drm/i915: Avoid NULL dereference with component_only tv_modes

In commit d2d9f2324, the guard for a valid video mode was removed. This
caused the regression:

  kernel crash during kms graphic boot on Intel GM4500 platform
  https://bugzilla.redhat.com/show_bug.cgi?id=540218

This patches changes the logic slightly not to rely on a coupled
variable, but to just check whether the video_modes is valid before
dereferencing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Zhenyu Wang <zhenyu.z.wang@intel.com>
[ickle: Actually reference the correct bug report]
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

1 files changed, 4 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
index a0e4bc47b0f7..d7465ca27b97 100644
--- a/drivers/gpu/drm/i915/intel_tv.c
+++ b/drivers/gpu/drm/i915/intel_tv.c
@@ -1213,20 +1213,17 @@ intel_tv_mode_set(struct drm_encoder *encoder, struct drm_display_mode *mode,
                 tv_ctl |= TV_TRILEVEL_SYNC;
         if (tv_mode->pal_burst)
                 tv_ctl |= TV_PAL_BURST;
+
         scctl1 = 0;
-        /* dda1 implies valid video levels */
-        if (tv_mode->dda1_inc) {
+        if (tv_mode->dda1_inc)
                 scctl1 |= TV_SC_DDA1_EN;
-        }
-
         if (tv_mode->dda2_inc)
                 scctl1 |= TV_SC_DDA2_EN;
-
         if (tv_mode->dda3_inc)
                 scctl1 |= TV_SC_DDA3_EN;
-
         scctl1 |= tv_mode->sc_reset;
-        scctl1 |= video_levels->burst << TV_BURST_LEVEL_SHIFT;
+        if (video_levels)
+                scctl1 |= video_levels->burst << TV_BURST_LEVEL_SHIFT;
         scctl1 |= tv_mode->dda1_inc << TV_SCDDA1_INC_SHIFT;
 
         scctl2 = tv_mode->dda2_size << TV_SCDDA2_SIZE_SHIFT |