drm/i915/bios: Don't parse the DPS panel type when the VBT does not have it

Older VBTs don't have all the stuff we've defined for the
LVDS options block (40). In particular we're currently parsing
the DPS panel type bits even though they may not exist, which
could mean we end up flagging the machine as supporting static
DRRS when the VBT declared no such thing.

We don't actually have a clear idea which VBT versions have
which bits so we rely on the block size instead.

Here's a quick list from my VBT stash:
mgm version 108 -> 4 bytes
alv version 120 -> 4 bytes
cst version 134 -> 14 bytes
pnv version 144 -> 14 bytes
cl  version 142 -> 16 bytes
ctg version 155 -> 24 bytes

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220615151445.8531-2-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
index 5ed8ceb..f37058f 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_bios.c
+++ b/drivers/gpu/drm/i915/display/intel_bios.c
@@ -757,6 +757,14 @@ parse_panel_options(struct drm_i915_private *i915,
 
        panel->vbt.lvds_dither = lvds_options->pixel_dither;
 
+       /*
+        * Empirical evidence indicates the block size can be
+        * either 4,14,16,24+ bytes. For older VBTs no clear
+        * relationship between the block size vs. BDB version.
+        */
+       if (get_blocksize(lvds_options) < 16)
+               return;
+
        drrs_mode = (lvds_options->dps_panel_type_bits
                                >> (panel_type * 2)) & MODE_MASK;
        /*