drivers/lcd: Add ST7796 TFT LCD framebuffer driver

Add support for ST7796 TFT LCD controller (320x480). The driver
implements the NuttX framebuffer interface for SPI-connected displays.

Features:
- SPI interface with CONFIG_SPI_CMDDATA for D/C pin control;
- RGB565 (16-bit) and RGB666 (18-bit) color formats;
- Runtime rotation support (0, 90, 180, 270 degrees) via MADCTL;
- Board-provided configuration via st7796_config_s structure;
- Partial screen update via updatearea for efficient rendering;
- Persistent swap buffer to avoid per-frame allocations;
- Proper ST7796S initialization sequence with documented timing;

The driver uses a board-provided configuration structure allowing
flexible setup of resolution, SPI frequency, color depth, and
initial MADCTL value without requiring Kconfig options in the
generic driver.

Architecture changes in this revision as per request:

- Moved internal register commands to .c file (private)
- Moved MADCTL bit definitions to .c file (private)
- Moved struct st7796_cmd_s to .c file (private)
- Converted MADCTL orientation macros to absolute values
- Moved CONFIG_SPI_CMDDATA error check to beginning of file
- Removed duplicated CONFIG_SPI_CMDDATA guards
- Public header contains only board configuration API
- Changed the Kconfig auto-select FB from 'select' to 'depends on'

Tested with LVGL graphics library on STM32H753ZI board (my own port).

Signed-off-by: Vinicius May <vmay.sweden@gmail.com>

drivers/lcd/CMakeLists.txt

@@ -145,6 +145,10 @@ if(CONFIG_LCD)
     list(APPEND SRCS st7789.c)
   endif()
 
+  if(CONFIG_LCD_ST7796)
+    list(APPEND SRCS st7796.c)
+  endif()
+
   if(CONFIG_LCD_GC9A01)
     list(APPEND SRCS gc9a01.c)
   endif()

drivers/lcd/Kconfig

@@ -827,6 +827,14 @@ config LCD_ST7789_DATA_ENDIAN_LITTLE
 
 endif # LCD_ST7789
 
+config LCD_ST7796
+	bool "ST7796 LCD Display"
+	default n
+	depends on SPI
+	depends on LCD_FRAMEBUFFER
+	---help---
+		Enable support for ST7796 LCD display driver.
+
 config LCD_GC9A01
 	bool "Galaxycore GC9A01 TFT Controller"
 	default n

drivers/lcd/Make.defs

@@ -143,6 +143,10 @@ ifeq ($(CONFIG_LCD_ST7789),y)
   CSRCS += st7789.c
 endif
 
+ifeq ($(CONFIG_LCD_ST7796),y)
+  CSRCS += st7796.c
+endif
+
 ifeq ($(CONFIG_LCD_GC9A01),y)
   CSRCS += gc9a01.c
 endif

