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From: Svyatoslav Ryhel <clamor95@gmail.com>
To: Neil Armstrong <neil.armstrong@linaro.org>,
	Jessica Zhang <jesszhan0024@gmail.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>,
	Simona Vetter <simona@ffwll.ch>,
	Rob Herring <robh@kernel.org>,
	Krzysztof Kozlowski <krzk+dt@kernel.org>,
	Conor Dooley <conor+dt@kernel.org>,
	Thierry Reding <thierry.reding@kernel.org>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Mikko Perttunen <mperttunen@nvidia.com>,
	Svyatoslav Ryhel <clamor95@gmail.com>
Cc: dri-devel@lists.freedesktop.org,
	devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-tegra@vger.kernel.org
Subject: [PATCH v1 4/6] drm/tegra: Add support for 8-bit CPU interface
Date: Wed, 30 Sep 2026 10:05:33 +0300
Message-ID: <20260930070535.47130-5-clamor95@gmail.com>
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The display controller in Tegra20/30 SoCs features an 8-bit SPI interface
that closely resembles the MIPI DBI Type B protocol and is referred to as
'8-bit CPU'. Each display controller provides two such interfaces, which
can be used to send MIPI DCS commands to initialize and control the panel
while image data is transmitted via 16/18/24-line RGB.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
---
 drivers/gpu/drm/tegra/Kconfig      |   7 +
 drivers/gpu/drm/tegra/Makefile     |   1 +
 drivers/gpu/drm/tegra/cpu-bridge.c | 427 +++++++++++++++++++++++++++++
 3 files changed, 435 insertions(+)
 create mode 100644 drivers/gpu/drm/tegra/cpu-bridge.c

diff --git a/drivers/gpu/drm/tegra/Kconfig b/drivers/gpu/drm/tegra/Kconfig
index 8a3b16aac5d68..db9839280b8aa 100644
--- a/drivers/gpu/drm/tegra/Kconfig
+++ b/drivers/gpu/drm/tegra/Kconfig
@@ -30,6 +30,13 @@ config DRM_TEGRA
 
 if DRM_TEGRA
 
+config DRM_TEGRA_CPU_BRIDGE
+	bool "Enable 8 bit panel communication protocol for Tegra 20/30"
+	help
+	  Tegra20 and Tegra30 feature 8 bit CPU driver panel control
+	  protocol (similar to MIPI DBI Type B). This option allows use
+	  it as a MIPI DBI bridge to set up and control compatible panel.
+
 config DRM_TEGRA_DEBUG
 	bool "NVIDIA Tegra DRM debug support"
 	help
diff --git a/drivers/gpu/drm/tegra/Makefile b/drivers/gpu/drm/tegra/Makefile
index e399b40d64a1d..88d5ed2f9ded0 100644
--- a/drivers/gpu/drm/tegra/Makefile
+++ b/drivers/gpu/drm/tegra/Makefile
@@ -31,5 +31,6 @@ tegra-drm-y := \
 tegra-drm-y += trace.o
 
 tegra-drm-$(CONFIG_DRM_FBDEV_EMULATION) += fbdev.o
+tegra-drm-$(CONFIG_DRM_TEGRA_CPU_BRIDGE) += cpu-bridge.o
 
 obj-$(CONFIG_DRM_TEGRA) += tegra-drm.o
diff --git a/drivers/gpu/drm/tegra/cpu-bridge.c b/drivers/gpu/drm/tegra/cpu-bridge.c
new file mode 100644
index 0000000000000..63ca87a0be909
--- /dev/null
+++ b/drivers/gpu/drm/tegra/cpu-bridge.c
@@ -0,0 +1,427 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/delay.h>
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/kernel.h>
+#include <linux/media-bus-format.h>
+#include <linux/module.h>
+#include <linux/of_platform.h>
