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

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Date:   Thu, 16 Nov 2023 12:37:48 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>,
        "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

On 15/11/2023 22:53, Chris Packham wrote:
> 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?

It's not a feature or property of Devicetree, but missing feature of OS.

Best regards,
Krzysztof