[PATCH v5 1/2] dt-bindings: i2c: Add support for ASPEED i2Cv2

[Krzysztof Kozlowski]

On 22/02/2023 03:59, Ryan Chen wrote:
> Hello Krzysztof,
> 
>> -----Original Message-----
>> From: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>
>> Sent: Tuesday, February 21, 2023 7:05 PM
>> To: Ryan Chen <ryan_chen at aspeedtech.com>; Rob Herring
>> <robh+dt at kernel.org>; Krzysztof Kozlowski
>> <krzysztof.kozlowski+dt at linaro.org>; Joel Stanley <joel at jms.id.au>; Andrew
>> Jeffery <andrew at aj.id.au>; Philipp Zabel <p.zabel at pengutronix.de>;
>> openbmc at lists.ozlabs.org; linux-arm-kernel at lists.infradead.org;
>> linux-aspeed at lists.ozlabs.org; linux-kernel at vger.kernel.org
>> Subject: Re: [PATCH v5 1/2] dt-bindings: i2c: Add support for ASPEED i2Cv2
>>
>> On 21/02/2023 11:42, Ryan Chen wrote:
>>>>>>> +    type: boolean
>>>>>>> +    description: Enable i2c bus timeout for master/slave (35ms)
>>>>>>
>>>>>> Why this is property for DT? It's for sure not bool, but proper
>>>>>> type coming from units.
>>>>> This is i2c controller feature for enable slave mode inactive
>>>>> timeout and also master mode sda/scl auto release timeout.
>>>>> So I will modify to
>>>>>   aspeed,timeout:
>>>>> 	type: boolean
>>>>>     description: I2C bus timeout enable for master/slave mode
>>>>
>>>> This does not answer my concerns. Why this is board specific?
>>> Sorry, can’t catch your point.
>>> It is not board specific. It is controller feature.
>>> ASPEED SOC chip is server product, master connect may have fingerprint
>>> connect to another board. And also support hotplug.
>>> For example I2C controller as slave mode, and suddenly disconnected.
>>> Slave state machine will keep waiting for master clock in for rx/tx transfer.
>>> So it need timeout setting to enable timeout unlock controller state.
>>> And in another side. As master mode, slave is clock stretching.
>>> The master will be keep waiting, until slave release cll stretching.
>>
>> OK, thanks for describing the feature. I still do not see how this is DT related.
> 
> Let me draw more about the board-specific. 
> The following is an example about i2c layout in board.
> Board A														Board B
> --------------------------------------------------------							--------------------------------------------------------
> |    i2c bus#1(master/slave)  <--------------------> fingerprint.(can be unplug)    <--------------------> i2c bus#x (master/slave) |
> |    i2c bus#2(master) -> tmp i2c device     |			     		|									|
> |    i2c bus#3(master) -> adc i2c device      |					|									|
> --------------------------------------------------------							--------------------------------------------------------
> In this case i2c bus#1 need enable timeout, avoid suddenly unplug the connector. That slave will keep state to drive clock stretching.
> So it is specific enable in i2c bus#1. Others is not needed enable timeout. 
> Does this draw is more clear in scenario?

I2C bus #1 works in slave mode? So you always need it for slave work?

> 
>>>
>>> So in those reason add this timeout design in controller.
>>
>> You need to justify why DT is correct place for this property. DT is not for
>> configuring OS, but to describe hardware. I gave you one possibility
>> - why different boards would like to set this property. You said it is not board
>> specific, thus all boards will have it (or none of them).
>> Without any other reason, this is not a DT property. Drop.
>>
>>>>
>>>>>
>>>>>>> +
>>>>>>> +  byte-mode:
>>>>>>> +    type: boolean
>>>>>>> +    description: Force i2c driver use byte mode transmit
>>>>>>
>>>>>> Drop, not a DT property.
>>>>>>
>>>>>>> +
>>>>>>> +  buff-mode:
>>>>>>> +    type: boolean
>>>>>>> +    description: Force i2c driver use buffer mode transmit
>>>>>>
>>>>>> Drop, not a DT property.
>>>>>>
>>>>> The controller support 3 different for transfer.
>>>>> Byte mode: it means step by step to issue transfer.
>>>>> Example i2c read, each step will issue interrupt then enable next step.
>>>>> Sr (start read) | D | D | D | P
>>>>> Buffer mode: it means, the data can prepare into buffer register,
>>>>> then Trigger transfer. So Sr D D D P, only have only 1 interrupt handling.
>>>>> The DMA mode most like with buffer mode, The differ is data prepare
>>>>> in DRAM, than trigger transfer.
>>>>>
>>>>> So, should I modify to
>>>>>   aspeed,byte:
>>>>> 	type: boolean
>>>>>     description: Enable i2c controller transfer with byte mode
>>>>>
>>>>>   aspeed,buff:
>>>>> 	type: boolean
>>>>>     description: Enable i2c controller transfer with buff mode
>>>>
>>>> 1. No, these are not bools but enum in such case.
>>>
>>> Thanks, will modify following.
>>> aspeed,xfer_mode:
>>>     enum: [0, 1, 2]
>>>     description:
>>>       0: byte mode, 1: buff_mode, 2: dma_mode
>>
>> Just keep it text - byte, buffered, dma
>>
>>>
>>>> 2. And why exactly this is board-specific?
>>>
>>> No, it not depends on board design. It is only for register control for
>> controller transfer behave.
>>> The controller support 3 different trigger mode for transfer.
>>> Assign bus#1 ~ 3 : dma tranfer and assign bus#4 ~ 6 : buffer mode
>>> transfer, That can reduce the dram usage.
>>
>> Then anyway it does not look like property for Devicetree. DT describes
>> hardware, not OS behavior.
> 
> The same draw, in this case, i2c bus#1 that is multi-master transfer architecture. 
> Both will inactive with trunk data. That cane enable i2c#1 use DMA transfer to reduce CPU utilized.
> Others (bus#2/3) can keep byte/buff mode. 

Isn't then current bus configuration for I2C#1 known to the driver?
Jeremy asked few other questions around here...

Best regards,
Krzysztof