linux-kernel - Re: [PATCH v6 1/2] dt-bindings: i2c: add bus-reset-gpios property

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Date:   Wed, 15 Nov 2023 21:53:45 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "wsa@...nel.org" <wsa@...nel.org>,
        "andi.shyti@...nel.org" <andi.shyti@...nel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "conor+dt@...nel.org" <conor+dt@...nel.org>,
        "gregory.clement@...tlin.com" <gregory.clement@...tlin.com>
CC:     "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/2] dt-bindings: i2c: add bus-reset-gpios property

Hi Krystof,

On 16/11/23 10:29, Krzysztof Kozlowski wrote:
> On 15/11/2023 04:57, Chris Packham wrote:
>> Add bus-reset-gpios and bus-reset-duration-us properties to the binding
>> description for i2c busses. These can be used to describe hardware where
>> a common reset GPIO is connected to all downstream devices on and I2C
>> bus. This reset will be asserted then released before the downstream
>> devices on the bus are probed.
>>
>> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
>> ---
>>
> ...
>
>>   Documentation/devicetree/bindings/i2c/i2c.txt | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/i2c/i2c.txt b/Documentation/devicetree/bindings/i2c/i2c.txt
>> index fc3dd7ec0445..3f95d71b9985 100644
>> --- a/Documentation/devicetree/bindings/i2c/i2c.txt
>> +++ b/Documentation/devicetree/bindings/i2c/i2c.txt
>> @@ -99,6 +99,14 @@ wants to support one of the below features, it should adapt these bindings.
>>   	indicates that the system is accessible via this bus as an endpoint for
>>   	MCTP over I2C transport.
>>   
>> +- bus-reset-gpios:
>> +	GPIO pin providing a common reset for all downstream devices. This GPIO
>> +	will be asserted then released before the downstream devices are probed.
> I initially reviewed it, but did not think enough about it. After more
> consideration, I believe this is not a property of the I2C bus
> controller. This is a property of each device, even if the GPIO is the same.
>
> Linux kernel already supports shared GPIO, so you only need
> enable-ref-counting on it.

That's the kind of breadcrumb I need. Although I can't see 
enable-ref-counting as any kind of DT property. Do you mean 
GPIOD_FLAGS_BIT_NONEXCLUSIVE?

> Putting it into the controller bindings looks like solving OS issue with
> incorrect hardware description.
Yes that's entirely whats happening here.
> Best regards,
> Krzysztof
>