include/nuttx/lcd/st7796.h

@@ -0,0 +1,178 @@
+/****************************************************************************
+ * include/nuttx/lcd/st7796.h
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+#ifndef __INCLUDE_NUTTX_LCD_ST7796_H
+#define __INCLUDE_NUTTX_LCD_ST7796_H
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/* Display raw dimensions (before orientation transform)
+ *
+ * These must be in the public header because board code needs them to
+ * calculate the effective xres/yres based on Kconfig orientation settings
+ * at compile-time. Without these, boards would need to hardcode dimensions.
+ */
+
+#define ST7796_XRES_RAW         320
+#define ST7796_YRES_RAW         480
+
+/* Default SPI frequency
+ *
+ * Must be in the public header because board code uses this as a safe
+ * default when the user hasn't specified a custom frequency via Kconfig.
+ * Centralizes the datasheet specification to prevent duplication.
+ */
+
+#define ST7796_SPI_MAXFREQUENCY 40000000
+
+/* Pre-defined MADCTL values for common orientations
+ *
+ * These must be in the public header because board code selects the
+ * appropriate MADCTL value based on hardware orientation and RGB/BGR
+ * panel type configured via Kconfig. This abstracts the internal MADCTL
+ * bit manipulation from board code. Values are absolute to eliminate
+ * dependency on internal bit definitions.
+ */
+
+#define ST7796_MADCTL_PORTRAIT           0x40
+#define ST7796_MADCTL_PORTRAIT_BGR       0x48
+#define ST7796_MADCTL_RPORTRAIT          0x80
+#define ST7796_MADCTL_RPORTRAIT_BGR      0x88
+#define ST7796_MADCTL_LANDSCAPE          0x20
+#define ST7796_MADCTL_LANDSCAPE_BGR      0x28
+#define ST7796_MADCTL_RLANDSCAPE         0xe0
+#define ST7796_MADCTL_RLANDSCAPE_BGR     0xe8
+
+/****************************************************************************
+ * Public Types
+ ****************************************************************************/
+
+#ifndef __ASSEMBLY__
+
+/* Board-specific configuration passed to driver at initialization.
+ *
+ * This structure must be in the public header because it defines the
+ * driver's configuration API. Board code instantiates and populates this
+ * structure to configure the driver without requiring board-specific
+ * Kconfig options in the generic driver.
+ */
+
+struct st7796_config_s
+{
+  uint32_t frequency;       /* SPI clock frequency in Hz */
+  uint16_t xres;            /* Horizontal resolution (after orientation) */
+  uint16_t yres;            /* Vertical resolution (after orientation) */
+  uint16_t rotation;        /* Initial rotation: 0, 90, 180, or 270 */
+  uint8_t bpp;              /* Bits per pixel: 16 (RGB565) or 18 (RGB666) */
+  uint8_t madctl;           /* Base MADCTL register value for orientation */
+};
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+/****************************************************************************
+ * Name: st7796_fbinitialize
+ *
+ * Description:
+ *   Initialize the ST7796 LCD controller as a framebuffer device.
+ *
+ *   This function initializes the ST7796 display controller using the
+ *   provided configuration and returns a framebuffer virtual table.
+ *   The driver uses CONFIG_SPI_CMDDATA to control the DC (Data/Command)
+ *   pin automatically via the SPI driver.
+ *
+ * Input Parameters:
+ *   spi    - SPI device instance configured for the ST7796
+ *   config - Board-specific configuration (frequency, resolution, etc.)
+ *
+ * Returned Value:
+ *   Pointer to framebuffer vtable on success; NULL on failure.
+ *
+ * Assumptions:
+ *   - CONFIG_SPI_CMDDATA is enabled
+ *   - DC pin has been registered with the SPI driver
+ *   - CS pin has been registered with the SPI driver
+ *   - RESET and backlight pins configured by board code
+ *
+ ****************************************************************************/
+
+FAR struct fb_vtable_s *st7796_fbinitialize(FAR struct spi_dev_s *spi,
+                                            FAR const struct st7796_config_s
+                                            *config);
+
+/****************************************************************************
+ * Name: st7796_setrotation
+ *
+ * Description:
+ *   Set display rotation at runtime. Valid rotation values are 0, 90,
+ *   180, and 270 degrees. The rotation is applied by modifying the
+ *   MADCTL register.
+ *
+ * Input Parameters:
+ *   vtable   - Reference to the framebuffer virtual table
+ *   rotation - Rotation angle in degrees (0, 90, 180, or 270)
+ *
+ * Returned Value:
+ *   OK on success; a negated errno value on failure.
+ *
+ ****************************************************************************/
+
+int st7796_setrotation(FAR struct fb_vtable_s *vtable, uint16_t rotation);
+
+/****************************************************************************
+ * Name: st7796_getrotation
+ *
+ * Description:
+ *   Get current display rotation.
+ *
+ * Input Parameters:
+ *   vtable - Reference to the framebuffer virtual table
+ *
+ * Returned Value:
+ *   Current rotation in degrees (0, 90, 180, or 270).
+ *
+ ****************************************************************************/
+
+uint16_t st7796_getrotation(FAR struct fb_vtable_s *vtable);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __ASSEMBLY__ */
+#endif /* __INCLUDE_NUTTX_LCD_ST7796_H */

include/nuttx/video/fb.h

@@ -313,6 +313,12 @@
 #define FBIOSET_VSYNCOFFSET   _FBIOC(0x001a)  /* Set VSync offset in usec
                                                * Argument:             int */
 
+#define FBIOSET_ROTATION      _FBIOC(0x001e)  /* Set display rotation
+                                               * Argument:             int */
+
+#define FBIOGET_ROTATION      _FBIOC(0x001f)  /* Get display rotation
+                                               * Argument:            int* */
+
 /* Linux Support ************************************************************/
 
 #define FBIOGET_VSCREENINFO   _FBIOC(0x001b)  /* Get video variable info */