linux-kernel - [PATCH 5.4 109/434] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller

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Message-Id: <20191229172708.876152596@linuxfoundation.org>
Date:   Sun, 29 Dec 2019 18:22:42 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Matthias Kaehlcke <mka@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Sean Paul <sean@...rly.run>,
        Neil Armstrong <narmstrong@...libre.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.4 109/434] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller

From: Matthias Kaehlcke <mka@...omium.org>

[ Upstream commit bee447e224b2645911c5d06e35dc90d8433fcef6 ]

The DDC/CI protocol involves sending a multi-byte request to the
display via I2C, which is typically followed by a multi-byte
response. The internal I2C controller only allows single byte
reads/writes or reads of 8 sequential bytes, hence DDC/CI is not
supported when the internal I2C controller is used. The I2C
transfers complete without errors, however the data in the response
is garbage. Abort transfers to/from slave address 0x37 (DDC) with
-EOPNOTSUPP, to make it evident that the communication is failing.

Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
Reviewed-by: Douglas Anderson <dianders@...omium.org>
Reviewed-by: Sean Paul <sean@...rly.run>
Acked-by: Neil Armstrong <narmstrong@...libre.com>
Signed-off-by: Neil Armstrong <narmstrong@...libre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002124354.v2.1.I709dfec496f5f0b44a7b61dcd4937924da8d8382@changeid
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index 521d689413c8..3e82d604201e 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -36,6 +36,7 @@
 #include "dw-hdmi-cec.h"
 #include "dw-hdmi.h"
 
+#define DDC_CI_ADDR		0x37
 #define DDC_SEGMENT_ADDR	0x30
 
 #define HDMI_EDID_LEN		512
@@ -398,6 +399,15 @@ static int dw_hdmi_i2c_xfer(struct i2c_adapter *adap,
 	u8 addr = msgs[0].addr;
 	int i, ret = 0;
 
+	if (addr == DDC_CI_ADDR)
+		/*
+		 * The internal I2C controller does not support the multi-byte
+		 * read and write operations needed for DDC/CI.
+		 * TOFIX: Blacklist the DDC/CI address until we filter out
+		 * unsupported I2C operations.
+		 */
+		return -EOPNOTSUPP;
+
 	dev_dbg(hdmi->dev, "xfer: num: %d, addr: %#x\n", num, addr);
 
 	for (i = 0; i < num; i++) {
-- 
2.20.1