Date:   Thu, 13 Jun 2019 04:33:50 -0700
From:   Bitan Biswas <bbiswas@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Thierry Reding <treding@...dia.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-i2c@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Peter Rosin <peda@...ntia.se>,
        Wolfram Sang <wsa@...-dreams.de>
CC:     Shardar Mohammed <smohammed@...dia.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        Mantravadi Karthik <mkarthik@...dia.com>
Subject: Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON



On 6/12/19 6:33 AM, Dmitry Osipenko wrote:
> 11.06.2019 21:22, Bitan Biswas пишет:
>>
>>
>> On 6/11/19 4:34 AM, Dmitry Osipenko wrote:
>>> 11.06.2019 10:38, Bitan Biswas пишет:
>>>>
>>>>
>>>> On 6/10/19 2:00 PM, Dmitry Osipenko wrote:
>>>>> 10.06.2019 22:41, Bitan Biswas пишет:
>>>>>>
>>>>>>
>>>>>> On 6/10/19 11:12 AM, Dmitry Osipenko wrote:
>>>>>>> 10.06.2019 20:08, Bitan Biswas пишет:
>>>>>>>> Remove redundant BUG_ON calls or replace with WARN_ON_ONCE
>>>>>>>> as needed. Remove BUG() and make Rx and Tx case handling
>>>>>>>> similar.
>>>>>>>>
>>>>>>>> Signed-off-by: Bitan Biswas <bbiswas@...dia.com>
>>>>>>>> ---
>>>>>>>>      drivers/i2c/busses/i2c-tegra.c | 11 ++++++-----
>>>>>>>>      1 file changed, 6 insertions(+), 5 deletions(-)
>>>>>>>
>>>>>>> Looks that this is still not correct. What if it transfer-complete
>>>>>>> flag
>>>>>>> is set and buffer is full on RX? In this case the transfer will
>>>>>>> succeed
>>>>>>> while it was a failure.
>>>>>>>
>>>>>>>> diff --git a/drivers/i2c/busses/i2c-tegra.c
>>>>>>>> b/drivers/i2c/busses/i2c-tegra.c
>>>>>>>> index 4dfb4c1..30619d6 100644
>>>>>>>> --- a/drivers/i2c/busses/i2c-tegra.c
>>>>>>>> +++ b/drivers/i2c/busses/i2c-tegra.c
>>>>>>>> @@ -515,7 +515,6 @@ static int tegra_i2c_empty_rx_fifo(struct
>>>>>>>> tegra_i2c_dev *i2c_dev)
>>>>>>>>           * prevent overwriting past the end of buf
>>>>>>>>           */
>>>>>>>>          if (rx_fifo_avail > 0 && buf_remaining > 0) {
>>>>>>>> -        BUG_ON(buf_remaining > 3);
>>>>>>>
>>>>>>> Actually error should be returned here since out-of-bounds memory
>>>>>>> accesses must be avoided, hence:
>>>>>>>
>>>>>>>        if (WARN_ON_ONCE(buf_remaining > 3))
>>>>>>>            return -EINVAL;
>>>>>> buf_remaining will be less than equal to 3 because of the expression
>>>>>> earlier
>>>>>> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L520
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> Ah yes, indeed!
>>>>>
>>>> I see that I am wrong and buf_remaining > 3 needs to be prevented at
>>>>
>>>> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L528
>>>>
>>>>
>>>>
>>>> because of word_to_transfer is limited to rx_fifo_avail:
>>>>
>>>> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L515
>>>>
>>>>
>>>>
>>>> I shall add the check for less than 3 in both RX and TX cases in a
>>>> separate patch in this series.
>>>
>>> When word_to_transfer is more than rx_fifo_avail, then the rx_fifo_avail
>>> becomes zero and hence the nibbles won't be copied. Please take a closer
>>> look, the current code is correct, but the buf_remaining > 3 is unneeded
>>> because it can't ever happen.
>>>
>>> The code is structured the way that it's difficult to follow, apparently
>>> the person who added the BUG_ON check in the first place couldn't follow
>>> it either. Maybe it's worth to invest some more effort into refactoring
>>> at least that part of the code. At minimum a clarifying comments would
