Re: [PATCH v2 3/4] iio: magnetometer: add a driver for Voltafield AF8133J magnetometer

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 14 Feb 2024 18:43:10 +0100
Ondřej Jirman <megi@xff.cz> wrote:

> Hi Jonathan,

Gah. Runtime pm always gives me a headache. I'd indeed misunderstood some
of what you are doing.
> 
> On Wed, Feb 14, 2024 at 05:01:36PM +0000, Jonathan Cameron wrote:
> > On Mon, 12 Feb 2024 18:53:55 +0100
> > Ondřej Jirman <megi@xff.cz> wrote:
> >   
> > > From: Icenowy Zheng <icenowy@aosc.io>
> > > 
> > > AF8133J is a simple I2C-connected magnetometer, without interrupts.
> > > 
> > > Add a simple IIO driver for it.
> > > 
> > > Co-developed-by: Icenowy Zheng <icenowy@aosc.io>
> > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > Signed-off-by: Dalton Durst <dalton@ubports.com>
> > > Signed-off-by: Shoji Keita <awaittrot@shjk.jp>
> > > Co-developed-by: Ondrej Jirman <megi@xff.cz>
> > > Signed-off-by: Ondrej Jirman <megi@xff.cz>  
> > 
> > 
> > Hi a few comments (mostly on changes)
> > 
> > The runtime_pm handling can be simplified somewhat if you
> > rearrange probe a little.
> >   
> > > diff --git a/drivers/iio/magnetometer/af8133j.c b/drivers/iio/magnetometer/af8133j.c
> > > new file mode 100644
> > > index 000000000000..1f64a2337f6e
> > > --- /dev/null
> > > +++ b/drivers/iio/magnetometer/af8133j.c
> > > @@ -0,0 +1,528 @@  
> > 
> >   
> > > +static int af8133j_take_measurement(struct af8133j_data *data)
> > > +{
> > > +	unsigned int val;
> > > +	int ret;
> > > +
> > > +	ret = regmap_write(data->regmap,
> > > +			   AF8133J_REG_STATE, AF8133J_REG_STATE_WORK);
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	/* The datasheet says "Mesaure Time <1.5ms" */
> > > +	ret = regmap_read_poll_timeout(data->regmap, AF8133J_REG_STATUS, val,
> > > +				       val & AF8133J_REG_STATUS_ACQ,
> > > +				       500, 1500);
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	ret = regmap_write(data->regmap,
> > > +			   AF8133J_REG_STATE, AF8133J_REG_STATE_STBY);  
> > 
> > return regmap_write()
> > 
> > regmap accesses return 0 or a negative error code enabling little code
> > reductions like this.  
> 
> Yeah, some reviewers dislike this, because modifying the code in the future
> creates a more unpleasant diff. But if you like this style, I don't mind
> changing it.

Always a gamble on chance of a modification coming.

In general I'd check regmap calls with if (ret) but don't feel that strongly
about that either.

So not really important either way.
> 
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int af8133j_read_measurement(struct af8133j_data *data, __le16 buf[3])
> > > +{
> > > +	struct device *dev = &data->client->dev;
> > > +	int ret;
> > > +
> > > +	ret = pm_runtime_resume_and_get(dev);
> > > +	if (ret) {
> > > +		/*
> > > +		 * Ignore EACCES because that happens when RPM is disabled
> > > +		 * during system sleep, while userspace leave eg. hrtimer
> > > +		 * trigger attached and IIO core keeps trying to do measurements.  
> > 
> > Yeah. We still need to fix that more elegantly :(
> >   
> > > +		 */
> > > +		if (ret != -EACCES)
> > > +			dev_err(dev, "Failed to power on (%d)\n", ret);
> > > +		return ret;
> > > +	}
> > > +
> > > +	scoped_guard(mutex, &data->mutex) {
> > > +		ret = af8133j_take_measurement(data);
> > > +		if (ret)
> > > +			goto out_rpm_put;
> > > +
> > > +		ret = regmap_bulk_read(data->regmap, AF8133J_REG_OUT,
> > > +				       buf, sizeof(__le16) * 3);
> > > +	}
> > > +
> > > +out_rpm_put:
> > > +	pm_runtime_mark_last_busy(dev);
> > > +	pm_runtime_put_autosuspend(dev);
> > > +
> > > +	return ret;
> > > +}
> > > +  
> > 
> >   
> > > +
> > > +static int af8133j_set_scale(struct af8133j_data *data,
> > > +			     unsigned int val, unsigned int val2)
> > > +{
> > > +	struct device *dev = &data->client->dev;
> > > +	u8 range;
> > > +	int ret = 0;
> > > +
> > > +	if (af8133j_scales[0][0] == val && af8133j_scales[0][1] == val2)
> > > +		range = AF8133J_REG_RANGE_12G;
> > > +	else if (af8133j_scales[1][0] == val && af8133j_scales[1][1] == val2)
> > > +		range = AF8133J_REG_RANGE_22G;
> > > +	else
> > > +		return -EINVAL;
> > > +
> > > +	pm_runtime_disable(dev);
> > > +
> > > +	/*
> > > +	 * When suspended, just store the new range to data->range to be
> > > +	 * applied later during power up.  
> > Better to just do
> > 	pm_runtime_resume_and_get() here
> >   
> > > +	 */
> > > +	if (!pm_runtime_status_suspended(dev))
> > > +		ret = regmap_write(data->regmap, AF8133J_REG_RANGE, range);
> > > +
> > > +	pm_runtime_enable(dev);  
> > and
> > 	pm_runtime_mark_last_busy(dev);
> > 	pm_runtime_put_autosuspend(dev);
> > here.
> > 
> > The userspace interface is only way this function is called so rearrange
> > probe a little so that you don't need extra complexity in these functions.  
> 
> It doesn't make sense to wakeup the device for range change, because it will
> forget the range the moment it's powered off again, after changing the range.

