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Subject: Re: [PATCH i2c-next v9 5/5] i2c: aspeed: Add bus idle waiting logic
 for multi-master use cases
To:     Wolfram Sang <wsa@the-dreams.de>
Cc:     Brendan Higgins <brendanhiggins@google.com>,
        Rob Herring <robh+dt@kernel.org>,
        Joel Stanley <joel@jms.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Jeffery <andrew@aj.id.au>, 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,
        Jarkko Nikula <jarkko.nikula@linux.intel.com>,
        James Feist <james.feist@linux.intel.com>,
        Vernon Mauery <vernon.mauery@linux.intel.com>
References: <20181030210917.32711-1-jae.hyun.yoo@linux.intel.com>
 <20181030210917.32711-6-jae.hyun.yoo@linux.intel.com>
 <20181128215434.GA1666@kunai>
From:   Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Message-ID: <2772b86e-6ffb-e289-40f2-f96d11b82349@linux.intel.com>
Date:   Wed, 28 Nov 2018 16:30:03 -0600
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On 11/28/2018 3:54 PM, Wolfram Sang wrote:
> On Tue, Oct 30, 2018 at 02:09:16PM -0700, Jae Hyun Yoo wrote:
>> In multi-master environment, this driver's master cannot know
>> exactly when a peer master sends data to this driver's slave so a
>> case can be happened that this master tries to send data through
>> the master_xfer function but slave data from peer master is still
>> being processed by this driver.
> 
> So, that I get this correct: your IP core cannot detect a bus busy state
> when its own slave address is acessed? Well, I know HW can have bugs,
> but I still wonder because a bus is busy as soon as another START has
> been detected, independent of which device is going to be accessed.
> 
> Even if so, why can't you use a mutex to prevent any master_xfer while
> slave_xfer is ongoing?
> 
> Shouldn't that be enough according to the above description? Why do you
> need the delay? I might be missing something...
> 

This IP has a bus busy state bit which can cover master and slave
operations but in case of slave operation, the bit is cleared little
bit earlier than its actual completion so it's the reason why I made
this patch. I agree with you that it still has a weak point since
the bus can be changed to busy state immediately just after finishing
the busy waiting delay.

As you suggested, I'll try to implement it using a mutex instead of
using delay. Thanks a lot for your comment.

Jae

>> To prevent any state corruption in the case, this patch adds
>> checking code if any slave operation is ongoing and it waits up to
>> the bus timeout duration before starting a master_xfer operation.
>>
>> Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
>> Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
>> ---
>>   drivers/i2c/busses/i2c-aspeed.c | 55 ++++++++++++++++++++++++---------
>>   1 file changed, 40 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
>> index 833b6b6a4c7e..30c3ab3a4844 100644
>> --- a/drivers/i2c/busses/i2c-aspeed.c
>> +++ b/drivers/i2c/busses/i2c-aspeed.c
>> @@ -12,6 +12,7 @@
>>   
>>   #include <linux/clk.h>
>>   #include <linux/completion.h>
>> +#include <linux/delay.h>
>>   #include <linux/err.h>
>>   #include <linux/errno.h>
>>   #include <linux/i2c.h>
>> @@ -115,6 +116,9 @@
>>   /* 0x18 : I2CD Slave Device Address Register   */
>>   #define ASPEED_I2CD_DEV_ADDR_MASK			GENMASK(6, 0)
>>   
>> +/* Busy checking */
>> +#define ASPEED_I2C_BUS_BUSY_CHECK_INTERVAL_US		(10 * 1000)
>> +
>>   enum aspeed_i2c_master_state {
>>   	ASPEED_I2C_MASTER_INACTIVE,
>>   	ASPEED_I2C_MASTER_START,
>> @@ -156,6 +160,8 @@ struct aspeed_i2c_bus {
>>   	int				cmd_err;
>>   	/* Protected only by i2c_lock_bus */
>>   	int				master_xfer_result;
>> +	/* Multi-master */
>> +	bool				multi_master;
>>   #if IS_ENABLED(CONFIG_I2C_SLAVE)
>>   	struct i2c_client		*slave;
>>   	enum aspeed_i2c_slave_state	slave_state;
>> @@ -596,27 +602,44 @@ static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
>>   	return irq_remaining ? IRQ_NONE : IRQ_HANDLED;
>>   }
>>   
>> +static int aspeed_i2c_check_bus_busy(struct aspeed_i2c_bus *bus)
>> +{
>> +	unsigned long timeout;
>> +
>> +	if (bus->multi_master) {
>> +		might_sleep();
>> +		/* Initialize it only when multi_master is set */
>> +		timeout = jiffies + bus->adap.timeout;
>> +	}
>> +
>> +	for (;;) {
>> +		if (!(readl(bus->base + ASPEED_I2C_CMD_REG) &
>> +		      ASPEED_I2CD_BUS_BUSY_STS))
>> +#if IS_ENABLED(CONFIG_I2C_SLAVE)
>> +			if (bus->slave_state == ASPEED_I2C_SLAVE_STOP)
>> +#endif
>> +				return 0;
>> +		if (!bus->multi_master)
>> +			break;
>> +		if (time_after(jiffies, timeout))
>> +			break;
>> +		usleep_range((ASPEED_I2C_BUS_BUSY_CHECK_INTERVAL_US >> 2) + 1,
>> +			     ASPEED_I2C_BUS_BUSY_CHECK_INTERVAL_US);
>> +	}
>> +
>> +	return aspeed_i2c_recover_bus(bus);
>> +}
>> +
>>   static int aspeed_i2c_master_xfer(struct i2c_adapter *adap,
>>   				  struct i2c_msg *msgs, int num)
>>   {
>>   	struct aspeed_i2c_bus *bus = i2c_get_adapdata(adap);
>>   	unsigned long time_left, flags;
>> -	int ret = 0;
>>   
>> -	spin_lock_irqsave(&bus->lock, flags);
>> -	bus->cmd_err = 0;
>> -
>> -	/* If bus is busy, attempt recovery. We assume a single master
>> -	 * environment.
>> -	 */
>> -	if (readl(bus->base + ASPEED_I2C_CMD_REG) & ASPEED_I2CD_BUS_BUSY_STS) {
>> -		spin_unlock_irqrestore(&bus->lock, flags);
>> -		ret = aspeed_i2c_recover_bus(bus);
>> -		if (ret)
>> -			return ret;
>> -		spin_lock_irqsave(&bus->lock, flags);
>> -	}
>> +	if (aspeed_i2c_check_bus_busy(bus))
>> +		return -EAGAIN;
>>   
>> +	spin_lock_irqsave(&bus->lock, flags);
>>   	bus->cmd_err = 0;
>>   	bus->msgs = msgs;
>>   	bus->msgs_index = 0;
>> @@ -827,7 +850,9 @@ static int aspeed_i2c_init(struct aspeed_i2c_bus *bus,
>>   	if (ret < 0)
>>   		return ret;
>>   
>> -	if (!of_property_read_bool(pdev->dev.of_node, "multi-master"))
>> +	if (of_property_read_bool(pdev->dev.of_node, "multi-master"))
>> +		bus->multi_master = true;
>> +	else
>>   		fun_ctrl_reg |= ASPEED_I2CD_MULTI_MASTER_DIS;
>>   
>>   	/* Enable Master Mode */
>> -- 
>> 2.19.1
>>