Re: [PATCH v5 11/11] drm/tegra: Add support for 8-bit CPU interface

[Mikko Perttunen <mperttunen@nvidia.com>]

On Friday, October 30, 2026 12:29 AM Svyatoslav Ryhel wrote:
> 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.
> 
> The '8-bit CPU', although being the DC interface, is modeled using a
> bridge framework for flexibility and convenience. This implementation does
> not change the existing codebase besides registering the bus.
> 
> Suggested-by: Mikko Perttunen <mperttunen@nvidia.com> # DC command transfer
> Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
> ---
>  drivers/gpu/drm/tegra/Makefile |   1 +
>  drivers/gpu/drm/tegra/dbi.c    | 632 +++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/tegra/dc.c     |   4 +
>  drivers/gpu/drm/tegra/dc.h     |   3 +
>  4 files changed, 640 insertions(+)
>  create mode 100644 drivers/gpu/drm/tegra/dbi.c
> 
> diff --git a/drivers/gpu/drm/tegra/Makefile b/drivers/gpu/drm/tegra/Makefile
> index e399b40d64a1d..ae7fccb3d6421 100644
> --- a/drivers/gpu/drm/tegra/Makefile
> +++ b/drivers/gpu/drm/tegra/Makefile
> @@ -18,6 +18,7 @@ tegra-drm-y := \
>  	hdmi.o \
>  	mipi-phy.o \
>  	dsi.o \
> +	dbi.o \
>  	sor.o \
>  	dpaux.o \
>  	gr2d.o \
> diff --git a/drivers/gpu/drm/tegra/dbi.c b/drivers/gpu/drm/tegra/dbi.c
> new file mode 100644
> index 0000000000000..e129be0f21da5
> --- /dev/null
> +++ b/drivers/gpu/drm/tegra/dbi.c
> @@ -0,0 +1,632 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Nvidia Tegra20/30 MIPI-DBI Type B host driver
> + *
> + * Copyright (C) 2026 Svyatoslav Ryhel <clamor95@gmail.com>
> + */
> +
> +#include <linux/bitfield.h>
> +#include <linux/cleanup.h>
> +#include <linux/delay.h>
> +#include <linux/device.h>
> +#include <linux/err.h>
> +#include <linux/kernel.h>
> +#include <linux/media-bus-format.h>
> +#include <linux/module.h>
> +#include <linux/of_platform.h>
> +#include <linux/property.h>
> +#include <linux/platform_device.h>
> +#include <linux/string.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_mipi_dbi_bus.h>
> +#include <drm/drm_of.h>
> +
> +#include <video/mipi_display.h>
> +
> +#include "dc.h"
> +
> +enum tegra_dbi_dc_signal {
> +	TEGRA_DBI_COMMAND,
> +	TEGRA_DBI_DATA
> +};
> +
> +#define TEGRA_DBI_DATA_PINS_MAX			8
> +#define TEGRA_DBI_INTERFACE_MAX			2
> +
> +struct reg_entry {
> +	u32 offset;
> +	u32 value;
> +};
> +
> +static const struct reg_entry com_pin_output[] = {
> +	{ DC_COM_PIN_OUTPUT_ENABLE(0),   0 },
> +	{ DC_COM_PIN_OUTPUT_ENABLE(1),   0 },
> +	{ DC_COM_PIN_OUTPUT_ENABLE(2),   0 },
> +	{ DC_COM_PIN_OUTPUT_ENABLE(3),   0 },
> +	{ DC_COM_PIN_OUTPUT_POLARITY(0), 0 },
> +	{ DC_COM_PIN_OUTPUT_POLARITY(1), 0 },
> +	{ DC_COM_PIN_OUTPUT_POLARITY(2), 0 },
> +	{ DC_COM_PIN_OUTPUT_POLARITY(3), 0 },
> +	{ DC_COM_PIN_OUTPUT_DATA(0),     0 },
> +	{ DC_COM_PIN_OUTPUT_DATA(1),     0 },
> +	{ DC_COM_PIN_OUTPUT_DATA(2),     0 },
> +	{ DC_COM_PIN_OUTPUT_DATA(3),     0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(0),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(1),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(2),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(3),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(4),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(5),   0 },
> +	{ DC_COM_PIN_OUTPUT_SELECT(6),   0 },
> +};

Instead of this table with unused repeated zero values, I think it would 
be cleaner to have just the register offsets:

  static const u32 tegra_dbi_pin_regs[] = {
  	DC_COM_PIN_OUTPUT_ENABLE(0),
  	// ...
  };

> +
> +struct tegra_dbi_priv {
> +	struct device *dev;
> +	struct tegra_dc *dc;
> +
> +	struct mipi_dbi dbi;
> +	struct mipi_dbi_host dbi_host;
> +
> +	struct drm_bridge bridge;
> +	struct drm_bridge *next_bridge;
> +
> +	const struct tegra_dbi_interface *index;

'index' implies an integer, can you rename this to 'interface'.