+#include <linux/platform_device.h>
+
+#include <drm/drm_atomic_helper.h>
+#include <drm/drm_bridge.h>
+#include <drm/drm_drv.h>
+#include <drm/drm_mipi_dbi.h>
+#include <drm/drm_of.h>
+#include <drm/drm_panel.h>
+#include <video/mipi_display.h>
+
+#include "dc.h"
+
+#define CPU_BRIDGE_COMMAND			0
+#define CPU_BRIDGE_DATA				1
+#define CPU_BRIDGE_DATA_PINS_MAX		8
+
+struct tegra_cpu_bridge_dbi_output {
+	struct drm_panel *panel;
+	struct drm_bridge *bridge;
+};
+
+struct tegra_cpu_bridge_priv {
+	struct mipi_dbi dbi;		/* must be first */
+	struct device *dev;
+	struct tegra_dc *dc;
+
+	struct gpio_desc *dc_gpio;
+	struct gpio_desc *rw_gpio;
+	struct gpio_desc *cs_gpio;
+	struct gpio_descs *data_gpios;
+
+	struct drm_bridge bridge;
+	struct tegra_cpu_bridge_dbi_output output;
+
+	u32 spi_init_seq[4];
+	u32 bus_width;
+	bool prepared;
+};
+
+static inline struct tegra_cpu_bridge_priv *mipi_dbi_to_tegra_cpu_bridge(struct mipi_dbi *dbi)
+{
+	return container_of(dbi, struct tegra_cpu_bridge_priv, dbi);
+}
+
+static inline struct tegra_cpu_bridge_priv *bridge_to_tegra_cpu_bridge(struct drm_bridge *bridge)
+{
+	return container_of(bridge, struct tegra_cpu_bridge_priv, bridge);
+}
+
+static void tegra_cpu_bridge_write(struct tegra_cpu_bridge_priv *priv, u8 type, u8 value)
+{
+	DECLARE_BITMAP(value_bitmap, 8);
+
+	value_bitmap[0] = value;
+
+	gpiod_set_value(priv->dc_gpio, type);
+
+	gpiod_set_value(priv->cs_gpio, 0);
+	gpiod_set_value(priv->rw_gpio, 0);
+
+	gpiod_set_array_value(priv->data_gpios->ndescs, priv->data_gpios->desc,
+			      priv->data_gpios->info, value_bitmap);
+
+	gpiod_set_value(priv->cs_gpio, 1);
+	gpiod_set_value(priv->rw_gpio, 1);
+
+	udelay(10);
+}
+
+static int tegra_cpu_bridge_dbi_command(struct mipi_dbi *dbi, u8 *cmd, u8 *param, size_t num)
+{
+	struct tegra_cpu_bridge_priv *priv = mipi_dbi_to_tegra_cpu_bridge(dbi);
+	u8 command = *cmd;
+
+	tegra_cpu_bridge_write(priv, CPU_BRIDGE_COMMAND, command);
+
+	for (int i = 0; i < num; i++)
+		tegra_cpu_bridge_write(priv, CPU_BRIDGE_DATA, param[i]);
+
+	return 0;
+}
+
+static int tegra_cpu_bridge_prepare_gpios(struct tegra_cpu_bridge_priv *priv)
+{
+	struct device *dev = priv->dev;
+
+	if (priv->prepared)
+		return 0;
+
+	/*
+	 * Control and data GPIOs are shared between SPI and DPI. The bridge
+	 * driver must get and put the required GPIOs each time DCS commands
+	 * are transmitted.
+	 */
+
+	priv->data_gpios = devm_gpiod_get_array_optional(dev, "data", GPIOD_OUT_LOW);
+	if (IS_ERR(priv->data_gpios)) {
+		dev_err(dev, "Failed to get data gpios %ld\n", PTR_ERR(priv->data_gpios));
+		return PTR_ERR(priv->data_gpios);
+	}
+
+	if (priv->data_gpios && priv->data_gpios->ndescs > CPU_BRIDGE_DATA_PINS_MAX) {
+		dev_err(dev, "Too many data gpios\n");
+		return -EINVAL;
+	}
+
+	priv->cs_gpio = devm_gpiod_get_optional(dev, "cs", GPIOD_OUT_HIGH);
+	if (IS_ERR(priv->cs_gpio)) {
+		dev_err(dev, "Failed to get CS GPIO: %ld\n", PTR_ERR(priv->cs_gpio));
+		return PTR_ERR(priv->cs_gpio);
+	}
+