Ah.  I'd missed understood that. Thanks for extra explanation.

I'm not keen on the enable / disable dance but anything else is probably worse
(delaying update until we actually using it etc).



> 
> Also this function has nothing to do with probe. data->range is authoritative
> value, not cache. It gets applied to HW on each power up.
> 
> >   
> > > +
> > > +	data->range = range;  
> > 
> > If the write failed, generally don't update the cached value.  
> 
> Right.
> 
> > > +	return ret;
> > > +}
> > > +
> > > +static int af8133j_write_raw(struct iio_dev *indio_dev,
> > > +			     struct iio_chan_spec const *chan,
> > > +			     int val, int val2, long mask)
> > > +{
> > > +	struct af8133j_data *data = iio_priv(indio_dev);
> > > +	int ret;
> > > +
> > > +	switch (mask) {
> > > +	case IIO_CHAN_INFO_SCALE:
> > > +		scoped_guard(mutex, &data->mutex)
> > > +			ret = af8133j_set_scale(data, val, val2);  
> > 
> > Look more closely at what scoped_guard() does.
> > 			return af8133j_set_scale(data, val, val2);
> > is fine and simpler as no local variable needed.  
> 
> I did, it will not work as you suggest. It's implemented as for loop with
> condition, and the compiler will complain about fallthrough.
> 
> I can do:
> 
> 		scoped_guard(mutex, &data->mutex)
> 			return af8133j_set_scale(data, val, val2);
> 		return 0;
> 
> but it looks weirder at first glance, at least to my eye.

I agree that bit is less than ideal, but with your code it should also
get confused about whether ret is ever set.

		scoped_guard(mutex, &data->mutex)
			return ...
		unreachable();

perhaps?  or just use a guard and add scope manually

	case IIO_CHAN_INFO_SCALE: {
		guard(mutex)(&data->mutex);

		return af8133j_set_scale(...);
	}

I'd go with this as the cleanest solution in this case.


> 
> > > +		return ret;
> > > +	default:
> > > +		return -EINVAL;
> > > +	}
> > > +}  
> >   
> > > +static void af8133j_power_down_action(void *ptr)
> > > +{
> > > +	struct af8133j_data *data = ptr;
> > > +	struct device *dev = &data->client->dev;
> > > +
> > > +	pm_runtime_disable(dev);  
> > You group together unwinding of calls that occur in very
> > different places in probe.  Don't do that as it leas
> > to disabling runtime pm having never enabled it
> > in some error paths.  That may be safe but if fails the
> > obviously correct test.  
> 
> This whole disable/enable dance is here so that pm_runtime_status_suspended can
> be trusted. Not for disabling PM during device remove or in error paths.
> 
> There's no imbalance here or problem with disabling PM when it's already
> disabled. Disable/enable is reference counted, and this function keeps the
> balance.

Whilst not buggy but I still want to be able to cleanly associate a given
bit of cleanup with what is being cleaned up.  That is the path to
maintainable code longer term.  Runtime PM does make a mess of doing this
but tends to have somewhat logical sets of calls that go together.

As long as we hold a reference, doesn't matter when we turn it on in probe()
Only the put_autosuspend has to come after we done talking to it.




	
> 
> > So this is a good solution to the normal dance of turning power on
> > just to turn it off shortly afterwards.
> >   
> > > +		af8133j_power_down(data);
> > > +	pm_runtime_enable(dev);  
> > Why?  
> 
> See above. To keep the disable ref count balanced.
> 
> Looks like actual RPM disable already happened at this point a bit earlier in
> another callback registered via devm_pm_runtime_enable(). I guess this
> pm_runtime_enable()/pm_runtime_disable() guard can just be skipped, because RPM
> is already disabled thanks to reverse ordering of devm callbacks during device
> remove. So while this is safe, it's redundant at this point and call to 
> pm_runtime_status_suspended() is safe without this.

