[Intel-gfx] [PATCH] drm/fb: Proper support of boundary conditions in bitmasks.

[Tomasz Lis]

The recently introduced patch changed behavior of masks when
the bit number is negative. Instead of no bits set, the new way
makes all bits set. Problematic patch:
  drm/i915: Avoid BIT(max) - 1 and use GENMASK(max - 1, 0)

This behaviour was not considered when making changes, and boundary
value of count (=0) is now resulting in a mask with all bits on, since
the value is directly decreased and therefore negative. Checking if
all bits are set leads to infinite loop.

This patch introduces an additional check to avoid empty masks. It
reverts the control flow to the exact same way it worked before
the problematic patch.

Signed-off-by: Tomasz Lis <tomasz.lis at intel.com>
---
 drivers/gpu/drm/i915/intel_fbdev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
index e6f3eb2d..bc65ecf 100644
--- a/drivers/gpu/drm/i915/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/intel_fbdev.c
@@ -496,7 +496,7 @@ static bool intel_fb_initial_config(struct drm_fb_helper *fb_helper,
 		conn_configured |= BIT(i);
 	}
 
-	if ((conn_configured & mask) != mask) {
+	if (count > 0 && (conn_configured & mask) != mask) {
 		pass++;
 		goto retry;
 	}
-- 
2.7.4