spi: move split xfers for CS_WORD emulation

This moves splitting transfers for CS_WORD software emulation to the
same place where we split transfers for controller-specific reasons.

This fixes a few subtle bugs.

The calculation for maxsize was wrong for bit sizes between 17 and 24.
This is fixed by making use of spi_split_transfers_maxwords() which
already has the correct calculation.

Also, since this indirectly calls spi_res_alloc(), to avoid leaking
resources, spi_finalize_current_message() would need to be called
on all error paths in __spi_validate() and callers of __spi_validate()
would need to do the same. This is fixed by moving the call to
__spi_pump_transfer_message() where it is already splitting transfers
for other reasons and correctly releases resources in the subsequent
error paths.

Fixes: cbaa62e0 ("spi: add software implementation for SPI_CS_WORD")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240126212358.3916280-2-dlechner@baylibre.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1743,13 +1743,37 @@ static int __spi_pump_transfer_message(struct spi_controller *ctlr,
 
 	trace_spi_message_start(msg);
 
-	ret = spi_split_transfers_maxsize(ctlr, msg,
-					  spi_max_transfer_size(msg->spi),
-					  GFP_KERNEL | GFP_DMA);
-	if (ret) {
-		msg->status = ret;
-		spi_finalize_current_message(ctlr);
-		return ret;
+	/*
+	 * If an SPI controller does not support toggling the CS line on each
+	 * transfer (indicated by the SPI_CS_WORD flag) or we are using a GPIO
+	 * for the CS line, we can emulate the CS-per-word hardware function by
+	 * splitting transfers into one-word transfers and ensuring that
+	 * cs_change is set for each transfer.
+	 */
+	if ((msg->spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
+					       spi_is_csgpiod(msg->spi))) {
+		ret = spi_split_transfers_maxwords(ctlr, msg, 1, GFP_KERNEL);
+		if (ret) {
+			msg->status = ret;
+			spi_finalize_current_message(ctlr);
+			return ret;
+		}
+
+		list_for_each_entry(xfer, &msg->transfers, transfer_list) {
+			/* Don't change cs_change on the last entry in the list */
+			if (list_is_last(&xfer->transfer_list, &msg->transfers))
+				break;
+			xfer->cs_change = 1;
+		}
+	} else {
+		ret = spi_split_transfers_maxsize(ctlr, msg,
+						  spi_max_transfer_size(msg->spi),
+						  GFP_KERNEL | GFP_DMA);
+		if (ret) {
+			msg->status = ret;
+			spi_finalize_current_message(ctlr);
+			return ret;
+		}
 	}
 
 	if (ctlr->prepare_message) {
@@ -4061,31 +4085,6 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 
 	message->spi = spi;
 
-	/*
-	 * If an SPI controller does not support toggling the CS line on each
-	 * transfer (indicated by the SPI_CS_WORD flag) or we are using a GPIO
-	 * for the CS line, we can emulate the CS-per-word hardware function by
-	 * splitting transfers into one-word transfers and ensuring that
-	 * cs_change is set for each transfer.
-	 */
-	if ((spi->mode & SPI_CS_WORD) && (!(ctlr->mode_bits & SPI_CS_WORD) ||
-					  spi_is_csgpiod(spi))) {
-		size_t maxsize = BITS_TO_BYTES(spi->bits_per_word);
-		int ret;
-
-		ret = spi_split_transfers_maxsize(ctlr, message, maxsize,
-						  GFP_KERNEL);
-		if (ret)
-			return ret;
-
-		list_for_each_entry(xfer, &message->transfers, transfer_list) {
-			/* Don't change cs_change on the last entry in the list */
-			if (list_is_last(&xfer->transfer_list, &message->transfers))
-				break;
-			xfer->cs_change = 1;
-		}
-	}
-
 	/*
 	 * Half-duplex links include original MicroWire, and ones with
 	 * only one data pin like SPI_3WIRE (switches direction) or where