drm/i915: Reduce ELD hex dumps a bit

Do the ELD hexdumps only up to the last differing byte.
The rest is typically all zeroes anyway so not much point
in dumping it.

Couldn't find anything for memcmp_diff_len() so
rolled my own.

v2: Use semantics and function name suggested by Jani

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215150129.13288-1-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_display.c

@@ -5344,6 +5344,20 @@ pipe_config_dp_vsc_sdp_mismatch(struct drm_i915_private *dev_priv,
 	}
 }
 
+/* Returns the length up to and including the last differing byte */
+static size_t
+memcmp_diff_len(const u8 *a, const u8 *b, size_t len)
+{
+	int i;
+
+	for (i = len - 1; i >= 0; i--) {
+		if (a[i] != b[i])
+			return i + 1;
+	}
+
+	return 0;
+}
+
 static void
 pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
 			    bool fastset, const char *name,
@@ -5353,6 +5367,9 @@ pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
 		if (!drm_debug_enabled(DRM_UT_KMS))
 			return;
 
+		/* only dump up to the last difference */
+		len = memcmp_diff_len(a, b, len);
+
 		drm_dbg_kms(&dev_priv->drm,
 			    "fastset mismatch in %s buffer\n", name);
 		print_hex_dump(KERN_DEBUG, "expected: ", DUMP_PREFIX_NONE,
@@ -5360,6 +5377,9 @@ pipe_config_buffer_mismatch(struct drm_i915_private *dev_priv,
 		print_hex_dump(KERN_DEBUG, "found: ", DUMP_PREFIX_NONE,
 			       16, 0, b, len, false);
 	} else {
+		/* only dump up to the last difference */
+		len = memcmp_diff_len(a, b, len);
+
 		drm_err(&dev_priv->drm, "mismatch in %s buffer\n", name);
 		print_hex_dump(KERN_ERR, "expected: ", DUMP_PREFIX_NONE,
 			       16, 0, a, len, false);