Date:	Fri, 7 Feb 2014 14:20:07 +0530
From:	Naveen Krishna Ch <naveenkrishna.ch@...il.com>
To:	Wolfram Sang <w.sang@...gutronix.de>
Cc:	Naveen Krishna Chatradhi <ch.naveen@...sung.com>,
	linux-arm-kernel@...ts.infradead.org,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, linux-i2c@...r.kernel.org,
	Naveen Krishna <naveen@...omium.org>, sjg@...omium.org,
	grundler@...omium.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c-s3c2410: Leave the bus disabled unless it is in use

Hello Wolfram,

Sorry for a replying after really long time.

On 24 January 2013 17:50, Wolfram Sang <w.sang@...gutronix.de> wrote:
> On Thu, Nov 29, 2012 at 10:35:34AM +0530, Naveen Krishna Chatradhi wrote:
>> From: Simon Glass <sjg@...omium.org>
>>
>> There is a rather odd feature of the exynos i2c controller that if it
>> is left enabled, it can lock itself up with the clk line held low.
>> This makes the bus unusable.
>>
>> Unfortunately, the s3c24xx_i2c_set_master() function does not notice
>> this, and reports a timeout. From then on the bus cannot be used until
>> the AP is rebooted.
>>
>> The problem happens when any sort of interrupt occurs (e.g. due to a
>> bus transition) when we are not in the middle of a transaction. We
>> have seen many instances of this when U-Boot leaves the bus apparently
>> happy, but Linux cannot access it.
>>
>> The current code is therefore pretty fragile.
>>
>> This fixes things by leaving the bus disabled unless we are actually
>> in a transaction. We enable the bus at the start of the transaction and
>> disable it at the end. That way we won't get interrupts and will not
>> lock up the bus.
>>
>> It might be possible to clear pending interrupts on start-up, but this
>> seems to be a more robust solution. We can't service interrupts when
>> we are not in a transaction, and anyway would rather not lock up the
>> bus while we try.
>>
>> Signed-off-by: Simon Glass <sjg@...omium.org>
>> Cc: Grant Grundler <grundler@...omium.org>
>> Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@...sung.com>
>
> So, I assume this patch is still needed despite the ongoing discussion
> about arbitration?
Yes, this is an i2c crontroller fix.
>
>> ---
>>  drivers/i2c/busses/i2c-s3c2410.c |   37 +++++++++++++++++++++++++++++++++----
>>  1 file changed, 33 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
>> index e93e7d6..2fd346d 100644
>> --- a/drivers/i2c/busses/i2c-s3c2410.c
>> +++ b/drivers/i2c/busses/i2c-s3c2410.c
>> @@ -186,6 +186,31 @@ static inline void s3c24xx_i2c_enable_irq(struct s3c24xx_i2c *i2c)
>>       writel(tmp | S3C2410_IICCON_IRQEN, i2c->regs + S3C2410_IICCON);
>>  }
>>
>> +/*
>> + * Disable the bus so that we won't get any interrupts from now on, or try
>> + * to drive any lines. This is the default state when we don't have
>> + * anything to send/receive.
>> + *
>> + * If there is an event on the bus, or we have a pre-existing event at
>
> Otherwise, if...
>
>> + * kernel boot time, we may not notice the event and the I2C controller
>> + * will lock the bus with the I2C clock line low indefinitely.
>> + */
>> +static inline void s3c24xx_i2c_disable_bus(struct s3c24xx_i2c *i2c)
>> +{
>> +     unsigned long tmp;
>> +
>> +     /* Stop driving the I2C pins */
>> +     tmp = readl(i2c->regs + S3C2410_IICSTAT);
>> +     tmp &= ~S3C2410_IICSTAT_TXRXEN;
>> +     writel(tmp, i2c->regs + S3C2410_IICSTAT);
>> +
>> +     /* We don't expect any interrupts now, and don't want send acks */
>> +     tmp = readl(i2c->regs + S3C2410_IICCON);
>> +     tmp &= ~(S3C2410_IICCON_IRQEN | S3C2410_IICCON_IRQPEND |
>> +             S3C2410_IICCON_ACKEN);
>> +     writel(tmp, i2c->regs + S3C2410_IICCON);
>> +}
>> +
>>
>>  /* s3c24xx_i2c_message_start
>>   *
>> @@ -646,7 +671,11 @@ static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
>>
>>       s3c24xx_i2c_wait_idle(i2c);
>>
>> +     s3c24xx_i2c_disable_bus(i2c);
>> +
>>   out:
>> +     i2c->state = STATE_IDLE;
>> +
>
> Why is the state change after the label?
As the interrupts are enabled in the beginning of this function.
and interrupts in STATE_IDLE needs handling in a different way.

This was added with the intention the irq routine will handle the cases
>
>>       return ret;
>>  }
>>
>> @@ -912,7 +941,6 @@ static void s3c24xx_i2c_dt_gpio_free(struct s3c24xx_i2c *i2c)
>>
>>  static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
>>  {
>> -     unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;
>>       struct s3c2410_platform_i2c *pdata;
>>       unsigned int freq;
>>
>> @@ -926,12 +954,12 @@ static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
>>
>>       dev_info(i2c->dev, "slave address 0x%02x\n", pdata->slave_addr);
>>
>> -     writel(iicon, i2c->regs + S3C2410_IICCON);
>> +     writel(0, i2c->regs + S3C2410_IICCON);
>> +     writel(0, i2c->regs + S3C2410_IICSTAT);
>>
>>       /* we need to work out the divisors for the clock... */
>>
>>       if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) {
>> -             writel(0, i2c->regs + S3C2410_IICCON);
>>               dev_err(i2c->dev, "cannot meet bus frequency required\n");
>>               return -EINVAL;
>>       }
>> @@ -939,7 +967,8 @@ static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
>>       /* todo - check that the i2c lines aren't being dragged anywhere */
>>
>>       dev_info(i2c->dev, "bus frequency set to %d KHz\n", freq);
>> -     dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02lx\n", iicon);
>> +     dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02x\n",
>> +             readl(i2c->regs + S3C2410_IICCON));
>>
>
> Regards,
>
>    Wolfram
Will re-base and post the patch..
>
> --
> Pengutronix e.K.                           | Wolfram Sang                |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |



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