linux-kernel - Re: [PATCH v3 2/7] i2c: bcm2835: Protect against unexpected TXW/RXR interrupts

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Date:   Tue, 4 Oct 2016 21:24:01 +0200
From:   Noralf Trønnes <noralf@...nnes.org>
To:     Eric Anholt <eric@...olt.net>, wsa@...-dreams.de,
        swarren@...dotorg.org
Cc:     linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/7] i2c: bcm2835: Protect against unexpected TXW/RXR
 interrupts


Den 03.10.2016 21:42, skrev Eric Anholt:
> Noralf Trønnes <noralf@...nnes.org> writes:
>
>> Den 29.09.2016 00:00, skrev Eric Anholt:
>>> Noralf Trønnes <noralf@...nnes.org> writes:
>>>
>>>> If an unexpected TXW or RXR interrupt occurs (msg_buf_remaining == 0),
>>>> the driver has no way to fill/drain the FIFO to stop the interrupts.
>>>> In this case the controller has to be disabled and the transfer
>>>> completed to avoid hang.
>>>>
>>>> (CLKT | ERR) and DONE interrupts are completed in their own paths, and
>>>> the controller is disabled in the transfer function after completion.
>>>> Unite the code paths and do disabling inside the interrupt routine.
>>>>
>>>> Clear interrupt status bits in the united completion path instead of
>>>> trying to do it on every interrupt which isn't necessary.
>>>> Only CLKT, ERR and DONE can be cleared that way.
>>>>
>>>> Add the status value to the error value in case of TXW/RXR errors to
>>>> distinguish them from the other S_LEN error.
>>> I was surprised that not writing the TXW/RXR bits on handling their
>>> interrupts was OK, given that we were doing so before, but it's a level
>>> interrupt and those bits are basically ignored on write.
>>>
>>> This patch and 3, 4, and 6 are:
>>>
>>> Reviewed-by: Eric Anholt <eric@...olt.net>
>>>
>>> Patch 5 is:
>>>
>>> Acked-by: Eric Anholt <eric@...olt.net>
>>>
>>> Note for future debug: The I2C_C_CLEAR on errors will take some time to
>>> resolve -- if you were in non-idle state and I2C_C_READ, it sets an
>>> abort_rx flag and runs through the state machine to send a NACK and a
>>> STOP, I think.  Since we're setting CLEAR without I2CEN, that NACK will
>>> be hanging around queued up for next time we start the engine.
>> Maybe you're able to explain the issues I had with reset:
>> https://github.com/raspberrypi/linux/issues/1653
> One of the questions I think you might have is "what state does the
> controller end up in after the various interrupts?"
>
> ERR:
> - produced if we get a nack that's not at the end of a read.
>
> - Proceeds to repeated start if BCM2835_I2C_C_ST is queued, otherwise
>    stop.
>
> CLKT:
> - Triggered by a counter outside of the state machine when stretching
>    happens and then times out.
>
> - Sets cs_override, which causes proceeding through the state machine as
>    if the clock wasn't getting stretched, until the end of the next byte
>    sent/received.
>
> - According to Wolfram we shouldn't be timing out on clock stretching
>    for i2c, just on the transfer as a whole
>    (https://patchwork.kernel.org/patch/9148431/), so I wrote
>    https://github.com/anholt/linux/commit/894200276239d2e4c60b378bdc52164fcb13af8d.
>    However, I don't see an obvious way to get back to IDLE while the
>    slave is still stretching, without triggering the clock stretching
>    timeout path.

If the transfer times out, whatever the reason, we clear the fifo
(and disable). Doesn't that get us back to IDLE?

Code with my patches:

static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
                 int num)
{
[...]
     bcm2835_i2c_start_transfer(i2c_dev);

     time_left = wait_for_completion_timeout(&i2c_dev->completion,
                         adap->timeout);
     if (!time_left) {
         bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_C,
                    BCM2835_I2C_C_CLEAR);
         dev_err(i2c_dev->dev, "i2c transfer timed out\n");
         return -ETIMEDOUT;
     }

> DONE:
> - Signaled at STOP, and just moves to IDLE state which keeps scl/sda
>    high and waits for a BCM2835_I2C_C_ST while we're not clearing the
>    FIFOs (if you do signal start while the fifos are clearing, the start
>    will hang around until the fifo clear is done).  This is the only way
>    to get to IDLE.
>
> I'm don't think I have an answer to the "what should I do?" question you
> had, but hopefully this helps.