Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
 Xiandong Wang (王先冬)
 <Xiandong.Wang@...iatek.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 Project_Global_Chrome_Upstream_Group
 <Project_Global_Chrome_Upstream_Group@...iatek.com>,
 "linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
 Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>,
 "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
 "fshao@...omium.org" <fshao@...omium.org>,
 "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
 Singo Chang (張興國) <Singo.Chang@...iatek.com>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
 "treapking@...omium.org" <treapking@...omium.org>
Subject: Re: [PATCH 03/12] dt-bindings: display: mediatek: add EXDMA yaml for
 MT8196

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