> +	struct reg_entry com_pin_backup[ARRAY_SIZE(com_pin_output)];
> +
> +	u32 bus_width;
> +	u32 reg;
> +	bool active;
> +};
> +
> +struct pin {
> +	u32 reg;
> +	u32 pos;
> +};

Can you rename this to struct tegra_dbi_pin?

> +
> +struct tegra_dbi_interface {
> +	struct pin dc;
> +	struct pin rw;
> +	struct pin cs;
> +	struct pin data[TEGRA_DBI_DATA_PINS_MAX];
> +};
> +
> +static const struct tegra_dbi_interface tegra_dbi_idx[TEGRA_DBI_INTERFACE_MAX] = {
> +	/* Nvidia Primary 8-bit CPU interface configuration */
> +	{
> +		.dc = { DC_COM_PIN_OUTPUT_DATA(3), PIN_LDC  },
> +		.rw = { DC_COM_PIN_OUTPUT_DATA(1), PIN_LSC0 },
> +		.cs = { DC_COM_PIN_OUTPUT_DATA(3), PIN_LCSN },
> +		.data = {
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD0 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD1 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD2 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD3 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD4 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD5 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD6 },
> +			{ DC_COM_PIN_OUTPUT_DATA(0), PIN_LD7 },
> +		},
> +	},
> +	/* Nvidia Secondary 8-bit CPU interface configuration */
> +	{
> +		.dc = { DC_COM_PIN_OUTPUT_DATA(2), PIN_LVP0 },
> +		.rw = { DC_COM_PIN_OUTPUT_DATA(1), PIN_LSC1 },
> +		.cs = { DC_COM_PIN_OUTPUT_DATA(2), PIN_LM0  },
> +		.data = {
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LHP1 },
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LM1  },
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LDI  },
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LPP  },
> +			{ DC_COM_PIN_OUTPUT_DATA(1), PIN_LPW0 },
> +			{ DC_COM_PIN_OUTPUT_DATA(1), PIN_LPW2 },
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LVP1 },
> +			{ DC_COM_PIN_OUTPUT_DATA(2), PIN_LHP0 },
> +		},
> +	},
> +};

If we include the information needed by init_primary/init_secondary in 
the tegra_dbi_interface tables as well, we can get rid of the 'reg' 
field.

