Message-ID: <57695F94.7030801@codeaurora.org>
Date:	Tue, 21 Jun 2016 21:09:00 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	Nicolas Boichat <drinkcat@...omium.org>,
	dri-devel@...ts.freedesktop.org
Cc:	marcheu@...omium.org, linux-kernel@...r.kernel.org,
	Enric Balletbo i Serra <enric.balletbo@...labora.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thierry Reding <treding@...dia.com>
Subject: Re: [RFC PATCH 1/2] drm: bridge: anx7688: Add anx7688 bridge driver
 support.

Hi,

On 6/20/2016 12:44 PM, Nicolas Boichat wrote:
> 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.

How does the chip know when to enable/disable itself? Does it shutoff
itself if there isn't anything on the HDMI link?

>
> Signed-off-by: Nicolas Boichat <drinkcat@...omium.org>
> ---
>
> 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


You should ideally have one DP connector at the end of the chain:

	- MTK HDMI bridge/encoder
             - ANX7688 bridge
                - Connector (DP)

In the dt-bindings for this board, hdmi's output port shouldn't be
connected to a hdmi connector, but the input port of the ANX7688
DP bridge. The output port of the bridge should be the one that
connects to the DP connector.

One way I can think of fixing this is to make make the MTK hdmi
encoder driver a bit smarter by observing the DT connections. If
it's output port is connected to just a hdmi-connector, then
things should be as before. If the output is connected to the DP
bridge, then it should create a DP connector. The connector ops
for the DP connector can still be the same as that of the HDMI
connector before, but the phandle to the DDC bus would be in the
DP device node in this case.

This way, you can get around having the correct layout.

Ideally, a bridge driver shouldn't be the one that creates a
connector. It may contain some of the connector functionality, but
the connector creation should be managed by the kms driver.
Almost all bridge drivers creating a connector in their .attach
callbacks since they own some of the connector functionality (like
reading EDID). That's something we're trying to fix by providing
some more bridge api that kms drivers can use.

Since this bridge driver doesn't have any connector functionality
anyway, you should be okay.

>
>      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.

I'm not clear about this either. Some drivers set a bus format
on the connector via drm_display_info_set_bus_formats in their
get_modes connector op, and then retrieve it later.

>
> 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>

The 3 headers above aren't needed.

> +#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;

It might make sense to use a u32 or long or something here to prevent
risk of overflow.

> +	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);

Who programmed these registers in the first place? Is the lane count
some sort of reset value? Or is it something that changes dynamically?

> +	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. */

s/compatiblity/compatibility

> +		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@...omium.org>");
> +MODULE_LICENSE("GPL v2");
>

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