linux-kernel - Re: [PATCH v5 1/4] dt-bindindgs: i2c: qcom,i2c-geni: Document shared flag

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Message-ID: <835ac8c6-3fbb-4a0d-aa07-716d1c8aad7c@gmail.com>
Date: Tue, 10 Dec 2024 10:09:22 +0100
From: Konrad Dybcio <konradybcio@...il.com>
To: Krzysztof Kozlowski <krzk@...nel.org>,
 Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
 Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>, konrad.dybcio@...aro.org,
 andersson@...nel.org, andi.shyti@...nel.org, linux-arm-msm@...r.kernel.org,
 dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-i2c@...r.kernel.org, conor+dt@...nel.org, agross@...nel.org,
 devicetree@...r.kernel.org, vkoul@...nel.org, linux@...blig.org,
 dan.carpenter@...aro.org, Frank.Li@....com, konradybcio@...nel.org,
 bryan.odonoghue@...aro.org, krzk+dt@...nel.org, robh@...nel.org
Cc: quic_vdadhani@...cinc.com
Subject: Re: [PATCH v5 1/4] dt-bindindgs: i2c: qcom,i2c-geni: Document shared
 flag



On 12/10/24 08:28, Krzysztof Kozlowski wrote:
> On 02/12/2024 15:04, Konrad Dybcio wrote:
>>>>>    >
>>>>> IP blocks like SE can be shared. Here we are talking about I2C sharing.
>>>>> In future it can be SPI sharing. But design wise it fits better to add
>>>>> flag per SE node. Same we shall be adding for SPI too in future.
>>>>
>>>>
>>>> How flag per SE node is relevant? I did not ask to move the property.
>>>>
>>>>>
>>>>> Please let me know your further suggestions.
>>>> We do not talk about I2C or SPI here only. We talk about entire SoC.
>>>> Since beginning. Find other patch proposals and align with rest of
>>>> Qualcomm developers so that you come with only one definition for this
>>>> feature/characteristic. Or do you want to say that I am free to NAK all
>>>> further properties duplicating this one?
>>
>> I'm not sure a single property name+description can fit all possible
>> cases here. The hardware being "shared" can mean a number of different
> 
> Existing property does not explain anything more, either. To recap -
> this block is SE and property is named "se-shared", so basically it is
> equal to just "shared". "shared" is indeed quite vague, so I was
> expecting some wider work here.
> 
> 
>> things, with some blocks having hardware provisions for that, while
>> others may have totally none and rely on external mechanisms (e.g.
>> a shared memory buffer) to indicate whether an external entity
>> manages power to them.
> 
> We have properties for that too. Qualcomm SoCs need once per year for
> such shared properties. BAM has two or three. IPA has two. There are
> probably even more blocks which I don't remember now.

So, the problem is "driver must not toggle GPIO states", because
"the bus controller must not be muxed away from the endpoint".
You can come up with a number of similar problems by swapping out
the quoted text.

We can either describe what the driver must do (A), or what the
reason for it is (B).


If we go with A, we could have a property like:

&i2c1 {
	externally-handled-resources = <(EHR_PINCTRL_STATE | EHR_CLOCK_RATE)>
};

which would be a generic list of things that the OS would have to
tiptoe around, fitting Linux's framework split quite well



or if we go with B, we could add a property like:

&i2c1 {
	qcom,shared-controller;
};

which would hide the implementation details into the driver

I could see both approaches having their place, but in this specific
instance I think A would be more fitting, as the problem is quite
simple.

Krzysztof, thoughts?

Konrad