Re: [PATCH v6 1/2] dt-bindings: i2c: aspeed: support for AST2600-i2cv2

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 03/03/2023 09:28, Ryan Chen wrote:
> Hello Krzysztof,
> 
>> -----Original Message-----
>> From: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>> Sent: Friday, March 3, 2023 4:20 PM
>> To: Ryan Chen <ryan_chen@aspeedtech.com>; Wolfram Sang
>> <wsa@kernel.org>
>> Cc: Joel Stanley <joel@jms.id.au>; Brendan Higgins
>> <brendan.higgins@linux.dev>; Krzysztof Kozlowski
>> <krzysztof.kozlowski+dt@linaro.org>; Andrew Jeffery <andrew@aj.id.au>;
>> devicetree@vger.kernel.org; Philipp Zabel <p.zabel@pengutronix.de>; Rob
>> Herring <robh+dt@kernel.org>; Benjamin Herrenschmidt
>> <benh@kernel.crashing.org>; linux-aspeed@lists.ozlabs.org;
>> linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
>> openbmc@lists.ozlabs.org; linux-i2c@vger.kernel.org
>> Subject: Re: [PATCH v6 1/2] dt-bindings: i2c: aspeed: support for AST2600-i2cv2
>>
>> On 01/03/2023 06:57, Ryan Chen wrote:
>>> Hello Krzysztof,
>>>
>>>> -----Original Message-----
>>>> From: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>>>> Sent: Monday, February 27, 2023 4:25 PM
>>>> To: Ryan Chen <ryan_chen@aspeedtech.com>; Andrew Jeffery
>>>> <andrew@aj.id.au>; Brendan Higgins <brendan.higgins@linux.dev>;
>>>> Benjamin Herrenschmidt <benh@kernel.crashing.org>; Joel Stanley
>>>> <joel@jms.id.au>; Rob Herring <robh+dt@kernel.org>; Krzysztof
>>>> Kozlowski <krzysztof.kozlowski+dt@linaro.org>; Philipp Zabel
>>>> <p.zabel@pengutronix.de>; linux-i2c@vger.kernel.org;
>>>> openbmc@lists.ozlabs.org; devicetree@vger.kernel.org;
>>>> linux-arm-kernel@lists.infradead.org;
>>>> linux-aspeed@lists.ozlabs.org; linux-kernel@vger.kernel.org
>>>> Subject: Re: [PATCH v6 1/2] dt-bindings: i2c: aspeed: support for
>>>> AST2600-i2cv2
>>>>
>>>> On 26/02/2023 04:13, Ryan Chen wrote:
>>>>> Add ast2600-i2cv2 compatible and aspeed,global-regs, aspeed,timeout
>>>>> aspeed,xfer-mode description for ast2600-i2cv2.
>>>>>
>>>>> Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
>>>>> ---
>>>>>  .../devicetree/bindings/i2c/aspeed,i2c.yaml   | 44
>> +++++++++++++++++++
>>>>>  1 file changed, 44 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/i2c/aspeed,i2c.yaml
>>>>> b/Documentation/devicetree/bindings/i2c/aspeed,i2c.yaml
>>>>> index f597f73ccd87..75de3ce41cf5 100644
>>>>> --- a/Documentation/devicetree/bindings/i2c/aspeed,i2c.yaml
>>>>> +++ b/Documentation/devicetree/bindings/i2c/aspeed,i2c.yaml
>>>>> @@ -49,6 +49,25 @@ properties:
>>>>>      description:
>>>>>        states that there is another master active on this bus
>>>>>
>>>>> +  aspeed,timeout:
>>>>> +    type: boolean
>>>>> +    description: I2C bus timeout enable for master/slave mode
>>>>
>>>> Nothing improved here in regards to my last comment.
>>>
>>> Yes, as I know your require is about " DT binding to represent hardware
>> setup"
>>> So I add more description about aspeed,timeout as blow.
>>>
>>> ASPEED SOC chip is server product, i2c bus may have fingerprint connect to
>> another board. And also support hotplug.
>>> The following is board-specific design example.
>>> Board A                                         Board B
>>> -------------------------                       ------------------------
>>> |i2c bus#1(master/slave)  <===fingerprint ===> i2c bus#x (master/slave)|
>>> |i2c bus#2(master)-> tmp i2c device |          |
>> |
>>> |i2c bus#3(master)-> adc i2c device |          |
>> |
>>> -------------------------                       ------------------------
>>>
>>> aspeed,timout properites:
>>> For example I2C controller as slave mode, and suddenly disconnected.
>>> Slave state machine will keep waiting for master clock in for rx/tx transmit.
>>> So it need timeout setting to enable timeout unlock controller state.
>>> And in another side. In Master side also need avoid suddenly slave
>> miss(un-plug), Master will timeout and release the SDA/SCL.
>>>
>>> Do you mean add those description into ore aspeed,timout properites
>> description?
>>
>> You are describing here one particular feature you want to enable in the driver
>> which looks non-scalable and more difficult to configure/use.
>> What I was looking for is to describe the actual configuration you have (e.g.
>> multi-master) which leads to enable or disable such feature in your hardware.
>> Especially that bool value does not scale later to actual timeout values in time
>> (ms)...
>>
>> I don't know I2C that much, but I wonder - why this should be specific to
>> Aspeed I2C and no other I2C controllers implement it? IOW, this looks quite
>> generic and every I2C controller should have it. Adding it specific to Aspeed
>> suggests that either we miss a generic property or this should not be in DT at
>> all (because no one else has it...).
>>
>> Also I wonder, why you wouldn't enable timeout always...
>>
>> +Cc Wolfram,
>> Maybe you know whether bool "timeout" property for one controller makes
>> sense? Why we do not have it for all controllers?
>>
> Because, i2c bus didn’t specific timeout.
> But SMBus defines a clock low time-out, TIMEOUT of 35 ms. 
> 
> It have definition in SMBus specification. 
> http://smbus.org/specs/SMBus_3_1_20180319.pdf
> You can check Page 18, Note3 that have timeout description.

Then you have already property for this - "smbus"?

Best regards,
Krzysztof


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