Re: Buiild error in i915/xe (was: [PATCH next 4/7] minmax.h: Use BUILD_BUG_ON_MSG() for the lo < hi test in clamp())

[Guenter Roeck <linux@roeck-us.net>]

On Sat, Jan 18, 2025 at 05:09:59PM +0000, David Laight wrote:
> On Sat, 18 Jan 2025 08:13:06 -0800
> Guenter Roeck <linux@roeck-us.net> wrote:
> 
> > Hi,
> > 
> > On Mon, Nov 18, 2024 at 07:13:31PM +0000, David Laight wrote:
> > > Use BUILD_BUG_ON_MSG(statically_true(ulo > uhi), ...) for the sanity
> > > check of the bounds in clamp().
> > > Gives better error coverage and one less expansion of the arguments.
> > > 
> > > Signed-off-by: David Laight <david.laight@aculab.com>  
> > 
> > This patch triggers a build error when trying to build parisc:allmodconfig.
> > See error message and bisect log below.
> > 
> > I don't think there is anything wrong with the patch. The underlying
> > problem seems to be that parisc:allmodconfig enables CONFIG_DRM_XE which
> > tries to build the affected file even though CONFIG_DRM_I915 is not
> > enabled/supported on parisc.
> 
> This has appeared before.
> Any idea which inlined copy of scale() is causing the problem.
> On the face of it they all look ok.
> 
> If you can reproduce it maybe try commenting out some of the calls.
> 

See diff below. All three changes are needed.
No idea why the compiler would know that the values are invalid.

Guenter

---
diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
index fc1e517e074a..3b2c8bdfcf8d 100644
--- a/drivers/gpu/drm/i915/display/intel_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_backlight.c
@@ -76,10 +76,14 @@ static u32 clamp_user_to_hw(struct intel_connector *connector,
 static u32 scale_hw_to_user(struct intel_connector *connector,
 			    u32 hw_level, u32 user_max)
 {
+#if 0
 	struct intel_panel *panel = &connector->panel;
 
 	return scale(hw_level, panel->backlight.min, panel->backlight.max,
 		     0, user_max);
+#else
+	return 0;
+#endif
 }
 
 u32 intel_backlight_invert_pwm_level(struct intel_connector *connector, u32 val)
@@ -119,8 +123,10 @@ u32 intel_backlight_level_to_pwm(struct intel_connector *connector, u32 val)
 	drm_WARN_ON_ONCE(&i915->drm,
 			 panel->backlight.max == 0 || panel->backlight.pwm_level_max == 0);
 
+#if 0
 	val = scale(val, panel->backlight.min, panel->backlight.max,
 		    panel->backlight.pwm_level_min, panel->backlight.pwm_level_max);
+#endif
 
 	return intel_backlight_invert_pwm_level(connector, val);
 }
@@ -138,8 +144,12 @@ u32 intel_backlight_level_from_pwm(struct intel_connector *connector, u32 val)
 	     intel_has_quirk(display, QUIRK_INVERT_BRIGHTNESS)))
 		val = panel->backlight.pwm_level_max - (val - panel->backlight.pwm_level_min);
 
+#if 0
 	return scale(val, panel->backlight.pwm_level_min, panel->backlight.pwm_level_max,
 		     panel->backlight.min, panel->backlight.max);
+#else
+	return 0;
+#endif
 }
 
 static u32 lpt_get_backlight(struct intel_connector *connector, enum pipe unused)