Yes, That's a side effect of only enabling it right at the end.

> 
> > > +}
> > > +
> > > +static int af8133j_probe(struct i2c_client *client)
> > > +{
> > > +	struct device *dev = &client->dev;
> > > +	struct af8133j_data *data;
> > > +	struct iio_dev *indio_dev;
> > > +	struct regmap *regmap;
> > > +	int ret, i;
> > > +
> > > +	indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
> > > +	if (!indio_dev)
> > > +		return -ENOMEM;
> > > +
> > > +	regmap = devm_regmap_init_i2c(client, &af8133j_regmap_config);
> > > +	if (IS_ERR(regmap))
> > > +		return dev_err_probe(dev, PTR_ERR(regmap),
> > > +				     "regmap initialization failed\n");
> > > +
> > > +	data = iio_priv(indio_dev);
> > > +	i2c_set_clientdata(client, indio_dev);
> > > +	data->client = client;
> > > +	data->regmap = regmap;
> > > +	data->range = AF8133J_REG_RANGE_12G;
> > > +	mutex_init(&data->mutex);
> > > +
> > > +	data->reset_gpiod = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
> > > +	if (IS_ERR(data->reset_gpiod))
> > > +		return dev_err_probe(dev, PTR_ERR(data->reset_gpiod),
> > > +				     "Failed to get reset gpio\n");
> > > +
> > > +	for (i = 0; i < ARRAY_SIZE(af8133j_supply_names); i++)
> > > +		data->supplies[i].supply = af8133j_supply_names[i];
> > > +	ret = devm_regulator_bulk_get(dev, ARRAY_SIZE(data->supplies),
> > > +				      data->supplies);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	ret = iio_read_mount_matrix(dev, &data->orientation);
> > > +	if (ret)
> > > +		return dev_err_probe(dev, ret, "Failed to read mount matrix\n");
> > > +
> > > +	ret = af8133j_power_up(data);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	pm_runtime_set_active(dev);
> > > +
> > > +	ret = devm_add_action_or_reset(dev, af8133j_power_down_action, data);  
> > 
> > As mentioned above, this should only undo things done before this point.
> > So just the af8133j_power_down() I think.  
> 
> The callback doesn't do anything else but power down. It leaves everything
> else as is after it exits.
> 
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	ret = af8133j_product_check(data);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	indio_dev->info = &af8133j_info;
> > > +	indio_dev->name = "af8133j";
> > > +	indio_dev->channels = af8133j_channels;
> > > +	indio_dev->num_channels = ARRAY_SIZE(af8133j_channels);
> > > +	indio_dev->modes = INDIO_DIRECT_MODE;
> > > +
> > > +	ret = devm_iio_triggered_buffer_setup(dev, indio_dev, NULL,
> > > +					      &af8133j_trigger_handler, NULL);
> > > +	if (ret < 0)
> > > +		return dev_err_probe(&client->dev, ret,
> > > +				     "Failed to setup iio triggered buffer\n");
> > > +
> > > +	ret = devm_iio_device_register(dev, indio_dev);
> > > +	if (ret)
> > > +		return dev_err_probe(dev, ret, "Failed to register iio device");
> > > +
> > > +	pm_runtime_get_noresume(dev);  
> >   
> > > +	pm_runtime_use_autosuspend(dev);
> > > +	pm_runtime_set_autosuspend_delay(dev, 500);
> > > +	ret = devm_pm_runtime_enable(dev);  
> > 
> > This already deals with pm_runtime_disable() so you shouldn't need do it manually.  
> 
> I'm not disabling RPM manually, it was just used as temporary guard to be able
> to read pm_runtime_status_suspended() safely.

I'd indeed misunderstood that. I forgot the oddity that runtime pm is effectively
reference counting in only one direction for enable / disable and the opposite
one for get and put.

pm_runtime_disable()
pm_runtime_disable()
pm_runtime_enable()
pm_runtime_enable()
pm_runtime_enable()
is fine, but

pm_runtime_enable()
pm_runtime_enable()
pm_runtime_disable()
pm_runtime_disable()
pm_runtime_enable()
is not.

Which makes sense when you realise it's all about ensuring it is off, but
never ensuring that it is turned on.




> 
> > Also you want to enable that before the devm_iio_device_register() to avoid
> > problems with it not being available as the userspace interfaces are used.
> >
> > So just move this up a few lines.  
> 
> Good idea, thanks.
> 
> kind regards,
> 	o.
> 
> > 
> >   
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	pm_runtime_put_autosuspend(dev);
> > > +
> > > +	return 0;
> > > +}  
> >