linux-kernel - Re: [PATCH 2/5] i2c: Add STM32F4 I2C driver

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Date:	Fri, 3 Jun 2016 09:44:43 +0200
From:	"M'boumba Cedric Madianga" <cedric.madianga@...il.com>
To:	Maxime Coquelin <mcoquelin.stm32@...il.com>
Cc:	Patrice Chotard <patrice.chotard@...com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-i2c@...r.kernel.org
Subject: Re: [PATCH 2/5] i2c: Add STM32F4 I2C driver

Hi Maxime,

2016-06-02 18:02 GMT+02:00 Maxime Coquelin <mcoquelin.stm32@...il.com>:
> Hi Cedric,
>
> 2016-06-02 17:35 GMT+02:00 M'boumba Cedric Madianga <cedric.madianga@...il.com>:
>> Hi,
>>
>>>> +
>>>> +/**
>>>> + * stm32f4_i2c_xfer() - Transfer combined I2C message
>>>> + * @i2c_adap: Adapter pointer to the controller
>>>> + * @msgs: Pointer to data to be written.
>>>> + * @num: Number of messages to be executed
>>>> + */
>>>> +static int stm32f4_i2c_xfer(struct i2c_adapter *i2c_adap, struct i2c_msg msgs[],
>>>> +                           int num)
>>>> +{
>>>> +       struct stm32f4_i2c_dev *i2c_dev = i2c_get_adapdata(i2c_adap);
>>>> +       int ret, i;
>>>> +
>>>> +       i2c_dev->busy = true;
>>>> +
>>>> +       ret = clk_prepare_enable(i2c_dev->clk);
>>>> +       if (ret) {
>>>> +               dev_err(i2c_dev->dev, "Failed to prepare_enable clock\n");
>>>> +               return ret;
>>>> +       }
>>>> +
>>>> +       stm32f4_i2c_hw_config(i2c_dev);
>>> Maybe you could call this only at probe and resume time?
>>> You would save some register accesses.
>> Some clarification about this point.
>> We need to call stm32f4_i2c_hw_config before each I2C transfer as at
>> the end of I2C communication the peripheral is automatically disabled
>> and configuration registers are reset.
>
> Ok, but I wonder how the IP knows this is the last i2c message to be sent?
When the IP is enabled we could generated a STOP at any moment by
setting a bit in CR1 register.
At this moment, the IP knows that this is the last byte to be sent.
After STOP generation, I notice that all registers are automatically
set with their reset values

> Or maybe it gets re-initialized as soon as the clk is disabled?
Not really because they are all re-initialized before disabling the clock

>
> Thanks for the inputs,
> Maxime