Re: [PATCH V5 6/7] i2c: tegra: fix PIO rx/tx residual transfer check

[Bitan Biswas <bbiswas@nvidia.com>]

On 6/12/19 6:55 AM, Dmitry Osipenko wrote:
> 11.06.2019 13:51, Bitan Biswas пишет:
>> Fix expression for residual bytes(less than word) transfer
>> in I2C PIO mode RX/TX.
>>
>> Signed-off-by: Bitan Biswas <bbiswas@nvidia.com>
>> ---
>>   drivers/i2c/busses/i2c-tegra.c | 11 ++++++-----
>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
>> index 4dfb4c1..0596c12 100644
>> --- a/drivers/i2c/busses/i2c-tegra.c
>> +++ b/drivers/i2c/busses/i2c-tegra.c
>> @@ -514,7 +514,8 @@ static int tegra_i2c_empty_rx_fifo(struct tegra_i2c_dev *i2c_dev)
>>   	 * If there is a partial word at the end of buf, handle it manually to
>>   	 * prevent overwriting past the end of buf
>>   	 */
>> -	if (rx_fifo_avail > 0 && buf_remaining > 0) {
>> +	if (rx_fifo_avail > 0 &&
>> +	    (buf_remaining > 0 && buf_remaining < BYTES_PER_FIFO_WORD)) {
> 
> The buf_remaining >= BYTES_PER_FIFO_WORD is not possible to happen
> because there are three possible cases:
> 
> 1) buf_remaining > rx_fifo_avail * 4:
> 
> 	In this case rx_fifo_avail = 0
> 
> 2) buf_remaining < rx_fifo_avail * 4;
> 
> 	In this case buf_remaining is always < 4 because
> 	words_to_transfer is a buf_remaining rounded down to 4
> 	and then divided by 4. Hence:
> 
> 	buf_remaining -= (buf_remaining / 4) * 4 always results
> 	into buf_remaining < 4.
> 
> 3) buf_remaining == rx_fifo_avail * 4:
> 
> 	In this case rx_fifo_avail = 0 and buf_remaining = 0.
> 
> Case 2 should never happen and means that something gone wrong.
> 
Yes I now agree with you. The first condition "rx_fifo_avail > 0" 
failure will take care and prevent need for additional checks.

>>   		BUG_ON(buf_remaining > 3);
>>   		val = i2c_readl(i2c_dev, I2C_RX_FIFO);
>>   		val = cpu_to_le32(val);
>> @@ -557,11 +558,10 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
>>   			words_to_transfer = tx_fifo_avail;
>>   
>>   		/*
>> -		 * Update state before writing to FIFO.  If this casues us
>> +		 * Update state before writing to FIFO.  If this causes us
>>   		 * to finish writing all bytes (AKA buf_remaining goes to 0) we
>>   		 * have a potential for an interrupt (PACKET_XFER_COMPLETE is
>> -		 * not maskable).  We need to make sure that the isr sees
>> -		 * buf_remaining as 0 and doesn't call us back re-entrantly.
>> +		 * not maskable).
>>   		 */
>>   		buf_remaining -= words_to_transfer * BYTES_PER_FIFO_WORD;
>>   		tx_fifo_avail -= words_to_transfer;
>> @@ -580,7 +580,8 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
>>   	 * prevent reading past the end of buf, which could cross a page
>>   	 * boundary and fault.
>>   	 */
>> -	if (tx_fifo_avail > 0 && buf_remaining > 0) {
>> +	if (tx_fifo_avail > 0 &&
>> +	    (buf_remaining > 0 && buf_remaining < BYTES_PER_FIFO_WORD)) {
>>   		BUG_ON(buf_remaining > 3);
>>   		memcpy(&val, buf, buf_remaining);
>>   		val = le32_to_cpu(val);
>>
> 
> Same as for RX.
> 
Yes shall discard this patch from the next update.

-Thanks,
  Bitan