Re: [PATCH v8 1/5] iio: magnetometer: ak8975: switch to using managed resources

[Joshua Crofts]

On Wed, 20 May 2026 at 17:31, Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
>
> On Mon, 18 May 2026 09:50:25 +0200
> Joshua Crofts via B4 Relay <devnull+joshua.crofts1.gmail.com@xxxxxxxxxx> wrote:
>
> > From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> >
> > Switch the driver to use managed resources (devm_*) which simplifier
> > error handling and allows removing ak8975_remove() method from
> > the driver.
> >
> > Note, on error path we now also set mode to POWER_DOWN state which is
> > fine. Even if the device is in that mode, there is no problem to set
> > that mode again, it should be no-op.
> >
> > Additionally, remove any pm_runtime_get/put*() function calls that
> > dummy cycled the counter to autosuspend the device.
> >
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > Co-developed-by: Joshua Crofts <joshua.crofts1@xxxxxxxxx>
> > Signed-off-by: Joshua Crofts <joshua.crofts1@xxxxxxxxx>
> > ---
> >  drivers/iio/magnetometer/ak8975.c | 77 ++++++++++++++++++++++-----------------
> >  1 file changed, 43 insertions(+), 34 deletions(-)
> >
> > diff --git a/drivers/iio/magnetometer/ak8975.c b/drivers/iio/magnetometer/ak8975.c
> > index bb74abb648f138352576a68c49d3c7dce7cc068e..de4402201783430fc69369f1accaab35145e9f0c 100644
> > --- a/drivers/iio/magnetometer/ak8975.c
> > +++ b/drivers/iio/magnetometer/ak8975.c
> > @@ -935,9 +935,25 @@ static const struct iio_buffer_setup_ops ak8975_buffer_setup_ops = {
> >       .preenable = ak8975_buffer_preenable,
> >       .postdisable = ak8975_buffer_postdisable,
> >  };
> > +
> > +static void devm_ak8975_power_off(void *data)
> > +{
> > +     struct ak8975_data *ak = data;
> > +     struct device *dev = &ak->client->dev;
> > +
> > +     /* Only power down if currently active */
> > +     if (pm_runtime_suspended(dev))
> I don't think we established that this actually works without underflowing
> the regular reference counts if in runtime pm suspsended state when
> we tear get to devm cleanup.
>
> Sashiko correctly moans about this and points out you refer to a different
> check anyway on the comment below.
> https://sashiko.dev/#/patchset/20260518-magnetometer-fixes-post-pickup-v8-0-088d610108a0%40gmail.com
>
> I'm not convinced pm_runtime_status_suspended() is sufficient either.
>
> The only path I'm 100% sure of is when we add tracking flag in the
> iio_priv() to indicate if we are already runtime suspended.  Ugly but
> at least we know that will not run into these odd race conditions.
>
> This is 'probably' a new bug rather than a latent one because of the
> unusual handling below.  I think there is a race in original code but
> it is narrow so unlikely anyone ever hit it.
>
> Whilst I do discuss what else you might do - I'd throw a flag in iio_priv()
> because it is a simpler solution.
>
> Now, if you happen to have some sort of real hardware or emulation to test
> the different flows, then I'd be interested to see if there are advantages
> to doing something closer to what this driver originally did.

I've thought about dropping this patch as power management in this driver
seems to be a complete rabbit hole, and that I don't have the actual hardware
to test this (IMO stubbing won't really help). Although the flag
solution is a bit
hacky, it could work.

