drm/i915/sdvo: Restore guess of the DDC bus in absence of VBIOS

If the VBIOS tells us the mapping of the SDVO device onto the DDC bus,
use it. However, if there is no VBIOS available that mapping is
uninitialised and we should fallback to our earlier guess.

Fix regression introduced in b1083333 (which in turn is a fix for the
regression caused by the introduction of this guess, 14571b4).

References:

  Bug 29499 - [945GM] Screen disconnected because of missing VBIOS
  https://bugs.freedesktop.org/show_bug.cgi?id=29499

  Bug 15109 - i945GM fails to detect EDID on DVI port
  https://bugzilla.kernel.org/show_bug.cgi?id=15109

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Paul Neumann <paul104x@yahoo.de>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@kernel.org

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