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Date:   Mon, 19 Jul 2021 14:55:58 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     Andrea Merello <andrea.merello@gmail.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Rob Herring <robh+dt@kernel.org>,
        Matt Ranostay <matt.ranostay@konsulko.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-iio <linux-iio@vger.kernel.org>,
        Andrea Merello <andrea.merello@iit.it>
Subject: Re: [PATCH 4/4] iio: imu: add BNO055 serdev driver
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On Mon, Jul 19, 2021 at 10:49:54AM +0200, Andrea Merello wrote:
> Il giorno sab 17 lug 2021 alle ore 17:48 Jonathan Cameron
> <jic23@kernel.org> ha scritto:
> > On Thu, 15 Jul 2021 16:17:42 +0200
> > Andrea Merello <andrea.merello@gmail.com> wrote:

...

> > > +/*
> > > + * Register writes cmd have the following format
> > > + * +------+------+-----+-----+----- ... ----+
> > > + * | 0xAA | 0xOO | REG | LEN | payload[LEN] |
> > > + * +------+------+-----+-----+----- ... ----+
> > > + *
> > > + * Register write responses have the following format
> > > + * +------+----------+
> > > + * | 0xEE | ERROCODE |
> > > + * +------+----------+
> > > + *
> > > + * Register read have the following format
> > > + * +------+------+-----+-----+
> > > + * | 0xAA | 0xO1 | REG | LEN |
> > > + * +------+------+-----+-----+
> > > + *
> > > + * Successful register read response have the following format
> > > + * +------+-----+----- ... ----+
> > > + * | 0xBB | LEN | payload[LEN] |
> > > + * +------+-----+----- ... ----+
> > > + *
> > > + * Failed register read response have the following format
> > > + * +------+--------+
> > > + * | 0xEE | ERRCODE|  (ERRCODE always > 1)
> > > + * +------+--------+
> > > + *
> > > + * Error codes are
> > > + * 01: OK
> > > + * 02: read/write FAIL
> > > + * 04: invalid address
> > > + * 05: write on RO
> > > + * 06: wrong start byte
> > > + * 07: bus overrun
> > > + * 08: len too high
> > > + * 09: len too low
> > > + * 10: bus RX byte timeout (timeout is 30mS)
> > > + *
> > > + *
> > > + * **WORKAROUND ALERT**
> > > + *
> > > + * Serial communication seems very fragile: the BNO055 buffer seems to overflow
> > > + * very easy; BNO055 seems able to sink few bytes, then it needs a brief pause.
> > > + * On the other hand, it is also picky on timeout: if there is a pause > 30mS in
> > > + * between two bytes then the transaction fails (IMU internal RX FSM resets).
> > > + *
> > > + * BMU055 has been seen also failing to process commands in case we send them
> > > + * too close each other (or if it is somehow busy?)
> > > + *
> > > + * One idea would be to split data in chunks, and then wait 1-2mS between
> > > + * chunks (we hope not to exceed 30mS delay for any reason - which should
> > > + * be pretty a lot of time for us), and eventually retry in case the BNO055
> > > + * gets upset for any reason. This seems to work in avoiding the overflow
> > > + * errors, but indeed it seems slower than just perform a retry when an overflow
> > > + * error occur.
> > > + * In particular I saw these scenarios:
> > > + * 1) If we send 2 bytes per time, then the IMU never(?) overflows.
> > > + * 2) If we send 4 bytes per time (i.e. the full header), then the IMU could
> > > + *    overflow, but it seem to sink all 4 bytes, then it returns error.
> > > + * 3) If we send more than 4 bytes, the IMU could overflow, and I saw it sending
> > > + *    error after 4 bytes are sent; we have troubles in synchronizing again,
> > > + *    because we are still sending data, and the IMU interprets it as the 1st
> > > + *    byte of a new command.
> > > + *
> > > + * So, we workaround all this in the following way:
> > > + * In case of read we don't split the header but we rely on retries; This seems
> > > + * convenient for data read (where we TX only the hdr).
> > > + * For TX we split the transmission in 2-bytes chunks so that, we should not
> > > + * only avoid case 2 (which is still manageable), but we also hopefully avoid
> > > + * case 3, that would be by far worse.
> >
> > Nice docs and this sounds terrible!
> 
> Indeed.. If anyone has nicer ideas, or is aware about better
> workaround, I would really love to know...

This needs somebody to go thru data sheet and check for possibilities, what you
described above is not gonna fly. Okay, "in a robust way".

I can't believe there is nothing in the communication protocol that may
increase a robustness.

> > > + */

...

> > > +/* Read operation overhead:
> > > + * 4 bytes req + 2byte resp hdr
> > > + * 6 bytes = 60 bit (considering 1start + 1stop bits).
> > > + * 60/115200 = ~520uS
> > > + * In 520uS we could read back about 34 bytes that means 3 samples, this means
> > > + * that in case of scattered read in which the gap is 3 samples or less it is
> > > + * still convenient to go for a burst.
> > > + * We have to take into account also IMU response time - IMU seems to be often
> > > + * reasonably quick to respond, but sometimes it seems to be in some "critical
> > > + * section" in which it delays handling of serial protocol.
> > > + * By experiment, it seems convenient to burst up to about 5/6-samples-long gap

Missed perial and entire comment needs proper style and space occupation ratio.

> > > + */

...

> > > +     enum {
> > > +             STATUS_OK = 0,  /* command OK */
> > > +             STATUS_FAIL = 1,/* IMU communicated an error */
> > > +             STATUS_CRIT = -1/* serial communication with IMU failed */

enum may be kernel doc described.

> > > +     } cmd_status;

...

> > > +static struct serdev_device_driver bno055_sl_driver = {
> > > +     .driver = {

> > > +             .name = BNO055_SL_DRIVER_NAME,

This is (semi-)ABI and preferably should be hard coded explicitly.

> > > +             .of_match_table = bno055_sl_of_match,
> > > +     },
> > > +     .probe = bno055_sl_probe,
> > > +};

-- 
With Best Regards,
Andy Shevchenko