+	priv->rw_gpio = devm_gpiod_get_optional(dev, "rw", GPIOD_OUT_HIGH);
+	if (IS_ERR(priv->rw_gpio)) {
+		dev_err(dev, "Failed to get RW GPIO: %ld\n", PTR_ERR(priv->rw_gpio));
+		return PTR_ERR(priv->rw_gpio);
+	}
+
+	priv->dc_gpio = devm_gpiod_get_optional(dev, "dc", GPIOD_OUT_LOW);
+	if (IS_ERR(priv->dc_gpio)) {
+		dev_err(dev, "Failed to get DC GPIO: %ld\n", PTR_ERR(priv->dc_gpio));
+		return PTR_ERR(priv->dc_gpio);
+	}
+
+	priv->prepared = true;
+
+	return 0;
+}
+
+static void tegra_cpu_bridge_atomic_enable(struct drm_bridge *bridge,
+					   struct drm_atomic_commit *state)
+{
+	struct tegra_cpu_bridge_priv *priv = bridge_to_tegra_cpu_bridge(bridge);
+	struct tegra_dc *dc = priv->dc;
+	u32 value;
+	int ret;
+
+	value = tegra_dc_readl(dc, DC_CMD_DISPLAY_COMMAND);
+	value &= ~DISP_CTRL_MODE_MASK;
+	tegra_dc_writel(dc, value, DC_CMD_DISPLAY_COMMAND);
+
+	/* Disable all options except cursor if enabled */
+	value = tegra_dc_readl(dc, DC_DISP_DISP_WIN_OPTIONS);
+	value &= CURSOR_ENABLE;
+	tegra_dc_writel(dc, value, DC_DISP_DISP_WIN_OPTIONS);
+
+	tegra_dc_commit(dc);
+
+	tegra_dc_writel(dc, V_PULSE1_ENABLE, DC_DISP_DISP_SIGNAL_OPTIONS0);
+	tegra_dc_writel(dc, PULSE_POLARITY_LOW, DC_DISP_V_PULSE1_CONTROL);
+
+	tegra_dc_writel(dc, PULSE_END(1), DC_DISP_V_PULSE0_POSITION_A);
+	tegra_dc_writel(dc, 0, DC_DISP_V_PULSE0_POSITION_B);
+	tegra_dc_writel(dc, 0, DC_DISP_V_PULSE0_POSITION_C);
+
+	value = 1 << FRAME_INIT_SEQ_CYCLES_SHIFT |
+		DC_SIGNAL_VPULSE1 << INIT_SEQ_DC_SIGNAL_SHIFT |
+		INIT_SEQUENCE_MODE_PLCD | SEND_INIT_SEQUENCE;
+	tegra_dc_writel(dc, value, DC_DISP_INIT_SEQ_CONTROL);
+
+	tegra_dc_writel(dc, priv->spi_init_seq[0], DC_DISP_SPI_INIT_SEQ_DATA_A);
+	tegra_dc_writel(dc, priv->spi_init_seq[1], DC_DISP_SPI_INIT_SEQ_DATA_B);
+	tegra_dc_writel(dc, priv->spi_init_seq[2], DC_DISP_SPI_INIT_SEQ_DATA_C);
+	tegra_dc_writel(dc, priv->spi_init_seq[3], DC_DISP_SPI_INIT_SEQ_DATA_D);
+
+	value = tegra_dc_readl(dc, DC_CMD_DISPLAY_COMMAND);
+	value &= ~DISP_CTRL_MODE_MASK;
+	value |= DISP_CTRL_MODE_C_DISPLAY;
+	tegra_dc_writel(dc, value, DC_CMD_DISPLAY_COMMAND);
+
+	/* set LDC pin to V Pulse 1 */
+	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(6));
+	value |= LDC_OUTPUT_SELECT_V_PULSE1;
+	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(6));
+
+	value = SC1_H_QUALIFIER(SC_H_QUALIFIER_NONE) |
+		SC0_V_QUALIFIER(SC_V_QUALIFIER_VACTIVE) |
+		SC0_H_QUALIFIER(SC_H_QUALIFIER_HACTIVE);
+	tegra_dc_writel(dc, value, DC_DISP_SHIFT_CLOCK_OPTIONS);
+
+	value = tegra_dc_readl(dc, DC_DISP_DISP_COLOR_CONTROL);
+	value &= ~BASE_COLOR_SIZE_MASK;
+
+	switch (priv->bus_width) {
+	case 16:
+		value |= BASE_COLOR_SIZE_565;
+		break;
+	case 18:
+		value |= BASE_COLOR_SIZE_666;
+		break;
+	case 24:
+	default:
+		value |= BASE_COLOR_SIZE_888;
+		break;
+	}
+
+	tegra_dc_writel(dc, value, DC_DISP_DISP_COLOR_CONTROL);
+
+	ret = tegra_cpu_bridge_prepare_gpios(priv);
+	if (ret)
+		return;
+
+	if (priv->output.panel)
+		drm_panel_enable(priv->output.panel);
+
+	gpiod_set_value(priv->cs_gpio, 0);
