linux-kernel - Re: [PATCH 2/2] phy: qcom-mipi-csi2: Add a CSI2 MIPI D-PHY driver

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Message-ID: <33d76d7f-ab14-4e76-8ffb-eb370901a046@linaro.org>
Date: Tue, 22 Jul 2025 10:32:15 +0200
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
 Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
 Vladimir Zapolskiy <vladimir.zapolskiy@...aro.org>,
 Vinod Koul <vkoul@...nel.org>, Kishon Vijay Abraham I <kishon@...nel.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
 linux-media@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] phy: qcom-mipi-csi2: Add a CSI2 MIPI D-PHY driver

On 21/07/2025 18:16, Bryan O'Donoghue wrote:
> On 21/07/2025 16:46, neil.armstrong@...aro.org wrote:
>> On 15/07/2025 11:33, Konrad Dybcio wrote:
>>> On 7/15/25 11:20 AM, Vladimir Zapolskiy wrote:
>>>> On 7/15/25 12:01, Konrad Dybcio wrote:
>>>>> On 7/15/25 8:35 AM, Vladimir Zapolskiy wrote:
>>>>>> On 7/15/25 03:13, Bryan O'Donoghue wrote:
>>>>>>> On 14/07/2025 16:30, Vladimir Zapolskiy wrote:
>>>>>>>>>
>>>>>>>>> I think that is genuinely something we should handle in camss-csid.c
>>>>>>>>> maybe with some meta-data inside of the ports/endpoints..
>>>>>>>>>
>>>>>>>>
>>>>>>>> This is a CSIPHY property, a CSIPHY hardware configuration and a wiring
>>>>>>>> of sensors to a CSIPHY. Where is the relation to CSID here? There is no.
>>>>>>>
>>>>>>> All the PHY really needs to know is the # of lanes in aggregate, which
>>>>>>> physical lanes to map to which logical lanes and the pixel clock.
>>>>>>>
>>>>>>> We should add additional support to the Kernel's D-PHY API parameters
>>>>>>> mechanism to support that physical-to-logical mapping but, that's not
>>>>>>> required for this series or for any currently know upstream user of CAMSS.
>>>>>>>
>>>>>>>> Please share at least a device tree node description, which supports
>>>>>>>> a connection of two sensors to a single CSIPHY, like it shall be done
>>>>>>>> expectedly.
>>>>>>> &camss {
>>>>>>>          port@0 {
>>>>>>>              csiphy0_lanes01_ep: endpoint0 {
>>>>>>>                  data-lanes = <0 1>;
>>>>>>>                  remote-endpoint = <&sensor0_ep>;
>>>>>>>              };
>>>>>>>
>>>>>>>              csiphy0_lanes23_ep: endpoint0 {
>>>>>>>                  data-lanes = <2 3>;
>>>>>>>                  remote-endpoint = <&sensor1_ep>;
>>>>>>>              };
>>>>>>>           };
>>>>>>> };
>>>>>>
>>>>>> Don't you understand that this is broken?.. That's no good.
>>>>>>
>>>>>> Please listen and reread the messages given to you above, your proposed
>>>>>> "solution" does not support by design a valid hardware setup of two
>>>>>> sensors connected to the same CSIPHY.
>>>>>>
>>>>>> I would propose to stop force pushing an uncorrectable dt scheme, it
>>>>>> makes no sense.
>>>>>
>>>>> If all you're asking for is an ability to grab an of_graph reference
>>>>> from the camss (v4l2) driver, you can simply do something along the
>>>>> lines of of_graph_get_remote_port(phy->dev->of_node)
>>>>>
>>>>
>>>> It's not about the driver specifics, my comment is about a proper
>>>> hardware description in dts notation, please see the device tree node
>>>> names.
>>>
>>> I'm a little lost on what you're trying to argue for..
>>>
>>> I could make out:
>>>
>>> 1. "the phy should be a multimedia device"
>>> 2. "There is no ports at all, which makes the device tree node unusable,
>>>     since you can not provide a way to connect any sensors to the phy."
>>>
>>> I don't really understand #1.. maybe that could be the case if the PHY
>>> has a multitude of tunables (which I don't know if it does, but wouldn't
>>> be exactly surprised if it did) that may be usecase/pipeline-specific
>>>
>>> As for #2, I do think it makes sense to connect the sensors to the PHY,
>>> as that's a representation of electrical signals travelling from the
>>> producer to the consumer (plus the data passed in e.g. data-lanes is
>>> directly related to the PHY and necessarily consumed by its driver)
>>
>> The port/endpoint should represent the data flow, and if the signal is the following:
>>
>> sensor -> csiphy -> csid
> 
> I'll be honest.
> 
> I looked at your upstreamed code
> 
> drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c Documentation/devicetree/bindings/parch/arm64/boot/dts/amlogic/meson-khadas-vim3-ts050.dtsoc/meson-axg.dtsi
> 
> And didn't really think CSIPHY needed to be included in the data-graph.

This is DSI, but I understand your point.

The whole key point here is the combo mode, as I understood the combo mode feature
makes the PHY lanes available as 2 separate streams, like if you got 2 "controllers"
attached to the same PHY. So in fact, the PHY should have a single node, but 2 PHY
interfaces in combo mode.

This makes all this controller/phy model very complex to handle and add a lot of
logic in the camss side. Moving the "csiphy" as an independent media device that
can declare up to 2 endpoints in combo mode makes things much simpler, and allows
us to attach each "csiphy" stream to any "controller" side of camss.

Neil

> 
> ---
> bod