1 files changed, 39 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_sdvo.c b/drivers/gpu/drm/i915/intel_sdvo.c
index 62d22ae4a1d1..e3b7a7ee39cb 100644
--- a/drivers/gpu/drm/i915/intel_sdvo.c
+++ b/drivers/gpu/drm/i915/intel_sdvo.c
@@ -1929,6 +1929,41 @@ static const struct drm_encoder_funcs intel_sdvo_enc_funcs = {
         .destroy = intel_sdvo_enc_destroy,
 };
 
+static void
+intel_sdvo_guess_ddc_bus(struct intel_sdvo *sdvo)
+{
+        uint16_t mask = 0;
+        unsigned int num_bits;
+
+        /* Make a mask of outputs less than or equal to our own priority in the
+         * list.
+         */
+        switch (sdvo->controlled_output) {
+        case SDVO_OUTPUT_LVDS1:
+                mask |= SDVO_OUTPUT_LVDS1;
+        case SDVO_OUTPUT_LVDS0:
+                mask |= SDVO_OUTPUT_LVDS0;
+        case SDVO_OUTPUT_TMDS1:
+                mask |= SDVO_OUTPUT_TMDS1;
+        case SDVO_OUTPUT_TMDS0:
+                mask |= SDVO_OUTPUT_TMDS0;
+        case SDVO_OUTPUT_RGB1:
+                mask |= SDVO_OUTPUT_RGB1;
+        case SDVO_OUTPUT_RGB0:
+                mask |= SDVO_OUTPUT_RGB0;
+                break;
+        }
+
+        /* Count bits to find what number we are in the priority list. */
+        mask &= sdvo->caps.output_flags;
+        num_bits = hweight16(mask);
+        /* If more than 3 outputs, default to DDC bus 3 for now. */
+        if (num_bits > 3)
+                num_bits = 3;
+
+        /* Corresponds to SDVO_CONTROL_BUS_DDCx */
+        sdvo->ddc_bus = 1 << num_bits;
+}
 
 /**
  * Choose the appropriate DDC bus for control bus switch command for this
@@ -1948,7 +1983,10 @@ intel_sdvo_select_ddc_bus(struct drm_i915_private *dev_priv,
         else
                 mapping = &(dev_priv->sdvo_mappings[1]);
 
-        sdvo->ddc_bus = 1 << ((mapping->ddc_pin & 0xf0) >> 4);
+        if (mapping->initialized)
+                sdvo->ddc_bus = 1 << ((mapping->ddc_pin & 0xf0) >> 4);
+        else
+                intel_sdvo_guess_ddc_bus(sdvo);
 }
 
 static bool