[PATCH 4.14 021/164] iio: stm32: fix adc/trigger link error

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnd Bergmann <arnd@xxxxxxxx>

commit 6d745ee8b5e81f3a33791e3c854fbbfd6f3e585e upstream.

The ADC driver can trigger on either the timer or the lptim
trigger, but it only uses a Kconfig 'select' statement
to ensure that the first of the two is present. When the lptim
trigger is enabled as a loadable module, and the adc driver
is built-in, we now get a link error:

drivers/iio/adc/stm32-adc.o: In function `stm32_adc_get_trig_extsel':
stm32-adc.c:(.text+0x4e0): undefined reference to `is_stm32_lptim_trigger'

We could use a second 'select' statement and always have both
trigger drivers enabled when the adc driver is, but it seems that
the lptimer trigger was intentionally left optional, so it seems
better to keep it that way.

This adds a hack to use 'IS_REACHABLE()' rather than 'IS_ENABLED()',
which avoids the link error, but instead leads to the lptimer trigger
not being used in the broken configuration. I've added a runtime
warning for this case to help users figure out what they did wrong
if this should ever be done by accident.

Fixes: f0b638a7f6db ("iio: adc: stm32: add support for lptimer triggers")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/iio/timer/stm32-lptim-trigger.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/iio/timer/stm32-lptim-trigger.h
+++ b/include/linux/iio/timer/stm32-lptim-trigger.h
@@ -16,11 +16,14 @@
 #define LPTIM2_OUT	"lptim2_out"
 #define LPTIM3_OUT	"lptim3_out"
 
-#if IS_ENABLED(CONFIG_IIO_STM32_LPTIMER_TRIGGER)
+#if IS_REACHABLE(CONFIG_IIO_STM32_LPTIMER_TRIGGER)
 bool is_stm32_lptim_trigger(struct iio_trigger *trig);
 #else
 static inline bool is_stm32_lptim_trigger(struct iio_trigger *trig)
 {
+#if IS_ENABLED(CONFIG_IIO_STM32_LPTIMER_TRIGGER)
+	pr_warn_once("stm32 lptim_trigger not linked in\n");
+#endif
 	return false;
 }
 #endif