> +
> +static inline struct tegra_dbi_priv *mipi_dbi_to_tegra_dbi(struct mipi_dbi *dbi)
> +{
> +	return container_of(dbi, struct tegra_dbi_priv, dbi);
> +}
> +
> +static inline struct tegra_dbi_priv *dbi_host_to_tegra_dbi(struct mipi_dbi_host *host)
> +{
> +	return container_of(host, struct tegra_dbi_priv, dbi_host);
> +}
> +
> +static inline struct tegra_dbi_priv *bridge_to_tegra_dbi(struct drm_bridge *bridge)
> +{
> +	return container_of(bridge, struct tegra_dbi_priv, bridge);
> +}
> +
> +static void tegra_dbi_pin_set(struct tegra_dc *dc, const struct pin *pin, bool value)
> +{
> +	u32 val = BIT(pin->pos + 1); /* unmask is 1 bit above position */
> +
> +	if (value)
> +		val |= BIT(pin->pos);
> +
> +	tegra_dc_writel(dc, val, pin->reg);
> +}
> +
> +static void tegra_dbi_write(struct tegra_dbi_priv *priv,
> +			    enum tegra_dbi_dc_signal signal, u8 value)
> +{
> +	struct tegra_dc *dc = priv->dc;
> +	const struct tegra_dbi_interface *idx = priv->index;
> +
> +	tegra_dbi_pin_set(dc, &idx->dc, signal);
> +
> +	tegra_dbi_pin_set(dc, &idx->cs, 0);
> +	tegra_dbi_pin_set(dc, &idx->rw, 0);
> +
> +	for (int i = 0; i < TEGRA_DBI_DATA_PINS_MAX; i++)
> +		tegra_dbi_pin_set(dc, &idx->data[i], field_get(BIT(i), value));
> +
> +	tegra_dbi_pin_set(dc, &idx->cs, 1);
> +	tegra_dbi_pin_set(dc, &idx->rw, 1);
> +
> +	udelay(10);
> +}
> +
> +static int tegra_dbi_dbi_command(struct mipi_dbi *dbi, u8 *cmd, u8 *param, size_t num)
> +{
> +	struct tegra_dbi_priv *priv = mipi_dbi_to_tegra_dbi(dbi);
> +	u8 command = *cmd;
> +
> +	/* Blocks all commands when interface is not active */
> +	if (!priv->active)
> +		return -ENODEV;
> +
> +	tegra_dbi_write(priv, TEGRA_DBI_COMMAND, command);
> +
> +	for (int i = 0; i < num; i++)
> +		tegra_dbi_write(priv, TEGRA_DBI_DATA, param[i]);
> +
> +	return 0;
> +}
> +
> +static int tegra_dbi_host_attach(struct mipi_dbi_host *host, struct mipi_dbi_device *dbidev)
> +{
> +	struct tegra_dbi_priv *priv = dbi_host_to_tegra_dbi(host);
> +	struct device *dev = priv->dev;
> +	struct drm_bridge *bridge;
> +	struct device_node *ep;
> +	int ret;
> +
> +	bridge = of_drm_get_bridge_by_endpoint(dev_of_node(dev), 1, -1);
> +	if (IS_ERR(bridge))
> +		return PTR_ERR(bridge);
> +
> +	bridge->type = DRM_MODE_CONNECTOR_DPI;
> +	priv->next_bridge = bridge;
> +
> +	/* get input ep (port0/endpoint0) */
> +	ret = -EINVAL;
> +	ep = of_graph_get_endpoint_by_regs(dev_of_node(dev), 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;

I think this would look a bit cleaner like ..

  priv->bus_width = 24;

  ep = of_graph_get_endpoint_by_regs(dev_of_node(dev), 0, 0);
  if (ep) {
    of_property_read_u32(ep, "bus-width", &priv->bus_width);
    of_node_put(ep);
  }

> +
> +	return devm_drm_bridge_add(dev, &priv->bridge);
> +}
> +
> +static void tegra_dbi_host_detach(struct mipi_dbi_host *host, struct mipi_dbi_device *dbidev)
> +{
> +	struct tegra_dbi_priv *priv = dbi_host_to_tegra_dbi(host);
> +
> +	drm_bridge_remove(&priv->bridge);

The above calls devm_drm_bridge_add, so I believe drm_bridge_remove will 
happen automatically.

> +	drm_bridge_put(priv->next_bridge);
> +}
> +
> +static const struct mipi_dbi_host_ops tegra_dbi_host_ops = {
> +	.attach = tegra_dbi_host_attach,
> +	.detach = tegra_dbi_host_detach,
> +};
> +
> +static void tegra_dbi_init_primary_interface(struct tegra_dbi_priv *priv)
> +{
> +	struct tegra_dc *dc = priv->dc;
> +	u32 value;
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_ENABLE(3));
> +	value &= ~(BIT(PIN_LCSN) | BIT(PIN_LDC));
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_ENABLE(3));
> +
> +	/* Set LSC0/LCSN/LDC output polarity HIGH */
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_POLARITY(1));
> +	value &= ~BIT(PIN_LSC0);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_POLARITY(1));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_POLARITY(3));
> +	value &= ~(BIT(PIN_LCSN) | BIT(PIN_LDC));
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_POLARITY(3));
> +
> +	value = PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD0) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD1) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD2) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD3) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD4) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD5) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD6) |
> +		PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LD7);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(0));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(3));
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LSC0);
> +	value |= PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LSC0);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(3));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(6));
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LCSN);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LDC);
> +	value |= PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LCSN) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LDC);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(6));
> +}
> +
> +static void tegra_dbi_init_secondary_interface(struct tegra_dbi_priv *priv)
> +{
> +	struct tegra_dc *dc = priv->dc;
> +	u32 value;
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_ENABLE(2));
> +	value &= ~(BIT(PIN_LDI) | BIT(PIN_LPP));
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_ENABLE(2));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(3));
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LPW0);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LPW2);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LSC1);
> +	value |= PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LPW0) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LPW2) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LSC1);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(3));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(4));
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LHP0);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LHP1);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LVP0);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LVP1);
> +	value |= PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LHP0) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LHP1) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LVP0) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LVP1);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(4));
> +
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(5));
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LM0);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LM1);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LDI);
> +	value &= ~PIN_OUTPUT_SELECT_MASK(PIN_OUTPUT_SELECT_LPP);
> +	value |= PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LM0) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LM1) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LDI) |
> +		 PIN_OUTPUT_SELECT_OUT(PIN_OUTPUT_SELECT_LPP);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(5));
> +}
> +
> +static void tegra_dbi_atomic_enable(struct drm_bridge *bridge,
> +				    struct drm_atomic_commit *state)
> +{
> +	struct tegra_dbi_priv *priv = bridge_to_tegra_dbi(bridge);
> +	struct drm_bridge *next_bridge = priv->next_bridge;
> +	struct tegra_dc *dc = priv->dc;
> +	u32 value;
> +
> +	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_PULSE1_POSITION_A);
> +	tegra_dc_writel(dc, 0, DC_DISP_V_PULSE1_POSITION_B);
> +	tegra_dc_writel(dc, 0, DC_DISP_V_PULSE1_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);
> +
> +	 /* Init cycle 1: WRITE_MEMORY_START, set LCD_PCLK and COMMAND */

