drm/mipi-dbi: Remove mipi_dbi_machine_little_endian()

mipi_dbi_machine_little_endian() should really have been called
mipi_dbi_framebuffer_little_endian() because that's the function it
performs. When I added support for these SPI displays I thought that the
framebuffers on big endian machines were also big endian, but I have
later learned that this is not the case. There's a bit in the fourcc code
that controls this: DRM_FORMAT_BIG_ENDIAN.

Just remove the function to avoid confusion. We can add big endian support
later should the need arise and we have hardware to test on.

Instead of just amending the docs, expand it to explain the endianness
handling.
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604-panel-mipi-dbi-rgb666-v4-2-d7c2bcb9b78d@tronnes.orgSigned-off-by: Noralf Trønnes <noralf@tronnes.org>

drivers/gpu/drm/drm_mipi_dbi.c

@@ -824,15 +824,6 @@ u32 mipi_dbi_spi_cmd_max_speed(struct spi_device *spi, size_t len)
 }
 EXPORT_SYMBOL(mipi_dbi_spi_cmd_max_speed);
 
-static bool mipi_dbi_machine_little_endian(void)
-{
-#if defined(__LITTLE_ENDIAN)
-	return true;
-#else
-	return false;
-#endif
-}
-
 /*
  * MIPI DBI Type C Option 1
  *
@@ -855,7 +846,7 @@ static int mipi_dbi_spi1e_transfer(struct mipi_dbi *dbi, int dc,
 				   const void *buf, size_t len,
 				   unsigned int bpw)
 {
-	bool swap_bytes = (bpw == 16 && mipi_dbi_machine_little_endian());
+	bool swap_bytes = (bpw == 16);
 	size_t chunk, max_chunk = dbi->tx_buf9_len;
 	struct spi_device *spi = dbi->spi;
 	struct spi_transfer tr = {
@@ -1004,7 +995,7 @@ static int mipi_dbi_spi1_transfer(struct mipi_dbi *dbi, int dc,
 		size_t chunk = min(len, max_chunk);
 		unsigned int i;
 
-		if (bpw == 16 && mipi_dbi_machine_little_endian()) {
+		if (bpw == 16) {
 			for (i = 0; i < (chunk * 2); i += 2) {
 				dst16[i]     = *src16 >> 8;
 				dst16[i + 1] = *src16++ & 0xFF;
@@ -1218,11 +1209,23 @@ static int mipi_dbi_typec3_command(struct mipi_dbi *dbi, u8 *cmd,
  * If @dc is set, a Type C Option 3 interface is assumed, if not
  * Type C Option 1.
  *
- * If the SPI master driver doesn't support the necessary bits per word,
- * the following transformation is used:
+ * If the command is %MIPI_DCS_WRITE_MEMORY_START and the pixel format is RGB565, endianness has
+ * to be taken into account. The MIPI DBI serial interface is big endian and framebuffers are
+ * assumed stored in memory as little endian (%DRM_FORMAT_BIG_ENDIAN is not supported).
+ *
+ * This is how endianness is handled:
+ *
+ * Option 1 (D/C as a bit): The buffer is sent on the wire byte by byte so the 16-bit buffer is
+ *                          byteswapped before transfer.
+ *
+ * Option 3 (D/C as a gpio): If the SPI controller supports 16 bits per word the buffer can be
+ *                           sent as-is. If not the caller is responsible for swapping the bytes
+ *                           before calling mipi_dbi_command_buf() and the buffer is sent 8 bpw.
+ *
+ * This handling is optimised for %DRM_FORMAT_RGB565 framebuffers.
  *
- * - 9-bit: reorder buffer as 9x 8-bit words, padded with no-op command.
- * - 16-bit: if big endian send as 8-bit, if little endian swap bytes
+ * If the interface is Option 1 and the SPI controller doesn't support 9 bits per word,
+ * the buffer is sent as 9x 8-bit words, padded with MIPI DCS no-op commands if necessary.
  *
  * Returns:
  * Zero on success, negative error code on failure.
@@ -1257,7 +1260,7 @@ int mipi_dbi_spi_init(struct spi_device *spi, struct mipi_dbi *dbi,
 	if (dc) {
 		dbi->command = mipi_dbi_typec3_command;
 		dbi->dc = dc;
-		if (mipi_dbi_machine_little_endian() && !spi_is_bpw_supported(spi, 16))
+		if (!spi_is_bpw_supported(spi, 16))
 			dbi->swap_bytes = true;
 	} else {
 		dbi->command = mipi_dbi_typec1_command;