drm/dsi: Add a helper to get bits per pixel of MIPI DSI pixel format

BugLink: http://bugs.launchpad.net/bugs/1599109

Add a helper that can be used to obtain the number of bits per pixel
corresponding to a given MIPI DSI pixel format. This is useful in
bandwidth calculations, for example.
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
[treding@nvidia.com: add kerneldoc comment and commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>

(cherry picked from commit ec26d9e9)
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

include/drm/drm_mipi_dsi.h

@@ -163,6 +163,31 @@ static inline struct mipi_dsi_device *to_mipi_dsi_device(struct device *dev)
 	return container_of(dev, struct mipi_dsi_device, dev);
 }
 
+/**
+ * mipi_dsi_pixel_format_to_bpp - obtain the number of bits per pixel for any
+ *                                given pixel format defined by the MIPI DSI
+ *                                specification
+ * @fmt: MIPI DSI pixel format
+ *
+ * Returns: The number of bits per pixel of the given pixel format.
+ */
+static inline int mipi_dsi_pixel_format_to_bpp(enum mipi_dsi_pixel_format fmt)
+{
+	switch (fmt) {
+	case MIPI_DSI_FMT_RGB888:
+	case MIPI_DSI_FMT_RGB666:
+		return 24;
+
+	case MIPI_DSI_FMT_RGB666_PACKED:
+		return 18;
+
+	case MIPI_DSI_FMT_RGB565:
+		return 16;
+	}
+
+	return -EINVAL;
+}
+
 struct mipi_dsi_device *of_find_mipi_dsi_device_by_node(struct device_node *np);
 int mipi_dsi_attach(struct mipi_dsi_device *dsi);
 int mipi_dsi_detach(struct mipi_dsi_device *dsi);