[Intel-gfx] [PATCH v2] drm/i915: Reduce ELD hex dumps a bit

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

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 at intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 3479125fbda6..664322a58236 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -5348,6 +5348,20 @@ 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,
@@ -5357,6 +5371,9 @@ 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,
@@ -5364,6 +5381,9 @@ 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);
-- 
2.39.1