[PATCH] drm/i915: make sure we write all the DIP data bytes

[Paulo Zanoni <przanoni@gmail.com>]

From: Paulo Zanoni <paulo.r.zanoni@intel.com>

... even if the actual infoframe is smaller than the maximum possible
size.

If we don't write all the 32 DIP data bytes the InfoFrame ECC may not
be correctly calculated in some cases (e.g., when changing the port),
and this will lead to black screens on HDMI monitors. The ECC value is
generated by the hardware.

I don't see how this should break anything since we're writing 0 and
that should be the correct value, so this patch should be safe.

Notice that on IVB and older we actually have 64 bytes available for
VIDEO_DIP_DATA, but only bytes 0-31 actually store infoframe data: the
others are either read-only ECC values or marked as "reserved". On HSW
we only have 32 bytes, and the ECC value is stored on its own separate
read-only register. See BSpec.

This patch fixes bug #46761, which is marked as a regression
introduced by commit 4e89ee174bb2da341bf90a84321c7008a3c9210d:
    drm/i915: set the DIP port on ibx_write_infoframe

Before commit 4e89 we were just failing to send AVI infoframes when we
needed to change the port, which can lead to black screens in some
cases. After commit 4e89 we started sending infoframes, but with a
possibly wrong ECC value. After this patch I hope we start sending
correct infoframes.

Version 2:
  - Improve commit message
  - Try to make the code more clear

Cc: stable@vger.kernel.org
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=46761
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
---
 drivers/gpu/drm/i915/i915_reg.h   |    4 ++++
 drivers/gpu/drm/i915/intel_hdmi.c |   15 +++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index a828e90..7637824 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -1794,6 +1794,10 @@
 
 /* Video Data Island Packet control */
 #define VIDEO_DIP_DATA		0x61178
+/* Read the description of VIDEO_DIP_DATA (before Haswel) or VIDEO_DIP_ECC
+ * (Haswell and newer) to see which VIDEO_DIP_DATA byte corresponds to each byte
+ * of the infoframe structure specified by CEA-861. */
+#define   VIDEO_DIP_DATA_SIZE	32
 #define VIDEO_DIP_CTL		0x61170
 /* Pre HSW: */
 #define   VIDEO_DIP_ENABLE		(1 << 31)
diff --git a/drivers/gpu/drm/i915/intel_hdmi.c b/drivers/gpu/drm/i915/intel_hdmi.c
index f9fb47c..08f2b63 100644
--- a/drivers/gpu/drm/i915/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/intel_hdmi.c
@@ -151,6 +151,9 @@ static void g4x_write_infoframe(struct drm_encoder *encoder,
 		I915_WRITE(VIDEO_DIP_DATA, *data);
 		data++;
 	}
+	/* Write every possible data byte to force correct ECC calculation. */
+	for (; i < VIDEO_DIP_DATA_SIZE; i += 4)
+		I915_WRITE(VIDEO_DIP_DATA, 0);
 	mmiowb();
 
 	val |= g4x_infoframe_enable(frame);
@@ -186,6 +189,9 @@ static void ibx_write_infoframe(struct drm_encoder *encoder,
 		I915_WRITE(TVIDEO_DIP_DATA(intel_crtc->pipe), *data);
 		data++;
 	}
+	/* Write every possible data byte to force correct ECC calculation. */
+	for (; i < VIDEO_DIP_DATA_SIZE; i += 4)
+		I915_WRITE(TVIDEO_DIP_DATA(intel_crtc->pipe), 0);
 	mmiowb();
 
 	val |= g4x_infoframe_enable(frame);
@@ -224,6 +230,9 @@ static void cpt_write_infoframe(struct drm_encoder *encoder,
 		I915_WRITE(TVIDEO_DIP_DATA(intel_crtc->pipe), *data);
 		data++;
 	}
+	/* Write every possible data byte to force correct ECC calculation. */
+	for (; i < VIDEO_DIP_DATA_SIZE; i += 4)
+		I915_WRITE(TVIDEO_DIP_DATA(intel_crtc->pipe), 0);
 	mmiowb();
 
 	val |= g4x_infoframe_enable(frame);
@@ -259,6 +268,9 @@ static void vlv_write_infoframe(struct drm_encoder *encoder,
 		I915_WRITE(VLV_TVIDEO_DIP_DATA(intel_crtc->pipe), *data);
 		data++;
 	}
+	/* Write every possible data byte to force correct ECC calculation. */
+	for (; i < VIDEO_DIP_DATA_SIZE; i += 4)
+		I915_WRITE(VLV_TVIDEO_DIP_DATA(intel_crtc->pipe), 0);
 	mmiowb();
 
 	val |= g4x_infoframe_enable(frame);
@@ -292,6 +304,9 @@ static void hsw_write_infoframe(struct drm_encoder *encoder,
 		I915_WRITE(data_reg + i, *data);
 		data++;
 	}
+	/* Write every possible data byte to force correct ECC calculation. */
+	for (; i < VIDEO_DIP_DATA_SIZE; i += 4)
+		I915_WRITE(data_reg + i, 0);
 	mmiowb();
 
 	val |= hsw_infoframe_enable(frame);
-- 
1.7.10.4