SPI: omap2-mcspi: Fix SPI CS behaviour around

Merge series from Félix Piédallu <[email protected]>:

These patches fix the behaviour of the SPI Chip Select of the OMAP2 MCSPI
driver used on TI SoCs.

The omap2-mcspi driver supports the use of multi mode (multichannel in TI
documentation). In this mode, the CS is asserted and deasserted by the
hardware.

The multi mode is disabled for messages when cs_change=0 for all transfers
(e.g when CS is kept asserted between transfers of a same message).

The multi mode also needs to be disabled for messages when cs_change=1 on the
last transfer (e.g when CS is kept asserted after the WHOLE message), and the
message right after.

Currently, that is not the case and it CS is deasserted by hardware when it
shouldn't.
This breaks peripheral drivers that send multiple messages with the CS asserted
in between.

Patch 1 ensures that multi mode is disabled when cs_change=1 on the last
transfer of the message.

Patch 2 ensures that multi mode is disable on a message following one with
cs_change=1 on the last transfer.

This is the case for the TPM TIS SPI driver that uses this logic for flow
control purposes.

Tested on an AM6442 platform with a TPM ST33HTPH2X32AHE4.

Author: broonie

drivers/spi/spi-omap2-mcspi.c

@@ -134,6 +134,7 @@ struct omap2_mcspi {
 	size_t			max_xfer_len;
 	u32			ref_clk_hz;
 	bool			use_multi_mode;
+	bool			last_msg_kept_cs;
 };
 
 struct omap2_mcspi_cs {
@@ -1269,6 +1270,10 @@ static int omap2_mcspi_prepare_message(struct spi_controller *ctlr,
 	 * multi-mode is applicable.
 	 */
 	mcspi->use_multi_mode = true;
+
+	if (mcspi->last_msg_kept_cs)
+		mcspi->use_multi_mode = false;
+
 	list_for_each_entry(tr, &msg->transfers, transfer_list) {
 		if (!tr->bits_per_word)
 			bits_per_word = msg->spi->bits_per_word;
@@ -1287,18 +1292,19 @@ static int omap2_mcspi_prepare_message(struct spi_controller *ctlr,
 			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;
+		if (list_is_last(&tr->transfer_list, &msg->transfers)) {
+			/* Check if transfer asks to keep the CS status after the whole message */
+			if (tr->cs_change) {
+				mcspi->use_multi_mode = false;
+				mcspi->last_msg_kept_cs = true;
+			} else {
+				mcspi->last_msg_kept_cs = false;
+			}
+		} else {
+			/* Check if transfer asks to change the CS status after the transfer */
+			if (!tr->cs_change)
+				mcspi->use_multi_mode = false;
+		}
 	}
 
 	omap2_mcspi_set_mode(ctlr);