linux-kernel - Kconfig, Makefile and ifdef: mod as yes vs. no?

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Message-ID: <20090712000625.7e1bffc2@daedalus.pq.iki.fi>
Date:	Sun, 12 Jul 2009 00:06:25 +0300
From:	Pekka Paalanen <pq@....fi>
To:	linux-kernel@...r.kernel.org
Subject: Kconfig, Makefile and ifdef: mod as yes vs. no?

Hi,

I'm having hard time making a patch that would compile nouveau_backlight.o
only if CONFIG_BACKLIGHT_CLASS_DEVICE is y or m, and define just stubs
when it is n.

I'm trying this in the nouveau/linux-2.6 kernel tree, which is basically
2.6.31-rc2.

I attempted the following:

--- a/drivers/gpu/drm/nouveau/Makefile
+++ b/drivers/gpu/drm/nouveau/Makefile
@@ -8,7 +8,7 @@ nouveau-y := nouveau_drv.o nouveau_state.o nouveau_fifo.o nouveau_mem.o \
              nouveau_sgdma.o nouveau_dma.o \
              nouveau_bo.o nouveau_fence.o nouveau_gem.o nouveau_ttm.o \
              nouveau_hw.o nouveau_calc.o nouveau_bios.o nouveau_i2c.o \
-            nouveau_display.o nouveau_fbcon.o nouveau_backlight.o \
+             nouveau_display.o nouveau_fbcon.o \
              nv04_timer.o \
              nv04_mc.o nv40_mc.o nv50_mc.o \
              nv04_fb.o nv10_fb.o nv40_fb.o \
@@ -19,8 +19,9 @@ nouveau-y := nouveau_drv.o nouveau_state.o nouveau_fifo.o nouveau_mem.o \
              nv50_crtc.o nv50_dac.o nv50_sor.o nv50_connector.o \
              nv50_cursor.o nv50_display.o nv50_fbcon.o \
              nv04_display.o nv04_output.o nv04_crtc.o nv04_cursor.o \
-            nv04_fbcon.o
+             nv04_fbcon.o

 nouveau-$(CONFIG_COMPAT) += nouveau_ioc32.o
+nouveau-$(CONFIG_BACKLIGHT_CLASS_DEVICE) += nouveau_backlight.o

 obj-$(CONFIG_DRM_NOUVEAU)+= nouveau.o

--- a/drivers/gpu/drm/nouveau/nouveau_drv.h
+++ b/drivers/gpu/drm/nouveau/nouveau_drv.h
@@ -708,8 +708,21 @@ extern int  nouveau_dma_init(struct nouveau_channel *);
 extern int  nouveau_dma_wait(struct nouveau_channel *, int size);

 /* nouveau_backlight.c */
+#ifdef CONFIG_BACKLIGHT_CLASS_DEVICE
 extern int nouveau_backlight_init(struct drm_device *);
 extern void nouveau_backlight_exit(struct drm_device *);
+#else
+static inline int nouveau_backlight_init(struct drm_device *dev)
+{
+       (void)dev;
+       return 0;
+}
+
+static inline void nouveau_backlight_exit(struct drm_device *dev)
+{
+       (void)dev;
+}
+#endif

When CONFIG_BACKLIGHT_CLASS_DEVICE=m this behaves as if it was n, i.e.
I get the stubs and no nouveau_backlight.o.

For CONFIG_BACKLIGHT_CLASS_DEVICE=y and n I get what I want.

What is the proper way to get m behave like y in this situation?

Oh, and should I use the following instead of (void)dev;?
static inline void nouveau_backlight_exit(struct drm_device *dev __used) { }

Please, Cc me.

Thanks.

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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