spi: omap2-mcpsi: Enable MULTI-mode in more situations

Enable multimode when a transfer of multiple small words can be
transformed in a transfer with a single bigger word. This is allowed as
long as the result on the cable is the same, so word_delay must be zero.
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://msgid.link/r/20240327-spi-omap2-mcspi-multi-mode-v3-3-c4ac329dd5a2@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-omap2-mcspi.c

@@ -1277,11 +1277,24 @@ static int omap2_mcspi_prepare_message(struct spi_controller *ctlr,
 
 		/*
 		 * Check if this transfer contains only one word;
+		 * OR contains 1 to 4 words, with bits_per_word == 8 and no delay between each word
+		 * OR contains 1 to 2 words, with bits_per_word == 16 and no delay between each word
+		 *
+		 * If one of the two last case is true, this also change the bits_per_word of this
+		 * transfer to make it a bit faster.
+		 * It's not an issue to change the bits_per_word here even if the multi-mode is not
+		 * applicable for this message, the signal on the wire will be the same.
 		 */
 		if (bits_per_word < 8 && tr->len == 1) {
 			/* multi-mode is applicable, only one word (1..7 bits) */
+		} else if (tr->word_delay.value == 0 && bits_per_word == 8 && tr->len <= 4) {
+			/* multi-mode is applicable, only one "bigger" word (8,16,24,32 bits) */
+			tr->bits_per_word = tr->len * bits_per_word;
+		} else if (tr->word_delay.value == 0 && bits_per_word == 16 && tr->len <= 2) {
+			/* multi-mode is applicable, only one "bigger" word (16,32 bits) */
+			tr->bits_per_word = tr->len * bits_per_word / 2;
 		} else if (bits_per_word >= 8 && tr->len == bits_per_word / 8) {
-			/* multi-mode is applicable, only one word (8..32 bits) */
+			/* multi-mode is applicable, only one word (9..15,17..32 bits) */
 		} else {
 			/* multi-mode is not applicable: more than one word in the transfer */
 			mcspi->use_multi_mode = false;