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[91.79.162.197]) by smtp.googlemail.com with ESMTPSA id 17sm4630819wmx.47.2019.06.12.06.33.32 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 12 Jun 2019 06:33:33 -0700 (PDT) Subject: Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON To: Bitan Biswas , Laxman Dewangan , Thierry Reding , Jonathan Hunter , linux-i2c@vger.kernel.org, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Rosin , Wolfram Sang Cc: Shardar Mohammed , Sowjanya Komatineni , Mantravadi Karthik References: <1560186515-30797-1-git-send-email-bbiswas@nvidia.com> <1560186515-30797-6-git-send-email-bbiswas@nvidia.com> <06ab30b6-bf79-c628-0a04-d0307511a06f@gmail.com> <851d7837-5b98-228e-d8c9-3c41be1fb2e0@nvidia.com> From: Dmitry Osipenko Message-ID: <740f00e5-728a-e31d-28bb-193c7061753c@gmail.com> Date: Wed, 12 Jun 2019 16:33:28 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>>>>>> --- >>>>>>>     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! 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 > Can you please also review the newly added patch "V5 6/7 "that was newly > posted? I think it is needed. Sure.