From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bitan Biswas Subject: Re: [PATCH V4 6/6] i2c: tegra: remove BUG, BUG_ON Date: Thu, 13 Jun 2019 04:33:50 -0700 Message-ID: <71e9eb84-2d40-7ab5-1667-782b4f071c07@nvidia.com> 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> <740f00e5-728a-e31d-28bb-193c7061753c@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <740f00e5-728a-e31d-28bb-193c7061753c@gmail.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Dmitry Osipenko , 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 List-Id: linux-tegra@vger.kernel.org On 6/12/19 6:33 AM, Dmitry Osipenko wrote: > 11.06.2019 21:22, Bitan Biswas =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >> >> >> On 6/11/19 4:34 AM, Dmitry Osipenko wrote: >>> 11.06.2019 10:38, Bitan Biswas =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>> >>>> >>>> On 6/10/19 2:00 PM, Dmitry Osipenko wrote: >>>>> 10.06.2019 22:41, Bitan Biswas =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>>>> >>>>>> >>>>>> On 6/10/19 11:12 AM, Dmitry Osipenko wrote: >>>>>>> 10.06.2019 20:08, Bitan Biswas =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>>>>>> 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 >>>>>>>> --- >>>>>>>> =C2=A0=C2=A0=C2=A0 drivers/i2c/busses/i2c-tegra.c | 11 ++++++----= - >>>>>>>> =C2=A0=C2=A0=C2=A0 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) >>>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * prevent overwr= iting past the end of buf >>>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */ >>>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (rx_fifo_avail > 0 = && buf_remaining > 0) { >>>>>>>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 BUG_ON(buf_remaining >= 3); >>>>>>> >>>>>>> Actually error should be returned here since out-of-bounds memory >>>>>>> accesses must be avoided, hence: >>>>>>> >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (WARN_ON_ONCE(buf_remaining= > 3)) >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return -EIN= VAL; >>>>>> 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/i2= c-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/i2= c-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_avai= l >>> becomes zero and hence the nibbles won't be copied. Please take a close= r >>> look, the current code is correct, but the buf_remaining > 3 is unneede= d >>> because it can't ever happen. >>> >>> The code is structured the way that it's difficult to follow, apparentl= y >>> the person who added the BUG_ON check in the first place couldn't follo= w >>> 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: >>>>>>> >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (WARN_ON_ONCE(!i2c_dev->msg= _buf_remaining) || >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 tegra_i2c_e= mpty_rx_fifo(i2c_dev)) { >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 i2c_dev->ms= g_err |=3D I2C_ERR_UNKNOWN_INTERRUPT; >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 goto err; >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} >>>>>>> >>>>>> Can you please elaborate why the condition needs to be as follows >>>>>> instead of " if (WARN_ON_ONCE(i2c_dev->msg_buf_remaining)) " ? >>>>>> >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (WARN_ON_ONCE(!i2c_dev->ms= g_buf_remaining) || >>>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 tegra= _i2c_empty_rx_fifo(i2c_dev)) { >>>>> >>>>> Because this is a "receive" transfer and hence it is a error conditio= n >>>>> 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 the= re >>>>> 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 enoug= h >>>>> space in the buffer. Then it could be: >>>>> >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (tegra_i2c= _empty_rx_fifo(i2c_dev)) { >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 i2c_dev->msg_err |=3D I2C_ERR_UNKNOWN_= INTERRUPT; >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 goto err; >>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} >>>>> >>>> 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/i2= c-tegra.c#L860 >>>> >>>> >>>> >>>> So we can put the "WARN_ON_ONCE(i2c_dev->msg_buf_remaining) with msg_e= rr >>>> 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 i= t >>> with the first case. >>> >> Is the Overflow case pointed above corresponding to when >> msg_buf_remaining is zero? >=20 > Yes! >=20 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 b= uf. >> >> 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 >=20 > That addresses the "underflow" case. I'm not suggesting to replace it at > all. I was talking about replacing this and nothing else: >=20 > https://elixir.bootlin.com/linux/v5.2-rc4/source/drivers/i2c/busses/i2c-t= egra.c#L862 I would update the patch for the Overflow case around the line 862 pointed. >=20 >> Can you please also review the newly added patch "V5 6/7 "that was newly >> posted? I think it is needed. >=20 > Sure. >=20 Thanks. Based on your review plan to abandon the patch "V5 6/7". -regards, Bitan