> > +             return;
> > +
> > +     /* Soft-stop the chip before hard-stopping the regulators */
> > +     ak8975_set_mode(data, POWER_DOWN);
> > +     ak8975_power_off(data);
> > +}
> > +
> >  static int ak8975_probe(struct i2c_client *client)
> >  {
> >       const struct i2c_device_id *id = i2c_client_get_device_id(client);
> > +     struct device *dev = &client->dev;
> >       struct ak8975_data *data;
> >       struct iio_dev *indio_dev;
> >       struct gpio_desc *eoc_gpiod;
> > @@ -1005,10 +1021,21 @@ static int ak8975_probe(struct i2c_client *client)
> >       if (ret)
> >               return ret;
> >
> > +     /*
> > +      * Set device as active early so pm_runtime_status_suspended() works
> > +      * correctly if probe fails before pm_runtime is enabled. Do not attempt
> > +      * to move this line lower.
> > +      */
> > +     pm_runtime_set_active(dev);
> > +
> > +     ret = devm_add_action_or_reset(dev, devm_ak8975_power_off, data);
> > +     if (ret)
> > +             return ret;
> > +
> >       ret = ak8975_who_i_am(data, data->def->type);
> >       if (ret) {
> >               dev_err(&client->dev, "Unexpected device\n");
> > -             goto power_off;
> > +             return ret;
> >       }
> >       dev_dbg(&client->dev, "Asahi compass chip %s\n", name);
> >
> > @@ -1016,10 +1043,13 @@ static int ak8975_probe(struct i2c_client *client)
> >       ret = ak8975_setup(data);
> >       if (ret) {
> >               dev_err(&client->dev, "%s initialization fails\n", name);
> > -             goto power_off;
> > +             return ret;
> >       }
> >
> > -     mutex_init(&data->lock);
> > +     ret = devm_mutex_init(dev, &data->lock);
> > +     if (ret)
> > +             return ret;
> > +
> >       indio_dev->channels = ak8975_channels;
> >       indio_dev->num_channels = ARRAY_SIZE(ak8975_channels);
> >       indio_dev->info = &ak8975_info;
> > @@ -1027,52 +1057,32 @@ static int ak8975_probe(struct i2c_client *client)
> >       indio_dev->modes = INDIO_DIRECT_MODE;
> >       indio_dev->name = name;
> >
> > -     ret = iio_triggered_buffer_setup(indio_dev, NULL, ak8975_handle_trigger,
> > -                                      &ak8975_buffer_setup_ops);
> > +     ret = devm_iio_triggered_buffer_setup(dev, indio_dev, NULL,
> > +                                           ak8975_handle_trigger,
> > +                                           &ak8975_buffer_setup_ops);
> >       if (ret) {
> >               dev_err(&client->dev, "triggered buffer setup failed\n");
> > -             goto power_off;
> > +             return ret;
> >       }
> >
> > -     ret = iio_device_register(indio_dev);
> > +     ret = devm_pm_runtime_enable(dev);
> > +     if (ret)
> > +             return ret;
> > +
> > +     ret = devm_iio_device_register(dev, indio_dev);
> >       if (ret) {
> >               dev_err(&client->dev, "device register failed\n");
> > -             goto cleanup_buffer;
> > +             return ret;
> >       }
> >
> > -     /* Enable runtime PM */
> > -     pm_runtime_get_noresume(&client->dev);
>
> See below for comment on this.
>
> > -     pm_runtime_set_active(&client->dev);
> > -     pm_runtime_enable(&client->dev);
> >       /*
> >        * The device comes online in 500us, so add two orders of magnitude
> >        * of delay before autosuspending: 50 ms.
> >        */
> >       pm_runtime_set_autosuspend_delay(&client->dev, 50);
> >       pm_runtime_use_autosuspend(&client->dev);
> > -     pm_runtime_put(&client->dev);
> >
> >       return 0;
> > -
> > -cleanup_buffer:
> > -     iio_triggered_buffer_cleanup(indio_dev);
> > -power_off:
> > -     ak8975_power_off(data);
> > -     return ret;
> > -}
> > -
> > -static void ak8975_remove(struct i2c_client *client)
> > -{
> > -     struct iio_dev *indio_dev = i2c_get_clientdata(client);
> > -     struct ak8975_data *data = iio_priv(indio_dev);
> > -
> > -     pm_runtime_get_sync(&client->dev);
> This forces device to wake up.
>
> > -     pm_runtime_put_noidle(&client->dev);
> This forces the counter down to 0 but doesn't trigger a runtime
> pm suspend.   I'm nervous though as userspace is still in place
> (before iio_device_register() - so it's possible another
> get / put cycle can occur before the next call).
>
> > -     pm_runtime_disable(&client->dev);
> Hence when we disable runtime pm the device is powered up and
> the rest won't underflow.
>
> Now interestingly there is a :
> devm_pm_runtime_get_noresume() which calls
> pm_runtime_get_noresume() and schedules the
> pm_runtime_put_noidle() in the tear down path..
>
> However it is vanishingly rarely used and the sequence to me
> is non obvious - the two users are inconsistent :(

Hmm, I guess I could reorder the set to get things moving a bit more,
the rest of the patches are trivial.

-- 
Kind regards

CJD