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

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 <[email protected]>
Suggested-by: Linus Torvalds <[email protected]>
Suggested-by: David Laight <[email protected]>
Cc: David Laight <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Signed-off-by: Guenter Roeck <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Rodrigo Vivi <[email protected]>
(cherry picked from commit 6f71507)
Signed-off-by: Rodrigo Vivi <[email protected]>

Author: groeck

drivers/gpu/drm/i915/display/intel_backlight.c

@@ -41,8 +41,9 @@ static u32 scale(u32 source_val,
 {
 	u64 target_val;
 
-	WARN_ON(source_min > source_max);
-	WARN_ON(target_min > target_max);
+	if (WARN_ON(source_min >= source_max) ||
+	    WARN_ON(target_min > target_max))
+		return target_min;
 
 	/* defensive */
 	source_val = clamp(source_val, source_min, source_max);