Extra space here before '/*'

> +	tegra_dc_writel(dc, MIPI_DCS_WRITE_MEMORY_START, DC_DISP_SPI_INIT_SEQ_DATA_A);
> +	tegra_dc_writel(dc, 0x0, DC_DISP_SPI_INIT_SEQ_DATA_B);
> +	tegra_dc_writel(dc, 0x0, DC_DISP_SPI_INIT_SEQ_DATA_C);
> +	tegra_dc_writel(dc, BIT(12) | BIT(14), DC_DISP_SPI_INIT_SEQ_DATA_D);
> +
> +	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:

Why the default case? My assumption would be that if the bus width is 
not something that's expected, that's an error.

> +		value |= BASE_COLOR_SIZE_888;
> +		break;
> +	}
> +
> +	tegra_dc_writel(dc, value, DC_DISP_DISP_COLOR_CONTROL);
> +
> +	/*
> +	 * Backup registers before unshadowing since actual register values
> +	 * may differ and their state backed up now is the correct one.
> +	 */
> +	for (int i = 0; i < ARRAY_SIZE(com_pin_output); i++)
> +		priv->com_pin_backup[i].value = tegra_dc_readl(dc,
> +			priv->com_pin_backup[i].offset);
> +
> +	/* Unshadow to set up command and data pins */
> +	tegra_dc_writel(dc, WRITE_MUX | READ_MUX, DC_CMD_STATE_ACCESS);
> +
> +	if (priv->reg)
> +		tegra_dbi_init_secondary_interface(priv);
> +	else
> +		tegra_dbi_init_primary_interface(priv);
> +
> +	priv->active = true;
> +
> +	/*
> +	 * DBI requires that the next bridge be enabled here in order to
> +	 * send DCS commands, since the interface pins will subsequently be
> +	 * reconfigured for video data.
> +	 */
> +	if (next_bridge->funcs && next_bridge->funcs->atomic_enable)
> +		next_bridge->funcs->atomic_enable(next_bridge, state);

Instead of hijacking the flow, could we move the panel init commands 
into the prepare callback, and deinit into the unprepare callback? 
Command mode would then stay active until this function (host-side 
atomic_enable) is called, and again become active at the end of 
atomic_disable. AFAICT that should allow for a normal lifecycle.

