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Date:   Mon, 22 May 2023 17:58:26 +0300
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Subject: Re: [PATCH] i2c: designware: fix idx_write_cnt in read loop
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To:     David Zheng <david.zheng@intel.com>, linux-i2c@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     andriy.shevchenko@linux.intel.com, mika.westerberg@linux.intel.com,
        jsd@semihalf.com
References: <ZGZpEyITTuoBUEAM@davidzhe-DESK>
From:   Jarkko Nikula <jarkko.nikula@linux.intel.com>
In-Reply-To: <ZGZpEyITTuoBUEAM@davidzhe-DESK>
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Hi

On 5/18/23 21:06, David Zheng wrote:
> With IC_INTR_RX_FULL slave interrupt handler reads data in a loop until
> RX FIFO is empty. When testing with the slave-eeprom, each transaction
> has 2 bytes for address/index and 1 byte for value, the address byte
> can be written as data byte due to dropping STOP condition.
> 
> In the test below, the master continuously writes to the slave, first 2
> bytes are index, 3rd byte is value and follow by a STOP condition.
> 
>   i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D1-D1]
>   i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D2-D2]
>   i2c_write: i2c-3 #0 a=04b f=0000 l=3 [00-D3-D3]
> 
> Upon receiving STOP condition slave eeprom would reset `idx_write_cnt` so
> next 2 bytes can be treated as buffer index for upcoming transaction.
> Supposedly the slave eeprom buffer would be written as
> 
>   EEPROM[0x00D1] = 0xD1
>   EEPROM[0x00D2] = 0xD2
>   EEPROM[0x00D3] = 0xD3
> 
> When CPU load is high the slave irq handler may not read fast enough,
> the interrupt status can be seen as 0x204 with both DW_IC_INTR_STOP_DET
> (0x200) and DW_IC_INTR_RX_FULL (0x4) bits. The slave device may see
> the transactions below.
> 
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1794 : INTR_STAT=0x204
>   0x1 STATUS SLAVE_ACTIVITY=0x0 : RAW_INTR_STAT=0x1790 : INTR_STAT=0x200
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
>   0x1 STATUS SLAVE_ACTIVITY=0x1 : RAW_INTR_STAT=0x1594 : INTR_STAT=0x4
> 
> After `D1` is received, read loop continues to read `00` which is the
> first bype of next index. Since STOP condition is ignored by the loop,
> eeprom buffer index increased to `D2` and `00` is written as value.
> 
> So the slave eeprom buffer becomes
> 
>   EEPROM[0x00D1] = 0xD1
>   EEPROM[0x00D2] = 0x00
>   EEPROM[0x00D3] = 0xD3
> 
> The fix is to use `FIRST_DATA_BYTE` (bit 11) in `IC_DATA_CMD` to split
> the transactions. The first index byte in this case would have bit 11
> set. Check this indication to inject I2C_SLAVE_WRITE_REQUESTED event
> which will reset `idx_write_cnt` in slave eeprom.
> 
> Signed-off-by: David Zheng <david.zheng@intel.com>
> ---
>   drivers/i2c/busses/i2c-designware-core.h  | 2 ++
>   drivers/i2c/busses/i2c-designware-slave.c | 6 ++++--
>   2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-core.h b/drivers/i2c/busses/i2c-designware-core.h
> index c5d87aae39c6..8b85147bd518 100644
> --- a/drivers/i2c/busses/i2c-designware-core.h
> +++ b/drivers/i2c/busses/i2c-designware-core.h
> @@ -123,6 +123,8 @@
>   #define DW_IC_COMP_PARAM_1_SPEED_MODE_HIGH	(BIT(2) | BIT(3))
>   #define DW_IC_COMP_PARAM_1_SPEED_MODE_MASK	GENMASK(3, 2)
>   
> +#define DW_IC_DATA_CMD_FIRST_DATA_BYTE		BIT(11)
> +
>   /*
>    * Sofware status flags
>    */
> diff --git a/drivers/i2c/busses/i2c-designware-slave.c b/drivers/i2c/busses/i2c-designware-slave.c
> index cec25054bb24..9549cbcf50aa 100644
> --- a/drivers/i2c/busses/i2c-designware-slave.c
> +++ b/drivers/i2c/busses/i2c-designware-slave.c
> @@ -170,12 +170,14 @@ static irqreturn_t i2c_dw_isr_slave(int this_irq, void *dev_id)
>   		if (!(dev->status & STATUS_WRITE_IN_PROGRESS)) {
>   			dev->status |= STATUS_WRITE_IN_PROGRESS;
>   			dev->status &= ~STATUS_READ_IN_PROGRESS;
> -			i2c_slave_event(dev->slave, I2C_SLAVE_WRITE_REQUESTED,
> -					&val);
>   		}
>   
>   		do {
>   			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
> +			if (tmp & DW_IC_DATA_CMD_FIRST_DATA_BYTE)
> +				i2c_slave_event(dev->slave,
> +						I2C_SLAVE_WRITE_REQUESTED,
> +						&val);
>   			val = tmp;
>   			i2c_slave_event(dev->slave, I2C_SLAVE_WRITE_RECEIVED,
>   					&val);
I fear this might cause regression on some use case on HW that doesn't 
have the FIRST_DATA_BYTE bit in IC_DATA_CMD. That is available on newer 
Synopsys I2C IPs only. For example my test HW doesn't have it.

This means the I2C_SLAVE_WRITE_REQUESTED is never delivered on these HWs 
that don't implement the FIRST_DATA_BYTE.

My quick tests using i2c-slave-eeprom didn't show regression but I'm 
sure there is a case that will regress because of that.