Re: [PATCH v4 17/17] staging: iio: resolver: ad2s1210: simplify code with guard(mutex)

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 10 Oct 2023 12:40:03 -0500
David Lechner <dlechner@baylibre.com> wrote:

> On Tue, Oct 10, 2023 at 11:17 AM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > On Thu,  5 Oct 2023 19:50:34 -0500
> > David Lechner <dlechner@baylibre.com> wrote:
> >  
> > > We can simplify the code and get rid of most of the gotos by using
> > > guard(mutex) from cleanup.h.  
> > You could consider scoped_guard() for a few cases in here, but perhaps
> > it's better to be consistent and always use the guard() version.  
> 
> Yes, there it doesn't look like there are any cases where there is any
> long-running operation that could be done after unlocking the mutex,
> so I went with the simpler approach everywhere.
> 
> >
> > There is a small timing question wrt to the gpio manipulation inline.
> >  
> > >
> > > Signed-off-by: David Lechner <dlechner@baylibre.com>
> > > ---
> > >
> > > v4 changes: New patch in v4.
> > >
> > >  drivers/staging/iio/resolver/ad2s1210.c | 157 ++++++++++----------------------
> > >  1 file changed, 50 insertions(+), 107 deletions(-)
> > >
> > > diff --git a/drivers/staging/iio/resolver/ad2s1210.c b/drivers/staging/iio/resolver/ad2s1210.c
> > > index c4e1bc22e8b0..c4e0ffa92dc2 100644
> > > --- a/drivers/staging/iio/resolver/ad2s1210.c
> > > +++ b/drivers/staging/iio/resolver/ad2s1210.c
> > > @@ -47,6 +47,7 @@
> > >
> > >  #include <linux/bitfield.h>
> > >  #include <linux/bits.h>
> > > +#include <linux/cleanup.h>
> > >  #include <linux/clk.h>
> > >  #include <linux/delay.h>
> > >  #include <linux/device.h>
> > > @@ -404,11 +405,13 @@ static int ad2s1210_single_conversion(struct iio_dev *indio_dev,
> > >       s64 timestamp;
> > >       int ret;
> > >
> > > -     mutex_lock(&st->lock);
> > > +     guard(mutex)(&st->lock);
> > > +
> > >       gpiod_set_value(st->sample_gpio, 1);
> > >       timestamp = iio_get_time_ns(indio_dev);
> > >       /* delay (6 * tck + 20) nano seconds */
> > >       udelay(1);
> > > +     gpiod_set_value(st->sample_gpio, 0);
> > >
> > >       switch (chan->type) {
> > >       case IIO_ANGL:
> > > @@ -418,14 +421,13 @@ static int ad2s1210_single_conversion(struct iio_dev *indio_dev,
> > >               ret = ad2s1210_set_mode(st, MOD_VEL);
> > >               break;
> > >       default:
> > > -             ret = -EINVAL;
> > > -             break;
> > > +             return -EINVAL;
> > >       }
> > >       if (ret < 0)
> > > -             goto error_ret;
> > > +             return ret;
> > >       ret = spi_read(st->sdev, &st->sample, 3);
> > >       if (ret < 0)
> > > -             goto error_ret;
> > > +             return ret;
> > >
> > >       switch (chan->type) {
> > >       case IIO_ANGL:
> > > @@ -437,17 +439,11 @@ static int ad2s1210_single_conversion(struct iio_dev *indio_dev,
> > >               ret = IIO_VAL_INT;
> > >               break;
> > >       default:
> > > -             ret = -EINVAL;
> > > -             break;
> > > +             return -EINVAL;
> > >       }
> > >
> > >       ad2s1210_push_events(indio_dev, st->sample.fault, timestamp);
> > >
> > > -error_ret:
> > > -     gpiod_set_value(st->sample_gpio, 0);
> > > -     /* delay (2 * tck + 20) nano seconds */
> > > -     udelay(1);  
> >
> > Dropping this delay isn't obviously safe (though it probably is given stuff done before we exit).
> > I assume there are no rules on holding the gpio down for the register read.  
> 
> Correct. The SAMPLE gpio only needs to be held for a short time (~350
> nanoseconds) to latch in the current values, then it doesn't matter
> when it is released. (Figure 35 in datasheet)
> 
> >
> > If nothing else I think the patch description needs to made an argument for why it is fine.  
> 
> The longest possible delay needed after releasing the SAMPLE line
> before reasserting is ~350 nanoseconds. Is there a rule of thumb for
> deciding when there are enough instructions that no processor could
> execute them faster than this vs. when we should add an explicit
> delay?
Almost always use an explicit delay as then we don't have to think about it.

> 
> I think I will consider adding a patch in the next round to refactor
> the SAMPLE toggle to a separate function so we can be sure it is
> handled the same in all cases.
Sounds good.

Jonathan

> 
> >  
> > > -     mutex_unlock(&st->lock);
> > >       return ret;
> > >  }
> > >