Re: [PATCH 02/16] iio: adc: at91-sama5d2_adc: lock around oversampling and sample freq
From: Jonathan Cameron <jic23@kernel.org>
Date: 2022-06-11 17:21:54
Also in:
linux-devicetree, linux-iio, lkml
On Thu, 9 Jun 2022 11:31:59 +0300 Claudiu Beznea [off-list ref] wrote:
.read_raw()/.write_raw() could be called asynchronously from user space or other in kernel drivers. Without locking on st->lock these could be called asynchronously while there is a conversion in progress. Read will be harmless but changing registers while conversion is in progress may lead to inconsistent results. Thus, to avoid this lock st->lock.
The patch makes sense, but I'm not convinced all of the changes below involve any changes to registers. E.g. at91_adc_adjust_val_osr() is using the cached value of something in a register, but not the register itself, so please update the description to mention cached state. Other comments inline.
quoted hunk ↗ jump to hunk
Fixes: 27e177190891 ("iio:adc:at91_adc8xx: introduce new atmel adc driver") Fixes: 6794e23fa3fe ("iio: adc: at91-sama5d2_adc: add support for oversampling resolution") Signed-off-by: Claudiu Beznea <redacted> --- drivers/iio/adc/at91-sama5d2_adc.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-)diff --git a/drivers/iio/adc/at91-sama5d2_adc.c b/drivers/iio/adc/at91-sama5d2_adc.c index 32b6f157b803..a672a520cdc0 100644 --- a/drivers/iio/adc/at91-sama5d2_adc.c +++ b/drivers/iio/adc/at91-sama5d2_adc.c@@ -1542,10 +1542,11 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev, ret = at91_adc_read_position(st, chan->channel, &tmp_val); *val = tmp_val; + ret = at91_adc_adjust_val_osr(st, val); mutex_unlock(&st->lock); iio_device_release_direct_mode(indio_dev); - return at91_adc_adjust_val_osr(st, val); + return ret; } if (chan->type == IIO_PRESSURE) { ret = iio_device_claim_direct_mode(indio_dev);@@ -1556,10 +1557,11 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev, ret = at91_adc_read_pressure(st, chan->channel, &tmp_val); *val = tmp_val; + ret = at91_adc_adjust_val_osr(st, val); mutex_unlock(&st->lock); iio_device_release_direct_mode(indio_dev); - return at91_adc_adjust_val_osr(st, val); + return ret; } /* in this case we have a voltage channel */@@ -1620,11 +1622,15 @@ static int at91_adc_read_raw(struct iio_dev *indio_dev, return IIO_VAL_FRACTIONAL_LOG2; case IIO_CHAN_INFO_SAMP_FREQ: + mutex_lock(&st->lock); *val = at91_adc_get_sample_freq(st);
So this is a straight read of a cached value. The only thing you 'might' arguably be protecting against is read/write tearing due to it in theory being possible to write part of the value whilst reading. I don't see that being a concern for st->current_sample_rate
+ mutex_unlock(&st->lock); return IIO_VAL_INT; case IIO_CHAN_INFO_OVERSAMPLING_RATIO: + mutex_lock(&st->lock); *val = st->oversampling_ratio;
Likewise, what are you protecting against racing with this that can't just occur before or after the lock?
quoted hunk ↗ jump to hunk
+ mutex_unlock(&st->lock); return IIO_VAL_INT; default:@@ -1644,18 +1650,23 @@ static int at91_adc_write_raw(struct iio_dev *indio_dev, (val != AT91_OSR_16SAMPLES)) return -EINVAL; /* if no change, optimize out */ + mutex_lock(&st->lock); if (val == st->oversampling_ratio) - return 0;
It should be race free to check this outside the lock. Definitely valid to lock around the cached value write and the config write though.
+ goto unlock;
If you did want to have locking as now then flip the logic
if (val != st->oversampling_ratio) {
st->oversampling_ratio = val;
at91_adc_config_emr(st);
}
mutex_unlock()
..
Goto always have a cost in readability so if you can avoid them with
a simple flip of logic like this it is usually a good idea.
(exception is error code which should always be out of line as
that is more common so what we expect to see).
st->oversampling_ratio = val; /* update ratio */ at91_adc_config_emr(st); +unlock: + mutex_unlock(&st->lock); return 0; case IIO_CHAN_INFO_SAMP_FREQ: if (val < st->soc_info.min_sample_rate || val > st->soc_info.max_sample_rate) return -EINVAL; + mutex_lock(&st->lock); at91_adc_setup_samp_freq(indio_dev, val); + mutex_unlock(&st->lock); return 0; default: return -EINVAL;
_______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel