[PATCH 02/12] drm/i915: Add a function to compute the EDID checksum for Displayport compliance

[Todd Previte <tprevite@gmail.com>]

This function computes the EDID checksum for a block of EDID data. This function
is necessary for Displayport compliance testing as it does not not require the
complete EDID checking functionality provided by the DRM layer functions.

Signed-off-by: Todd Previte <tprevite@gmail.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 0d11145..f61502e 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -3328,6 +3328,29 @@ intel_dp_get_sink_irq(struct intel_dp *intel_dp, u8 *sink_irq_vector)
 				       sink_irq_vector, 1) == 1;
 }
 
+static bool intel_dp_compute_edid_checksum(uint8_t *edid_data, uint8_t *edid_checksum)
+{
+	uint32_t byte_total = 0;
+	uint8_t i = 0;
+	bool edid_ok = true;
+
+	/* Compute byte total w/o the checksum value*/
+	for (i = 0; i < EDID_LENGTH - 2; i++)
+			byte_total += edid_data[i];
+
+	DRM_DEBUG_KMS("Displayport: EDID total = %d, EDID checksum =  %d\n", byte_total, edid_data[EDID_LENGTH - 1]);
+
+	/* Compute the checksum */
+	*edid_checksum = 256 - (byte_total % 256);
+
+	if (*edid_checksum != edid_data[EDID_LENGTH - 1]) {
+		DRM_DEBUG_KMS("Displayport: Invalid EDID checksum %d, should be %d\n", edid_data[EDID_LENGTH - 1], *edid_checksum);
+		edid_ok = false;
+	}
+
+	return edid_ok;
+}
+
 /* Displayport compliance testing - Link training */
 static uint8_t
 intel_dp_autotest_link_training(struct intel_dp *intel_dp)
-- 
1.9.1