drm/mipi-dbi: Make bits per word configurable for pixel transfers

MIPI DCS write/set commands have 8 bit parameters except for the
write_memory commands where it depends on the pixel format.
drm_mipi_dbi does currently only support RGB565 which is 16-bit and it
has to make sure that the pixels enters the SPI bus in big endian format
since the MIPI DBI spec doesn't have support for little endian.

drm_mipi_dbi is optimized for DBI interface option 3 which means that the
16-bit bytes are swapped by the upper layer if the SPI bus does not
support 16 bits per word, signified by the swap_bytes member.

In order to support both 16-bit and 24-bit pixel transfers we need a way
to tell the DBI command layer the format of the buffer. Add a
write_memory_bpw member that the upper layer can use to tell how many
bits per word to use for the SPI transfer.

v4:
- Expand the commit message (Dmitry)
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-3-d7c2bcb9b78d@tronnes.orgSigned-off-by: Noralf Trønnes <noralf@tronnes.org>

drivers/gpu/drm/drm_mipi_dbi.c

@@ -1079,7 +1079,7 @@ static int mipi_dbi_typec1_command_read(struct mipi_dbi *dbi, u8 *cmd,
 static int mipi_dbi_typec1_command(struct mipi_dbi *dbi, u8 *cmd,
 				   u8 *parameters, size_t num)
 {
-	unsigned int bpw = (*cmd == MIPI_DCS_WRITE_MEMORY_START) ? 16 : 8;
+	unsigned int bpw = 8;
 	int ret;
 
 	if (mipi_dbi_command_is_read(dbi, *cmd))
@@ -1091,6 +1091,9 @@ static int mipi_dbi_typec1_command(struct mipi_dbi *dbi, u8 *cmd,
 	if (ret || !num)
 		return ret;
 
+	if (*cmd == MIPI_DCS_WRITE_MEMORY_START)
+		bpw = dbi->write_memory_bpw;
+
 	return mipi_dbi_spi1_transfer(dbi, 1, parameters, num, bpw);
 }
 
@@ -1184,8 +1187,8 @@ static int mipi_dbi_typec3_command(struct mipi_dbi *dbi, u8 *cmd,
 	if (ret || !num)
 		return ret;
 
-	if (*cmd == MIPI_DCS_WRITE_MEMORY_START && !dbi->swap_bytes)
-		bpw = 16;
+	if (*cmd == MIPI_DCS_WRITE_MEMORY_START)
+		bpw = dbi->write_memory_bpw;
 
 	spi_bus_lock(spi->controller);
 	gpiod_set_value_cansleep(dbi->dc, 1);
@@ -1256,12 +1259,15 @@ int mipi_dbi_spi_init(struct spi_device *spi, struct mipi_dbi *dbi,
 
 	dbi->spi = spi;
 	dbi->read_commands = mipi_dbi_dcs_read_commands;
+	dbi->write_memory_bpw = 16;
 
 	if (dc) {
 		dbi->command = mipi_dbi_typec3_command;
 		dbi->dc = dc;
-		if (!spi_is_bpw_supported(spi, 16))
+		if (!spi_is_bpw_supported(spi, 16)) {
+			dbi->write_memory_bpw = 8;
 			dbi->swap_bytes = true;
+		}
 	} else {
 		dbi->command = mipi_dbi_typec1_command;
 		dbi->tx_buf9_len = SZ_16K;

include/drm/drm_mipi_dbi.h

@@ -56,6 +56,11 @@ struct mipi_dbi {
 	 */
 	struct spi_device *spi;
 
+	/**
+	 * @write_memory_bpw: Bits per word used on a MIPI_DCS_WRITE_MEMORY_START transfer
+	 */
+	unsigned int write_memory_bpw;
+
 	/**
 	 * @dc: Optional D/C gpio.
 	 */