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 <torvalds@linux-foundation.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: David Laight <david.laight.linux@gmail.com>
Cc: David Laight <david.laight.linux@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250121145203.2851237-1-linux@roeck-us.net
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit 6f71507415841d1a6d38118e5fa0eaf0caab9c17)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
index fc1e517e074a30295707809debde0b124437a7e8..7e6ce905bdafa2e4905386af3dcbe544da49ef36 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_backlight.c
+++ b/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);