drm/i915: fix gpio register detection logic for BIOS without VBT

if no VBT is present, crt_ddc_bus will be left at 0, and cause us
to use that for the GPIO register offset. That's never a valid register
offset, so let the "undefined" value be 0 instead of -1.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
[anholt: clarified the commit message a bit]

3 files changed, 2 insertions, 6 deletions

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index dcc061cdc9a5..13ede75ff6ce 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -266,7 +266,7 @@ typedef struct drm_i915_private {
 
         struct notifier_block lid_notifier;
 
-        int crt_ddc_bus; /* -1 = unknown, else GPIO to use for CRT DDC */
+        int crt_ddc_bus; /* 0 = unknown, else GPIO to use for CRT DDC */
         struct drm_i915_fence_reg fence_regs[16]; /* assume 965 */
         int fence_reg_start; /* 4 if userland hasn't ioctl'd us yet */
         int num_fence_regs; /* 8 on pre-965, 16 otherwise */
diff --git a/drivers/gpu/drm/i915/intel_bios.c b/drivers/gpu/drm/i915/intel_bios.c
index 10806cd5e4d3..800d13bd9e6a 100644
--- a/drivers/gpu/drm/i915/intel_bios.c
+++ b/drivers/gpu/drm/i915/intel_bios.c
@@ -283,10 +283,6 @@ parse_general_definitions(struct drm_i915_private *dev_priv,
                 GPIOF,
         };
 
-        /* Set sensible defaults in case we can't find the general block
-           or it is the wrong chipset */
-        dev_priv->crt_ddc_bus = -1;
-
         general = find_section(bdb, BDB_GENERAL_DEFINITIONS);
         if (general) {
                 u16 block_size = get_blocksize(general);
diff --git a/drivers/gpu/drm/i915/intel_crt.c b/drivers/gpu/drm/i915/intel_crt.c
index 0864a2c40856..477a61c5402b 100644
--- a/drivers/gpu/drm/i915/intel_crt.c
+++ b/drivers/gpu/drm/i915/intel_crt.c
@@ -526,7 +526,7 @@ void intel_crt_init(struct drm_device *dev)
         else {
                 i2c_reg = GPIOA;
                 /* Use VBT information for CRT DDC if available */
-                if (dev_priv->crt_ddc_bus != -1)
+                if (dev_priv->crt_ddc_bus != 0)
                         i2c_reg = dev_priv->crt_ddc_bus;
         }
         intel_output->ddc_bus = intel_i2c_create(dev, i2c_reg, "CRTDDC_A");