On Tue, Oct 10, 2023 at 11:17 AM Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
>
> On Thu, 5 Oct 2023 19:50:34 -0500
> David Lechner <dlechner@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > ---
> >
> > 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?
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.
>
> > - mutex_unlock(&st->lock);
> > return ret;
> > }
> >
