drm/mipi-dbi: Add support for DRM_FORMAT_RGB888

DRM_FORMAT_RGB888 is 24 bits per pixel and it would be natural to send it
on the SPI bus using a 24 bits per word transfer. The problem with this
is that not all SPI controllers support 24 bpw.

Since DRM_FORMAT_RGB888 is stored in memory as little endian and the SPI
bus is big endian we use 8 bpw to always get the same pixel format on the
bus: b8g8r8.

The MIPI DCS specification lists the standard commands that can be sent
over the MIPI DBI interface. The set_address_mode (36h) command has one
bit in the parameter that controls RGB/BGR order. This means that the
controller can be configured to receive the pixel as BGR.

RGB888 is rarely supported on these controllers but RGB666 is very common.
All datasheets I have seen do at least support the pixel format option
where each color is sent as one byte and the 6 MSB's are used.

All this put together means that we can send each pixel as b8g8r8 and an
RGB666 capable controller sees this as b6x2g6x2r6x2.

v4:
- s/emulation_format/pixel_format/ (Dmitry)

Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Noralf Trønnes <[e-mail]>

drivers/gpu/drm/drm_mipi_dbi.c

···
 		      struct drm_rect *clip, bool swap,
 		      struct drm_format_conv_state *fmtcnv_state)
 {
+	struct mipi_dbi_dev *dbidev = drm_to_mipi_dbi_dev(fb->dev);
 	struct drm_gem_object *gem = drm_gem_fb_get_obj(fb, 0);
 	struct iosys_map dst_map = IOSYS_MAP_INIT_VADDR(dst);
 	int ret;
···
 		else
 			drm_fb_memcpy(&dst_map, NULL, src, fb, clip);
 		break;
+	case DRM_FORMAT_RGB888:
+		drm_fb_memcpy(&dst_map, NULL, src, fb, clip);
+		break;
 	case DRM_FORMAT_XRGB8888:
-		drm_fb_xrgb8888_to_rgb565(&dst_map, NULL, src, fb, clip, fmtcnv_state, swap);
+		switch (dbidev->pixel_format) {
+		case DRM_FORMAT_RGB565:
+			drm_fb_xrgb8888_to_rgb565(&dst_map, NULL, src, fb, clip, fmtcnv_state, swap);
+			break;
+		case DRM_FORMAT_RGB888:
+			drm_fb_xrgb8888_to_rgb888(&dst_map, NULL, src, fb, clip, fmtcnv_state);
+			break;
+		}
 		break;
 	default:
 		drm_err_once(fb->dev, "Format is not supported: %p4cc\n",
···
 	struct mipi_dbi_dev *dbidev = drm_to_mipi_dbi_dev(fb->dev);
 	unsigned int height = rect->y2 - rect->y1;
 	unsigned int width = rect->x2 - rect->x1;
+	const struct drm_format_info *dst_format;
 	struct mipi_dbi *dbi = &dbidev->dbi;
 	bool swap = dbi->swap_bytes;
 	int ret = 0;
+	size_t len;
 	bool full;
 	void *tr;
 
···
 	mipi_dbi_set_window_address(dbidev, rect->x1, rect->x2 - 1, rect->y1,
 				    rect->y2 - 1);
 
-	ret = mipi_dbi_command_buf(dbi, MIPI_DCS_WRITE_MEMORY_START, tr,
-				   width * height * 2);
+	if (fb->format->format == DRM_FORMAT_XRGB8888)
+		dst_format = drm_format_info(dbidev->pixel_format);
+	else
+		dst_format = fb->format;
+	len = drm_format_info_min_pitch(dst_format, 0, width) * height;
+
+	ret = mipi_dbi_command_buf(dbi, MIPI_DCS_WRITE_MEMORY_START, tr, len);
 err_msg:
 	if (ret)
 		drm_err_once(fb->dev, "Failed to update display %d\n", ret);
···
  * has one fixed &drm_display_mode which is rotated according to @rotation.
  * This mode is used to set the mode config min/max width/height properties.
  *
- * Use mipi_dbi_dev_init() if you don't need custom formats.
+ * Use mipi_dbi_dev_init() if you want native RGB565 and emulated XRGB8888 format.
  *
  * Note:
  * Some of the helper functions expects RGB565 to be the default format and the
···
 	drm->mode_config.min_height = dbidev->mode.vdisplay;
 	drm->mode_config.max_height = dbidev->mode.vdisplay;
 	dbidev->rotation = rotation;
+	dbidev->pixel_format = formats[0];
+	if (formats[0] == DRM_FORMAT_RGB888)
+		dbidev->dbi.write_memory_bpw = 8;
 
 	DRM_DEBUG_KMS("rotation = %u\n", rotation);
 

include/drm/drm_mipi_dbi.h

···
 	struct drm_display_mode mode;
 
 	/**
+	 * @pixel_format: Native pixel format (DRM_FORMAT\_\*)
+	 */
+	u32 pixel_format;
+
+	/**
 	 * @tx_buf: Buffer used for transfer (copy clip rect area)
 	 */
 	u16 *tx_buf;