linux-kernel - Re: [PATCH -next] iio: st_accel: Fix unused variable warning

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Date:   Sat, 2 Nov 2019 21:15:35 +0100
From:   Ladislav Michl <ladis@...ux-mips.org>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     YueHaibing <yuehaibing@...wei.com>, knaack.h@....de,
        lars@...afoo.de, pmeerw@...erw.net, denis.ciocca@...com,
        rfontana@...hat.com, tglx@...utronix.de, heiko.stuebner@...com,
        rjones@...eworks.com, drake@...lessm.com, colin.king@...onical.com,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] iio: st_accel: Fix unused variable warning

On Sat, Nov 02, 2019 at 02:08:10PM +0000, Jonathan Cameron wrote:
> On Sat, 2 Nov 2019 11:41:25 +0100
> Ladislav Michl <ladis@...ux-mips.org> wrote:
> 
> > On Fri, Nov 01, 2019 at 09:47:41PM +0800, YueHaibing wrote:
> > > drivers/iio/accel/st_accel_core.c:1005:44: warning:
> > >  mount_matrix_ext_info defined but not used [-Wunused-const-variable=]
> > > 
> > > Move it to ifdef to mute this warning.
> > > 
> > > Signed-off-by: YueHaibing <yuehaibing@...wei.com>
> > > ---
> > >  drivers/iio/accel/st_accel_core.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/iio/accel/st_accel_core.c b/drivers/iio/accel/st_accel_core.c
> > > index 2e37f8a..bba0717 100644
> > > --- a/drivers/iio/accel/st_accel_core.c
> > > +++ b/drivers/iio/accel/st_accel_core.c
> > > @@ -1002,10 +1002,12 @@ get_mount_matrix(const struct iio_dev *indio_dev,
> > >  	return adata->mount_matrix;
> > >  }
> > >  
> > > +#ifdef CONFIG_ACPI
> > >  static const struct iio_chan_spec_ext_info mount_matrix_ext_info[] = {
> > >  	IIO_MOUNT_MATRIX(IIO_SHARED_BY_ALL, get_mount_matrix),  
> > 
> > So now you do not get any warning for unused get_mount_matrix?
> > (Then it would make more sense to put all that stuff under one ifdef
> > and provide empty apply_acpi_orientation for non ACPI case)
> 
> Does the __maybe_unused marking make this go away?
> 
> I'd assume that the compiler will manage to drop this either way
> but I guess we should check that.
> 
> ifdef magic is always harder to read and potentially fragile in the
> long run.  Here we simply want to indicate that in some build
> configurations we might not use this.

This suggestion implies we'll get rid of CONFIG_ACPI completely, which
seems inapproriate looking at size of apply_acpi_orientation function.
And having both CONFIG_ACPI and __maybe_unused does not make much
sense. I had something like that in mind (+COMPILE_TEST perhaps):

diff --git a/drivers/iio/accel/st_accel_core.c b/drivers/iio/accel/st_accel_core.c
index 2e37f8a6d8cf..0e7eac62d618 100644
--- a/drivers/iio/accel/st_accel_core.c
+++ b/drivers/iio/accel/st_accel_core.c
@@ -993,6 +993,7 @@ static const struct iio_trigger_ops st_accel_trigger_ops = {
 #define ST_ACCEL_TRIGGER_OPS NULL
 #endif
 
+#ifdef CONFIG_ACPI
 static const struct iio_mount_matrix *
 get_mount_matrix(const struct iio_dev *indio_dev,
 		 const struct iio_chan_spec *chan)
@@ -1013,7 +1014,6 @@ static const struct iio_chan_spec_ext_info mount_matrix_ext_info[] = {
 static int apply_acpi_orientation(struct iio_dev *indio_dev,
 				  struct iio_chan_spec *channels)
 {
-#ifdef CONFIG_ACPI
 	struct st_sensor_data *adata = iio_priv(indio_dev);
 	struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
 	struct acpi_device *adev;
@@ -1141,10 +1141,14 @@ static int apply_acpi_orientation(struct iio_dev *indio_dev,
 out:
 	kfree(buffer.pointer);
 	return ret;
-#else /* !CONFIG_ACPI */
+}
+#else
+static int apply_acpi_orientation(struct iio_dev *indio_dev,
+				  struct iio_chan_spec *channels)
+{
 	return 0;
-#endif
 }
+#endif
 
 /*
  * st_accel_get_settings() - get sensor settings from device name
> Thanks,
> 
> Jonathan
> 
> 
> > 
> > >  	{ },
> > >  };
> > > +#endif
> > >  
> > >  /* Read ST-specific _ONT orientation data from ACPI and generate an
> > >   * appropriate mount matrix.
> > > -- 
> > > 2.7.4
> > >