Add multi mode support for omap-mcspi

Merge series from Louis Chauvet <louis.chauvet@bootlin.com>:

This series adds the support for the omap-mcspi multi mode which allows
sending SPI messages with a shorter delay between CS and the message.

One drawback of the multi-mode is that the CS is raised between each word,
so it can only be used with messages containing 1 word transfers and
asking for cs_change. Few devices, like FPGAs, may easily workaround this
limitation.

The first patch removes the current implementation, which is working, but
don't comply with what is asked in the spi transfer (The CS is raised by
the hardware regardless of cs_change state). No drivers or board file use this
implementation upstream.

The second patch adds the implementation of the multi-mode, which complies
with what is asked in the SPI message.

The third patch is the suggested optimization for using MULTI mode in more
situations.

drivers/spi/spi-omap2-mcspi.c

@@ -131,6 +131,7 @@ struct omap2_mcspi {
 	unsigned int		pin_dir:1;
 	size_t			max_xfer_len;
 	u32			ref_clk_hz;
+	bool			use_multi_mode;
 };
 
 struct omap2_mcspi_cs {
@@ -256,10 +257,15 @@ static void omap2_mcspi_set_cs(struct spi_device *spi, bool enable)
 
 		l = mcspi_cached_chconf0(spi);
 
-		if (enable)
+		/* Only enable chip select manually if single mode is used */
+		if (mcspi->use_multi_mode) {
 			l &= ~OMAP2_MCSPI_CHCONF_FORCE;
-		else
-			l |= OMAP2_MCSPI_CHCONF_FORCE;
+		} else {
+			if (enable)
+				l &= ~OMAP2_MCSPI_CHCONF_FORCE;
+			else
+				l |= OMAP2_MCSPI_CHCONF_FORCE;
+		}
 
 		mcspi_write_chconf0(spi, l);
 
@@ -283,7 +289,12 @@ static void omap2_mcspi_set_mode(struct spi_controller *ctlr)
 		l |= (OMAP2_MCSPI_MODULCTRL_MS);
 	} else {
 		l &= ~(OMAP2_MCSPI_MODULCTRL_MS);
-		l |= OMAP2_MCSPI_MODULCTRL_SINGLE;
+
+		/* Enable single mode if needed */
+		if (mcspi->use_multi_mode)
+			l &= ~OMAP2_MCSPI_MODULCTRL_SINGLE;
+		else
+			l |= OMAP2_MCSPI_MODULCTRL_SINGLE;
 	}
 	mcspi_write_reg(ctlr, OMAP2_MCSPI_MODULCTRL, l);
 
@@ -1175,13 +1186,6 @@ static int omap2_mcspi_transfer_one(struct spi_controller *ctlr,
 		    t->bits_per_word == spi->bits_per_word)
 			par_override = 0;
 	}
-	if (cd && cd->cs_per_word) {
-		chconf = mcspi->ctx.modulctrl;
-		chconf &= ~OMAP2_MCSPI_MODULCTRL_SINGLE;
-		mcspi_write_reg(ctlr, OMAP2_MCSPI_MODULCTRL, chconf);
-		mcspi->ctx.modulctrl =
-			mcspi_read_cs_reg(spi, OMAP2_MCSPI_MODULCTRL);
-	}
 
 	chconf = mcspi_cached_chconf0(spi);
 	chconf &= ~OMAP2_MCSPI_CHCONF_TRM_MASK;
@@ -1240,14 +1244,6 @@ static int omap2_mcspi_transfer_one(struct spi_controller *ctlr,
 		status = omap2_mcspi_setup_transfer(spi, NULL);
 	}
 
-	if (cd && cd->cs_per_word) {
-		chconf = mcspi->ctx.modulctrl;
-		chconf |= OMAP2_MCSPI_MODULCTRL_SINGLE;
-		mcspi_write_reg(ctlr, OMAP2_MCSPI_MODULCTRL, chconf);
-		mcspi->ctx.modulctrl =
-			mcspi_read_cs_reg(spi, OMAP2_MCSPI_MODULCTRL);
-	}
-
 	omap2_mcspi_set_enable(spi, 0);
 
 	if (spi_get_csgpiod(spi, 0))
@@ -1265,15 +1261,72 @@ static int omap2_mcspi_prepare_message(struct spi_controller *ctlr,
 	struct omap2_mcspi	*mcspi = spi_controller_get_devdata(ctlr);
 	struct omap2_mcspi_regs	*ctx = &mcspi->ctx;
 	struct omap2_mcspi_cs	*cs;
+	struct spi_transfer	*tr;
+	u8 bits_per_word;
+
+	/*
+	 * The conditions are strict, it is mandatory to check each transfer of the list to see if
+	 * multi-mode is applicable.
+	 */
+	mcspi->use_multi_mode = true;
+	list_for_each_entry(tr, &msg->transfers, transfer_list) {
+		if (!tr->bits_per_word)
+			bits_per_word = msg->spi->bits_per_word;
+		else
+			bits_per_word = tr->bits_per_word;
 
-	/* Only a single channel can have the FORCE bit enabled
+		/*
+		 * 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 (9..15,17..32 bits) */
+		} else {
+			/* multi-mode is not applicable: more than one word in the transfer */
+			mcspi->use_multi_mode = false;
+		}
+
+		/* Check if transfer asks to change the CS status after the transfer */
+		if (!tr->cs_change)
+			mcspi->use_multi_mode = false;
+
+		/*
+		 * If at least one message is not compatible, switch back to single mode
+		 *
+		 * The bits_per_word of certain transfer can be different, but it will have no
+		 * impact on the signal itself.
+		 */
+		if (!mcspi->use_multi_mode)
+			break;
+	}
+
+	omap2_mcspi_set_mode(ctlr);
+
+	/* In single mode only a single channel can have the FORCE bit enabled
 	 * in its chconf0 register.
 	 * Scan all channels and disable them except the current one.
 	 * A FORCE can remain from a last transfer having cs_change enabled
+	 *
+	 * In multi mode all FORCE bits must be disabled.
 	 */
 	list_for_each_entry(cs, &ctx->cs, node) {
-		if (msg->spi->controller_state == cs)
+		if (msg->spi->controller_state == cs && !mcspi->use_multi_mode) {
 			continue;
+		}
 
 		if ((cs->chconf0 & OMAP2_MCSPI_CHCONF_FORCE)) {
 			cs->chconf0 &= ~OMAP2_MCSPI_CHCONF_FORCE;

include/linux/platform_data/spi-omap2-mcspi.h

@@ -16,9 +16,6 @@ struct omap2_mcspi_platform_config {
 
 struct omap2_mcspi_device_config {
 	unsigned turbo_mode:1;
-
-	/* toggle chip select after every word */
-	unsigned cs_per_word:1;
 };
 
 #endif