+
+	devm_gpiod_put(priv->dev, priv->dc_gpio);
+	devm_gpiod_put(priv->dev, priv->rw_gpio);
+	devm_gpiod_put(priv->dev, priv->cs_gpio);
+
+	devm_gpiod_put_array(priv->dev, priv->data_gpios);
+	priv->prepared = false;
+}
+
+static void tegra_cpu_bridge_atomic_disable(struct drm_bridge *bridge,
+					    struct drm_atomic_commit *state)
+{
+	struct tegra_cpu_bridge_priv *priv = bridge_to_tegra_cpu_bridge(bridge);
+	int ret;
+
+	ret = tegra_cpu_bridge_prepare_gpios(priv);
+	if (ret)
+		return;
+
+	if (priv->output.panel)
+		drm_panel_disable(priv->output.panel);
+
+	gpiod_set_value(priv->cs_gpio, 0);
+
+	devm_gpiod_put(priv->dev, priv->dc_gpio);
+	devm_gpiod_put(priv->dev, priv->rw_gpio);
+	devm_gpiod_put(priv->dev, priv->cs_gpio);
+
+	devm_gpiod_put_array(priv->dev, priv->data_gpios);
+	priv->prepared = false;
+}
+
+static int tegra_cpu_bridge_atomic_check(struct drm_bridge *bridge,
+					 struct drm_bridge_state *bridge_state,
+					 struct drm_crtc_state *crtc_state,
+					 struct drm_connector_state *conn_state)
+{
+	struct drm_display_mode *adjusted_mode = &crtc_state->adjusted_mode;
+
+	/* Set safe default bus flags if panel does not specify them */
+	if (!bridge_state->input_bus_cfg.flags)
+		bridge_state->input_bus_cfg.flags = bridge->timings->input_bus_flags;
+
+	/* Default to positive sync */
+	if (!(adjusted_mode->flags &
+	      (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_NHSYNC)))
+		adjusted_mode->flags |= DRM_MODE_FLAG_PHSYNC;
+
+	if (!(adjusted_mode->flags &
+	      (DRM_MODE_FLAG_PVSYNC | DRM_MODE_FLAG_NVSYNC)))
+		adjusted_mode->flags |= DRM_MODE_FLAG_PVSYNC;
+
+	return 0;
+}
+
+static int tegra_cpu_bridge_attach(struct drm_bridge *bridge,
+				   struct drm_encoder *encoder,
+				   enum drm_bridge_attach_flags flags)
+{
+	struct tegra_cpu_bridge_priv *priv = bridge_to_tegra_cpu_bridge(bridge);
+	struct device_node *rgb_node, *dc_node;
+	struct platform_device *dc_pdev;
+
+	rgb_node = of_graph_get_remote_node(dev_of_node(priv->dev), 0, 0);
+	if (!rgb_node)
+		return -ENODEV;
+
+	dc_node = of_get_parent(rgb_node);
+	of_node_put(rgb_node);
+	if (!dc_node)
+		return -ENODEV;
+
+	dc_pdev = of_find_device_by_node(dc_node);
+	of_node_put(dc_node);
+	if (!dc_pdev)
+		return -ENODEV;
+
+	priv->dc = platform_get_drvdata(dc_pdev);
+	if (!priv->dc)
+		return -ENODEV;
+
+	return drm_bridge_attach(bridge->encoder, priv->output.bridge, bridge,
+				 flags);
+}
+
+static enum drm_mode_status
+tegra_cpu_bridge_mode_valid(struct drm_bridge *bridge,
+			    const struct drm_display_info *info,
+			    const struct drm_display_mode *mode)
+{
+	if (mode->hdisplay > 800)
+		return MODE_H_ILLEGAL;
+
+	if (mode->vdisplay > 800)
+		return MODE_V_ILLEGAL;
+
+	return MODE_OK;
+}
+
+static const struct drm_bridge_funcs tegra_cpu_bridge_funcs = {
+	.attach = tegra_cpu_bridge_attach,
+	.mode_valid = tegra_cpu_bridge_mode_valid,
+
+	.atomic_enable = tegra_cpu_bridge_atomic_enable,
+	.atomic_disable = tegra_cpu_bridge_atomic_disable,
