[PATCH] drm/i915/backlight: Return immediately when scale() finds invalid parameters

[Guenter Roeck]

The scale() functions detects invalid parameters, but continues
its calculations anyway. This causes bad results if negative values
are used for unsigned operations. Worst case, a division by 0 error
will be seen if source_min == source_max.

On top of that, after v6.13, the sequence of WARN_ON() followed by clamp()
may result in a build error with gcc 13.x.

drivers/gpu/drm/i915/display/intel_backlight.c: In function 'scale':
include/linux/compiler_types.h:542:45: error:
	call to '__compiletime_assert_415' declared with attribute error:
	clamp() low limit source_min greater than high limit source_max

This happens if the compiler decides to rearrange the code as follows.

        if (source_min > source_max) {
                WARN(..);
                /* Do the clamp() knowing that source_min > source_max */
                source_val = clamp(source_val, source_min, source_max);
        } else {
                /* Do the clamp knowing that source_min <= source_max */
                source_val = clamp(source_val, source_min, source_max);
        }

Fix the problem by evaluating the return values from WARN_ON and returning
immediately after a warning. While at it, fix divide by zero error seen
if source_min == source_max.

Analyzed-by: Linus Torvalds <torvalds at linux-foundation.org>
Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
Suggested-by: David Laight <david.laight.linux at gmail.com>
Cc: David Laight <david.laight.linux at gmail.com>
Cc: Jani Nikula <jani.nikula at linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Signed-off-by: Guenter Roeck <linux at roeck-us.net>
---
 drivers/gpu/drm/i915/display/intel_backlight.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
index 3f81a726cc7d..ad49bd4a1c12 100644
--- a/drivers/gpu/drm/i915/display/intel_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_backlight.c
@@ -40,8 +40,11 @@ static u32 scale(u32 source_val,
 {
 	u64 target_val;
 
-	WARN_ON(source_min > source_max);
-	WARN_ON(target_min > target_max);
+	if (WARN_ON(target_min > target_max))
+		return target_min;
+
+	if (WARN_ON(source_min > source_max) || source_min == source_max)
+		return target_min + (target_max - target_min) / 2;
 
 	/* defensive */
 	source_val = clamp(source_val, source_min, source_max);
-- 
2.45.2