>>> be helpful.
>>>
>> I shall try to add some comments near the BUG_ON check.
>>
>>> [snip]
>>>
>>>>>>> Then here:
>>>>>>>
>>>>>>>        if (WARN_ON_ONCE(!i2c_dev->msg_buf_remaining) ||
>>>>>>>            tegra_i2c_empty_rx_fifo(i2c_dev)) {
>>>>>>>            i2c_dev->msg_err |= I2C_ERR_UNKNOWN_INTERRUPT;
>>>>>>>            goto err;
>>>>>>>        }
>>>>>>>
>>>>>> Can you please elaborate why the condition needs to be as follows
>>>>>> instead of " if (WARN_ON_ONCE(i2c_dev->msg_buf_remaining)) " ?
>>>>>>
>>>>>>>         if (WARN_ON_ONCE(!i2c_dev->msg_buf_remaining) ||
>>>>>>>             tegra_i2c_empty_rx_fifo(i2c_dev)) {
>>>>>
>>>>> Because this is a "receive" transfer and hence it is a error condition
>>>>> if the data-message was already fully received and then there is
>>>>> another
>>>>> request from hardware to receive more data. So
>>>>> "!i2c_dev->msg_buf_remaining" is the error condition here because there
>>>>> is no more space in the buffer.
>>>>>
>>>>> Looking at this again, seems checking for "if
>>>>> (WARN_ON_ONCE(rx_fifo_avail))" in the above hunk [1] will be already
>>>>> enough since a not fully drained RX FIFO means that there is no enough
>>>>> space in the buffer. Then it could be:
>>>>>
>>>>>            if (tegra_i2c_empty_rx_fifo(i2c_dev)) {
>>>>>                    i2c_dev->msg_err |= I2C_ERR_UNKNOWN_INTERRUPT;
>>>>>                    goto err;
>>>>>       }
>>>>>
>>>> In the case "if (status & I2C_INT_PACKET_XFER_COMPLETE) {" , we do not
>>>> have any tegra_i2c_empty_rx_fifo call today. In this current driver I do
>>>> not see any code that checks for the buffer space and prevents RX FIFO
>>>> from being drained. The transfer complete when seen must have already
>>>> consumed all bytes of msg_buf_remaining in the call at the line
>>>>
>>>> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L860
>>>>
>>>>
>>>>
>>>> So we can put the "WARN_ON_ONCE(i2c_dev->msg_buf_remaining) with msg_err
>>>> assignment and goto err" to confirm if some corner case is not handled.
>>>>
>>>> Planning to share updated patch.
>>>
>>> There are two possible error conditions:
>>>
>>> 1) Underflow: the XFER_COMPLETE happens before message is fully sent.
>>>
>>> 2) Overflow: message is fully sent, but there is no XFER_COMPLETE and
>>> then hardware asks to transfer more.
>>>
>>> We are addressing the second case here, while you seems are confusing it
>>> with the first case.
>>>
>> Is the Overflow case pointed above corresponding to when
>> msg_buf_remaining is zero?
> 
> Yes!
> 
OK.

>   If no, what indicates that message is fully
>> sent? I see that if msg_buf_remaining is already zero, the call
>> tegra_i2c_empty_rx_fifo will not do any copy of the bytes from FIFO to buf.
>>
>> One more point that is not clear to me is are the above suggestions you
>> made is corresponding to replacing below line in linux-next ?
>>
>> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L888
> 
> That addresses the "underflow" case. I'm not suggesting to replace it at
> all. I was talking about replacing this and nothing else:
> 
> https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-tegra.c#L862
I would update the patch for the Overflow case around the line 862 pointed.


> 
>> Can you please also review the newly added patch "V5 6/7 "that was newly
>> posted? I think it is needed.
> 
> Sure.
> 
Thanks. Based on your review plan to abandon the patch "V5 6/7".

-regards,
  Bitan

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