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

["Paul-pl Chen (陳柏霖)" <Paul-pl.Chen@mediatek.com>]

On Fri, 2025-01-10 at 14:01 +0100, Krzysztof Kozlowski wrote:
> 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> On 10/01/2025 13:33, paul-pl.chen wrote:
> > From: "Paul-pl.Chen" <paul-pl.chen@mediatek.com>
> > 
> > Add mediatek,exdma.yaml to support EXDMA for MT8196.
> > 
> > Signed-off-by: Paul-pl.Chen <paul-pl.chen@mediatek.com>
> > ---
> > The header used in examples:
> > #include <dt-bindings/clock/mt8196-clk.h>
> > #include <dt-bindings/power/mt8196-power.h>
> > are not upstreamed yet.
> 
> Which makes this untestable and unmergeable.
> 
> This cannot be accepted. Fix your dependencies or decouple from them.
> 
> > It will be sent by related owner soon.
> 
> Still this won't build and won't be possible to apply.
> 
> > ---
> >  .../display/mediatek/mediatek,exdma.yaml      | 77
> > +++++++++++++++++++
> >  1 file changed, 77 insertions(+)
> >  create mode 100644
> > Documentation/devicetree/bindings/display/mediatek/mediatek,exdma.y
> > aml
> > 
> > diff --git
> > a/Documentation/devicetree/bindings/display/mediatek/mediatek,exdma
> > .yaml
> > b/Documentation/devicetree/bindings/display/mediatek/mediatek,exdma
> > .yaml
> > new file mode 100644
> > index 000000000000..385f5549dfaa
> > --- /dev/null
> > +++
> > 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
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.

> 
> > @@ -0,0 +1,77 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id:
> > https://urldefense.com/v3/__http://devicetree.org/schemas/display/mediatek/mediatek,exdma.yaml*__;Iw!!CTRNKA9wMg0ARbw!ldlqohpAoMyTt24UKnssKOk5Qvmc_wlvQPCdjneKDCeshDPwI5Uuuy4A2sI2RlfYLIFDKZx_-GGDOlX48Q$
> > +$schema:
> > https://urldefense.com/v3/__http://devicetree.org/meta-schemas/core.yaml*__;Iw!!CTRNKA9wMg0ARbw!ldlqohpAoMyTt24UKnssKOk5Qvmc_wlvQPCdjneKDCeshDPwI5Uuuy4A2sI2RlfYLIFDKZx_-GHpxO9DFQ$
> > +
> > +title: MediaTek EXDMA
> > +
> > +maintainers:
> > +  - Chun-Kuang Hu <chunkuang.hu@kernel.org>
> > +  - Philipp Zabel <p.zabel@pengutronix.de>
> > +
> > +description:
> > +  The MediaTek display overlap extended DMA engine, namely
> > OVL_EXDMA or EXDMA,
> > +  primarily functions as a DMA engine for reading data from DRAM
> > with various
> > +  DRAM footprints and data formats. For input sources in certain
> > color formats
> > +  and color domains, OVL_EXDMA also includes a color transfer
> > function
> > +  to process pixels into a consistent color domain.
> > +
> 
> Missing ref to dma schemas.

allOf:
- $ref: dma-controller.yaml#
> 
> > +properties:
> > +  compatible:
> > +    const: mediatek,mt8196-exdma
> > +
> > +  reg:
> > +    maxItems: 1
> > +
> > +  clocks:
> > +    items:
> > +      - description: EXDMA Clock
> > +
> > +  power-domains:
> > +    maxItems: 1
> > +
> > +  mediatek,larb:
> > +    $ref: /schemas/types.yaml#/definitions/phandle-array
> 
> Why array? And isn't the property named mediatek,larbs?
> 
Using "mediatek,larb" here is correct because the EXDMA hardware IP
will only have one mediatek,larb. In the next version, we will change
the phandle-array definition to a single phandle.
Please refer to this link:
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/Documentation/devicetree/bindings?id=6d0990e6e844cfa045b1a7348f58964caceb4de4
Since MT8196 use SMMU, the SMMU can no longer internally determine the
connection relationship between smi-larb and the consumer's
pm_runtime_get(_sync). Therefore, we need to add this information back.
> > 
> > 
> > Best, Paul