linux-kernel - Re: [PATCH v3] i2c: designware: fix idx_write

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Date:   Mon, 5 Jun 2023 13:48:51 +0300
From:   Jarkko Nikula <jarkko.nikula@...ux.intel.com>
To:     Wolfram Sang <wsa@...nel.org>, David Zheng <david.zheng@...el.com>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        andriy.shevchenko@...ux.intel.com, mika.westerberg@...ux.intel.com,
        jsd@...ihalf.com
Subject: Re: [PATCH v3] i2c: designware: fix idx_write_cnt in read loop

On 6/5/23 13:02, Wolfram Sang wrote:
> On Fri, May 26, 2023 at 04:58:26PM +0300, Jarkko Nikula wrote:
>> On 5/24/23 21:14, 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@...el.com>
> 
> Applied to for-current, thanks!
> 
> Someone maybe has a Fixes tag for it?
> 
In my opinion this patch is more improvement rather than a regression fix.

I see it's continuation to the commits dcf1bf648f94 ("i2c: designware: 
Empty receive FIFO in slave interrupt handler") and 3b5f7f10ff6e ("i2c: 
designware: slave should do WRITE_REQUESTED before WRITE_RECEIVED").