+	.atomic_check = tegra_cpu_bridge_atomic_check,
+
+	.atomic_create_state = drm_atomic_helper_bridge_create_state,
+	.atomic_duplicate_state = drm_atomic_helper_bridge_duplicate_state,
+	.atomic_destroy_state = drm_atomic_helper_bridge_destroy_state,
+};
+
+static const struct drm_bridge_timings default_tegra_cpu_bridge_timings = {
+	.input_bus_flags = DRM_BUS_FLAG_PIXDATA_SAMPLE_POSEDGE |
+			   DRM_BUS_FLAG_SYNC_SAMPLE_NEGEDGE |
+			   DRM_BUS_FLAG_DE_HIGH,
+};
+
+static int tegra_cpu_bridge_probe(struct platform_device *pdev)
+{
+	struct tegra_cpu_bridge_priv *priv;
+	struct device *dev = &pdev->dev;
+	struct device_node *np = dev->of_node;
+	struct drm_bridge *bridge;
+	struct drm_panel *panel;
+	struct device_node *ep;
+	int ret;
+
+	priv = devm_drm_bridge_alloc(dev, struct tegra_cpu_bridge_priv, bridge,
+				     &tegra_cpu_bridge_funcs);
+	if (IS_ERR(priv))
+		return PTR_ERR(priv);
+
+	priv->dev = dev;
+
+	ret = device_property_read_u32_array(dev, "nvidia,init-sequence",
+					     priv->spi_init_seq, 4);
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "Failed to get init sequence\n");
+
+	ret = devm_of_platform_populate(dev);
+	if (ret)
+		return dev_err_probe(dev, ret, "Failed to probe children\n");
+
+	/* Initialize MIPI DBI interface */
+	mutex_init(&priv->dbi.cmdlock);
+	priv->dbi.command = tegra_cpu_bridge_dbi_command;
+
+	ret = drm_of_find_panel_or_bridge(np, 1, 0, &panel, &bridge);
+	if (ret)
+		return ret;
+
+	if (panel) {
+		bridge = devm_drm_panel_bridge_add_typed(dev, panel,
+							 DRM_MODE_CONNECTOR_DPI);
+		if (IS_ERR(bridge))
+			return PTR_ERR(bridge);
+	}
+
+	priv->output.bridge = bridge;
+	priv->output.panel = panel;
+
+	/* get input ep (port0/endpoint0) */
+	ret = -EINVAL;
+	ep = of_graph_get_endpoint_by_regs(np, 0, 0);
+	if (ep) {
+		ret = of_property_read_u32(ep, "bus-width", &priv->bus_width);
+		of_node_put(ep);
+	}
+
+	if (ret)
+		priv->bus_width = 24;
+
+	priv->bridge.of_node = np;
+	priv->bridge.timings = &default_tegra_cpu_bridge_timings;
+
+	drm_bridge_add(&priv->bridge);
+
+	platform_set_drvdata(pdev, priv);
+
+	return 0;
+}
+
+static void tegra_cpu_bridge_remove(struct platform_device *pdev)
+{
+	struct tegra_cpu_bridge_priv *priv = platform_get_drvdata(pdev);
+
+	drm_bridge_remove(&priv->bridge);
+	if (priv->output.panel)
+		drm_panel_bridge_remove(priv->output.bridge);
+}
+
+static const struct of_device_id tegra_cpu_bridge_of_match[] = {
+	{ .compatible = "nvidia,tegra-8bit-cpu" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, tegra_cpu_bridge_of_match);
+
+static struct platform_driver tegra_cpu_bridge_driver = {
+	.driver = {
+		.name = "tegra-cpu-bridge",
+		.of_match_table = tegra_cpu_bridge_of_match,
+	},
+	.probe = tegra_cpu_bridge_probe,
+	.remove = tegra_cpu_bridge_remove,
+};
+module_platform_driver(tegra_cpu_bridge_driver);
+
+MODULE_AUTHOR("Svyatoslav Ryhel <clamor95@gmail.com>");
+MODULE_DESCRIPTION("Nvidia Tegra20/30 RGB to MIPI-DBI bridge driver");
+MODULE_LICENSE("GPL");
-- 
2.53.0