[media] Add and use IS_REACHABLE macro

In the media drivers, the v4l2 core knows about all submodules
and calls into them from a common function. However this cannot
work if the modules that get called are loadable and the
core is built-in. In that case we get

drivers/built-in.o: In function `set_type':
drivers/media/v4l2-core/tuner-core.c:301: undefined reference to `tea5767_attach'
drivers/media/v4l2-core/tuner-core.c:307: undefined reference to `tea5761_attach'
drivers/media/v4l2-core/tuner-core.c:349: undefined reference to `tda9887_attach'
drivers/media/v4l2-core/tuner-core.c:405: undefined reference to `xc4000_attach'

This was working previously, until the IS_ENABLED() macro was used
to replace the construct like

 #if defined(CONFIG_DVB_CX24110) || (defined(CONFIG_DVB_CX24110_MODULE) && defined(MODULE))

with the difference that the new code no longer checks whether it is being
built as a loadable module itself.

To fix this, this new patch adds an 'IS_REACHABLE' macro, which evaluates
true in exactly the condition that was used previously. The downside
of this is that this trades an obvious link error for a more subtle
runtime failure, but it is clear that the change that introduced the
link error was unintentional and it seems better to revert it for
now. Also, a similar change was originally created by Trent Piepho
and then reverted by teh change to the IS_ENABLED macro.

Ideally Kconfig would be used to avoid the case of a broken dependency,
or the code restructured in a way to turn around the dependency, but either
way would require much larger changes here.

Fixes: 7b34be71db53 ("[media] use IS_ENABLED() macro")
See-also: c5dec9fb248e ("V4L/DVB (4751): Fix DBV_FE_CUSTOMISE for card drivers compiled into kernel")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/media/tuners/mt2131.h b/drivers/media/tuners/mt2131.h
index 837c854b9c65..8267a6ae5d84 100644
--- a/drivers/media/tuners/mt2131.h
+++ b/drivers/media/tuners/mt2131.h
@@ -30,7 +30,7 @@ struct mt2131_config {
 	u8 clock_out; /* 0 = off, 1 = CLK/4, 2 = CLK/2, 3 = CLK/1 */
 };
 
-#if IS_ENABLED(CONFIG_MEDIA_TUNER_MT2131)
+#if IS_REACHABLE(CONFIG_MEDIA_TUNER_MT2131)
 extern struct dvb_frontend* mt2131_attach(struct dvb_frontend *fe,
 					  struct i2c_adapter *i2c,
 					  struct mt2131_config *cfg,