[RFC PATCH 1/2] drm: bridge: anx7688: Add anx7688 bridge driver support.

[Nicolas Boichat]

ANX7688 is a HDMI to DP converter (as well as USB-C port controller),
that has an internal microcontroller.

The only reason a Linux kernel driver is necessary is to reject
resolutions that require more bandwidth than what is available on
the DP side. DP bandwidth and lane count are reported by the bridge
via 2 registers on I2C.

Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
---

Hi,

I tested this driver using the Mediatek HDMI controller (MT8173) upstream
of the ANX bridge chip (Phillip sent a PULL request on June 13:
git://git.pengutronix.de/git/pza/linux.git tags/mediatek-drm-2016-06-13
).

I have 2 concerns, that I'm not sure how to address within the kernel DRM
framework:
 1. All other bridge drivers also have a connector attached to it. However, in
    this case, we cannot read the monitor EDID directly, so I'm not sure what
    I could put in a "get_modes" function. Instead, ANX7688 provides a I2C
    passthru/repeater, so the EDID is read on the Mediatek HDMI controller side.
    
    That leads to a somewhat strange layout, where we have:
    - MTK HDMI bridge/encoder
      - MTK connector (HDMI)
      - ANX7688 bridge
        - No connector

    Resolution filtering works fine though, thanks to mode_fixup callback on the
    bridge. It also helps that Mediatek HDMI bridge calls mode_fixup from its
    connector mode_valid callback, so that invalid modes are not even presented
    to userspace.

 2. In the bandwidth computation, I hard-code 8-bit per channel (bpc). bpc does
    not seem to be included in the mode setting itself. We could possibly iterate
    over connectors on the DRM device, but then, IIUC, connector->display_info.bpc
    indicates the _maximum_ bpc supported by the monitor.

Any pointers? Thanks!

Best,

Nicolas

 drivers/gpu/drm/bridge/Kconfig            |   9 ++
 drivers/gpu/drm/bridge/Makefile           |   1 +
 drivers/gpu/drm/bridge/analogix-anx7688.c | 227 ++++++++++++++++++++++++++++++
 3 files changed, 237 insertions(+)
 create mode 100644 drivers/gpu/drm/bridge/analogix-anx7688.c

diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
index 8f7423f..0c1eb41 100644
--- a/drivers/gpu/drm/bridge/Kconfig
+++ b/drivers/gpu/drm/bridge/Kconfig
@@ -7,6 +7,15 @@ config DRM_BRIDGE
 menu "Display Interface Bridges"
 	depends on DRM && DRM_BRIDGE
 
+config DRM_ANALOGIX_ANX7688
+	tristate "Analogix ANX7688 bridge"
+	depends on DRM
+	select DRM_KMS_HELPER
+	---help---
+	  ANX7688 is a transmitter to support DisplayPort over USB-C for
+	  smartphone and tablets.
+	  This driver only supports the HDMI to DP component of the chip.
+
 config DRM_ANALOGIX_ANX78XX
 	tristate "Analogix ANX78XX bridge"
 	select DRM_KMS_HELPER
diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
index 96b13b3..d744c6c 100644
--- a/drivers/gpu/drm/bridge/Makefile
+++ b/drivers/gpu/drm/bridge/Makefile
@@ -1,5 +1,6 @@
 ccflags-y := -Iinclude/drm
 