> +
> +	tegra_dbi_pin_set(dc, &priv->index->cs, 0);
> +
> +	priv->active = false;
> +
> +	tegra_dc_writel(dc, 0, DC_CMD_STATE_ACCESS);
> +
> +	for (int i = 0; i <  ARRAY_SIZE(com_pin_output); i++)
> +		tegra_dc_writel(dc, priv->com_pin_backup[i].value,
> +				priv->com_pin_backup[i].offset);
> +
> +	tegra_dc_commit(dc);
> +
> +	/* Set LDC pin to V Pulse 1 */
> +	value = tegra_dc_readl(dc, DC_COM_PIN_OUTPUT_SELECT(6));
> +	value |= PIN_OUTPUT_SELECT_F4(PIN_OUTPUT_SELECT_LDC);
> +	tegra_dc_writel(dc, value, DC_COM_PIN_OUTPUT_SELECT(6));
> +
> +	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);
> +}
> +
> +static void tegra_dbi_atomic_disable(struct drm_bridge *bridge,
> +				     struct drm_atomic_commit *state)
> +{
> +	struct tegra_dbi_priv *priv = bridge_to_tegra_dbi(bridge);
> +	struct drm_bridge *next_bridge = priv->next_bridge;
> +	struct tegra_dc *dc = priv->dc;
> +
> +	for (int i = 0; i < ARRAY_SIZE(com_pin_output); i++)
> +		priv->com_pin_backup[i].value = tegra_dc_readl(dc,
> +			priv->com_pin_backup[i].offset);
> +
> +	tegra_dc_writel(dc, WRITE_MUX, DC_CMD_STATE_ACCESS);
> +
> +	if (priv->reg)
> +		tegra_dbi_init_secondary_interface(priv);
> +	else
> +		tegra_dbi_init_primary_interface(priv);
> +
> +	priv->active = true;
> +
> +	/*
> +	 * DBI requires that the next bridge be disabled here in order to
> +	 * send DCS commands, since the interface pins will subsequently be
> +	 * reconfigured for video data.
> +	 */
> +	if (next_bridge->funcs && next_bridge->funcs->atomic_disable)
> +		next_bridge->funcs->atomic_disable(next_bridge, state);
> +
> +	priv->active = false;
> +
> +	tegra_dc_writel(dc, 0, DC_CMD_STATE_ACCESS);
> +
> +	for (int i = 0; i <  ARRAY_SIZE(com_pin_output); i++)
> +		tegra_dc_writel(dc, priv->com_pin_backup[i].value,
> +				priv->com_pin_backup[i].offset);
> +
> +	tegra_dc_commit(dc);
> +}
> +
> +static int tegra_dbi_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_dbi_attach(struct drm_bridge *bridge,
> +			    struct drm_encoder *encoder,
> +			    enum drm_bridge_attach_flags flags)
> +{
> +	struct tegra_dbi_priv *priv = bridge_to_tegra_dbi(bridge);
> +	struct device *dev = priv->dev;
> +
> +	priv->dc = dev_get_drvdata(dev->parent);
> +	if (!priv->dc)
> +		return -ENODEV;
> +
> +	return drm_bridge_attach(bridge->encoder, priv->next_bridge, bridge,
> +				 flags);
> +}
> +
> +static enum drm_mode_status tegra_dbi_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;

Is there a source for the 800x800 limit?

> +
> +	return MODE_OK;
> +}
> +
> +static const struct drm_bridge_funcs tegra_dbi_funcs = {
> +	.attach = tegra_dbi_attach,
> +	.mode_valid = tegra_dbi_mode_valid,
> +
> +	.atomic_enable = tegra_dbi_atomic_enable,
> +	.atomic_disable = tegra_dbi_atomic_disable,
> +	.atomic_check = tegra_dbi_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_dbi_timings = {
> +	.input_bus_flags = DRM_BUS_FLAG_PIXDATA_SAMPLE_POSEDGE |
> +			   DRM_BUS_FLAG_SYNC_SAMPLE_NEGEDGE |
> +			   DRM_BUS_FLAG_DE_HIGH,
> +};
> +
> +static int tegra_dbi_probe(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct mipi_dbi_device *dbidev;
> +	struct tegra_dbi_priv *priv;
> +	struct device_node *np;
> +	int ret;
> +
> +	priv = devm_drm_bridge_alloc(dev, struct tegra_dbi_priv, bridge,
> +				     &tegra_dbi_funcs);
> +	if (IS_ERR(priv))
> +		return PTR_ERR(priv);
> +
> +	priv->dev = dev;
> +
> +	/* Default to primaty */

The comment seems unnecessary now, as the code below will error out 
(correctly) if the property is not present.