+obj-$(CONFIG_DRM_ANALOGIX_ANX7688) += analogix-anx7688.o
 obj-$(CONFIG_DRM_ANALOGIX_ANX78XX) += analogix-anx78xx.o
 obj-$(CONFIG_DRM_DW_HDMI) += dw-hdmi.o
 obj-$(CONFIG_DRM_DW_HDMI_AHB_AUDIO) += dw-hdmi-ahb-audio.o
diff --git a/drivers/gpu/drm/bridge/analogix-anx7688.c b/drivers/gpu/drm/bridge/analogix-anx7688.c
new file mode 100644
index 0000000..2c34029
--- /dev/null
+++ b/drivers/gpu/drm/bridge/analogix-anx7688.c
@@ -0,0 +1,227 @@
+/*
+ * ANX7688 HDMI->DP bridge driver
+ *
+ * Copyright (C) 2016 Google, Inc.
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/delay.h>
+#include <linux/gpio.h>
+#include <linux/i2c.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/of_gpio.h>
+#include <drm/drmP.h>
+#include <drm/drm_crtc.h>
+
+/* Register addresses */
+#define VENDOR_ID_REG 0x00
+#define DEVICE_ID_REG 0x02
+
+#define FW_VERSION_REG 0x80
+
+#define DP_BANDWIDTH_REG 0x85
+#define DP_LANE_COUNT_REG 0x86
+
+#define VENDOR_ID 0x1f29
+#define DEVICE_ID 0x7688
+
+/* First supported firmware version (0.85) */
+#define MINIMUM_FW_VERSION 0x0085
+
+struct anx7688 {
+	struct drm_bridge bridge;
+	struct i2c_client *client;
+
+	bool filter;
+};
+
+static int anx7688_read(struct i2c_client *client, u8 reg, u8 *data,
+			u16 data_len)
+{
+	int ret;
+	struct i2c_msg msgs[] = {
+		{
+		 .addr = client->addr,
+		 .flags = 0,
+		 .len = 1,
+		 .buf = &reg,
+		 },
+		{
+		 .addr = client->addr,
+		 .flags = I2C_M_RD,
+		 .len = data_len,
+		 .buf = data,
+		 }
+	};
+
+	ret = i2c_transfer(client->adapter, msgs, 2);
+
+	if (ret == 2)
+		return 0;
+	if (ret < 0)
+		return ret;
+	else
+		return -EIO;
+}
+
+static inline struct anx7688 *bridge_to_anx7688(struct drm_bridge *bridge)
+{
+	return container_of(bridge, struct anx7688, bridge);
+}
+
+static bool anx7688_bridge_mode_fixup(struct drm_bridge *bridge,
+				      const struct drm_display_mode *mode,
+				      struct drm_display_mode *adjusted_mode)
+{
+	struct anx7688 *anx7688 = bridge_to_anx7688(bridge);
+	u8 regs[2];
+	int totalbw, requiredbw;
+	int ret;
+
+	if (!anx7688->filter)
+		return true;
+
+	/* Read both regs 0x85 (bandwidth) and 0x86 (lane count). */
+	ret = anx7688_read(anx7688->client, DP_BANDWIDTH_REG, regs, 2);
+	if (ret < 0) {
+		dev_err(&anx7688->client->dev,
+			"Failed to read bandwidth/lane count\n");
+		return false;
+	}
+
+	/* Compute available rate (kHz) */
+	totalbw = regs[0] * regs[1] * 270000 * 8 / 10;
+
+	/* Required bandwidth (8 bpc) */
+	requiredbw = mode->clock * 8 * 3;
+
+	dev_dbg(&anx7688->client->dev,
+		"DP bandwidth: %d kHz (%02x/%d); mode requires %d Khz\n",
+		totalbw, regs[0], regs[1], requiredbw);
+
+	if (totalbw == 0) {
+		dev_warn(&anx7688->client->dev,
+			 "Bandwidth/lane count are 0, not rejecting modes\n");
+		return true;
+	}
+
+	return totalbw >= requiredbw;
+}
+
+static const struct drm_bridge_funcs anx7688_bridge_funcs = {
+	.mode_fixup	= anx7688_bridge_mode_fixup,
+};
+
+static int anx7688_i2c_probe(struct i2c_client *client,
+			     const struct i2c_device_id *id)
+{
+	struct anx7688 *anx7688;
+	int ret;
+	u8 buffer[4];
+	u16 vendor, device, fwversion;
+
+	anx7688 = devm_kzalloc(&client->dev, sizeof(*anx7688), GFP_KERNEL);
+	if (!anx7688)
+		return -ENOMEM;
+
+#if IS_ENABLED(CONFIG_OF)
+	anx7688->bridge.of_node = client->dev.of_node;
+#endif
+
+	anx7688->client = client;
+	i2c_set_clientdata(client, anx7688);
+
+	/* Read both vendor and device id (4 bytes). */
+	ret = anx7688_read(client, VENDOR_ID_REG, buffer, 4);
+	if (ret) {
+		dev_err(&client->dev, "Failed to read chip vendor/device id\n");
+		return ret;
+	}
+
+	vendor = (u16)buffer[1] << 8 | buffer[0];
+	device = (u16)buffer[3] << 8 | buffer[2];
+	if (vendor != VENDOR_ID || device != DEVICE_ID) {
+		dev_err(&client->dev,
+			"Invalid vendor/device id %04x/%04x\n",
+			vendor, device);
+		return -ENODEV;
+	}
+
+	ret = anx7688_read(client, FW_VERSION_REG, buffer, 2);
+	if (ret) {
+		dev_err(&client->dev, "Failed to read firmware version\n");
+		return ret;
+	}
+
+	fwversion = (u16)buffer[0] << 8 | buffer[1];
+	dev_info(&client->dev, "ANX7688 firwmare version %02x.%02x\n",
+		 buffer[0], buffer[1]);
+
+	/* FW version >= 0.85 supports bandwidth/lane count registers */
+	if (fwversion >= MINIMUM_FW_VERSION) {
+		anx7688->filter = true;
+	} else {
+		/* Warn, but not fail, for backwards compatiblity. */
+		dev_warn(&client->dev,
+			 "Old ANX7688 FW version (%04x), not filtering\n",
+			 fwversion);
+	}
+
+	anx7688->bridge.funcs = &anx7688_bridge_funcs;
+	ret = drm_bridge_add(&anx7688->bridge);
+	if (ret < 0) {
+		dev_err(&client->dev, "Failed to add drm bridge\n");
+		return ret;
+	}
+
+	return 0;
+}
+
+static int anx7688_i2c_remove(struct i2c_client *client)
+{
+	struct anx7688 *anx7688 = i2c_get_clientdata(client);
+
+	drm_bridge_remove(&anx7688->bridge);
+
+	return 0;
+}
+
+static const struct i2c_device_id anx7688_id[] = {
+	{ "anx7688", 0 },
+	{ /* sentinel */ }
+};
+
+MODULE_DEVICE_TABLE(i2c, anx7688_id);
+
+#if IS_ENABLED(CONFIG_OF)
+static const struct of_device_id anx7688_match_table[] = {
+	{ .compatible = "analogix,anx7688", },
+	{ /* sentinel */ },
+};
+MODULE_DEVICE_TABLE(of, anx7688_match_table);
+#endif
+
+static struct i2c_driver anx7688_driver = {
+	.driver = {
+		   .name = "anx7688",
+		   .of_match_table = of_match_ptr(anx7688_match_table),
+		  },
+	.probe = anx7688_i2c_probe,
+	.remove = anx7688_i2c_remove,
+	.id_table = anx7688_id,
+};
+
+module_i2c_driver(anx7688_driver);
+
+MODULE_DESCRIPTION("ANX7688 SlimPort Transmitter driver");
+MODULE_AUTHOR("Nicolas Boichat <drinkcat@xxxxxxxxxxxx>");
+MODULE_LICENSE("GPL v2");
-- 
2.8.0.rc3.226.g39d4020