> +	ret = device_property_read_u32(dev, "reg", &priv->reg);
> +	if (ret)
> +		return dev_err_probe(dev, ret, "Failed to get reg\n");
> +
> +	if (priv->reg != 0 && priv->reg != 1)
> +		return dev_err_probe(dev, -EINVAL, "Wrong reg value: %d\n", priv->reg);
> +
> +	priv->index = &tegra_dbi_idx[priv->reg];
> +	memcpy(priv->com_pin_backup, com_pin_output, sizeof(com_pin_output));
> +
> +	/* Initialize MIPI DBI interface */
> +	mutex_init(&priv->dbi.cmdlock);
> +	priv->dbi.command = tegra_dbi_dbi_command;
> +
> +	priv->dbi_host.dev = dev;
> +	priv->dbi_host.dbi = &priv->dbi;
> +	priv->dbi_host.ops = &tegra_dbi_host_ops;
> +
> +	priv->bridge.of_node = dev_of_node(dev);
> +	priv->bridge.timings = &default_tegra_dbi_timings;
> +
> +	np = of_graph_get_remote_node(dev_of_node(dev), 1, -1);
> +	if (!np)
> +		return -EINVAL;
> +
> +	dbidev = devm_mipi_dbi_device_register(dev, &priv->dbi_host, np);
> +	of_node_put(np);
> +	if (IS_ERR(dbidev))
> +		return dev_err_probe(dev, PTR_ERR(dbidev),
> +				     "Failed to register DBI device\n");
> +
> +	return 0;
> +}
> +
> +static struct platform_driver tegra_dbi_driver = {
> +	.driver.name = "tegra-dbi",
> +	.probe = tegra_dbi_probe,
> +};

I don't see the need for the separate driver and probe. You should be 
able to just use the tegra-dc platform device. If there's some issue, 
please point it out.

> +
> +static void tegra_dbi_release(void *pdev)
> +{
> +	platform_device_unregister(pdev);
> +	platform_driver_unregister(&tegra_dbi_driver);
> +}
> +
> +int devm_tegra_dc_dbi_init(struct device *dev)

No need to have the devm_ prefix, for driver internal initialization 
functions it's expected that they can set up devm actions on the device.

> +{
> +	struct device_node *dbi_np __free(device_node) =
> +		of_get_available_child_by_name(dev_of_node(dev), "dbi");
> +	struct platform_device_info dbi_info = { 0 };
> +	struct platform_device *dbi;
> +	int ret;
> +
> +	/* Missing node means that DBI is not used, continue */
> +	if (!dbi_np)
> +		return 0;
> +
> +	ret = platform_driver_register(&tegra_dbi_driver);
> +	if (ret)
> +		return dev_err_probe(dev, ret,
> +				     "Failed to register DBI driver\n");
> +
> +	dbi_info.parent = dev;
> +	dbi_info.name = "tegra-dbi";
> +	dbi_info.id = PLATFORM_DEVID_AUTO;
> +	dbi_info.fwnode = of_fwnode_handle(dbi_np);
> +	dbi_info.of_node_reused = true;
> +
> +	dbi = platform_device_register_full(&dbi_info);
> +	if (IS_ERR(dbi)) {
> +		platform_driver_unregister(&tegra_dbi_driver);
> +		return dev_err_probe(dev, PTR_ERR(dbi),
> +				     "Failed to create DBI device\n");
> +	}
> +
> +	ret = devm_add_action_or_reset(dev, tegra_dbi_release, dbi);
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
> index 5bbf2c028edd8..768cf82776632 100644
> --- a/drivers/gpu/drm/tegra/dc.c
> +++ b/drivers/gpu/drm/tegra/dc.c
> @@ -3382,6 +3382,10 @@ static int tegra_dc_probe(struct platform_device *pdev)
>  		}
>  	}
>  
> +	err = devm_tegra_dc_dbi_init(&pdev->dev);
> +	if (err)
> +		return dev_err_probe(&pdev->dev, err, "failed to init DBI\n");
> +
>  	err = tegra_dc_rgb_probe(dc);
>  	if (err < 0 && err != -ENODEV)
>  		return dev_err_probe(&pdev->dev, err,
> diff --git a/drivers/gpu/drm/tegra/dc.h b/drivers/gpu/drm/tegra/dc.h
> index 2b87bb675d4cf..3df49422a2bd1 100644
> --- a/drivers/gpu/drm/tegra/dc.h
> +++ b/drivers/gpu/drm/tegra/dc.h
> @@ -177,6 +177,9 @@ void tegra_dc_rgb_remove(struct tegra_dc *dc);
>  int tegra_dc_rgb_init(struct drm_device *drm, struct tegra_dc *dc);
>  int tegra_dc_rgb_exit(struct tegra_dc *dc);
>  
> +/* from dbi.c */
> +int devm_tegra_dc_dbi_init(struct device *dev);
> +
>  #define DC_CMD_GENERAL_INCR_SYNCPT		0x000
>  
>  #define DC_CMD_GENERAL_INCR_SYNCPT_CNTRL	0x001
> -- 
> 2.53.0
> 
>