drivers: spi: mcux_flexcomm: fix chip select bug w/ dma transfers

Fix for bug:
zephyrproject-rtos/zephyr#59575

The dma version of the version of the driver can
invoke multiple intermediate dma transfers, like
when the spi_buf_set count is greater than one.
However, there is a bug where chip select is not kept
asserted for all intermediate dma transfers required
to process the entire spi_buf_set.

Signed-off-by: Mike J. Chen <mjchen@google.com>

drivers/spi/spi_mcux_flexcomm.c

@@ -610,6 +610,10 @@ static int transceive_dma(const struct device *dev,
 
 	while (data->ctx.rx_len > 0 || data->ctx.tx_len > 0) {
 		size_t dma_len;
+
+		/* last is used to deassert chip select if this
+		 * is the last transfer in the set.
+		 */
 		bool last = false;
 
 		if (data->ctx.rx_len == 0) {
@@ -626,6 +630,34 @@ static int transceive_dma(const struct device *dev,
 			last = false;
 		}
 
+		/* at this point, last just means whether or not
+		 * this transfer will completely cover
+		 * the current tx/rx buffer in data->ctx
+		 * or require additional transfers because the
+		 * the two buffers are not the same size.
+		 *
+		 * if it covers the current ctx tx/rx buffers, then
+		 * we'll move to the next pair of buffers (if any)
+		 * after the transfer, but if there are
+		 * no more buffer pairs, then this is the last
+		 * transfer in the set and we need to deassert CS.
+		 */
+		if (last) {
+			/* this dma transfer should cover
+			 * the entire current data->ctx set
+			 * of buffers. if there are more
+			 * buffers in the set, then we don't
+			 * want to deassert CS.
+			 */
+			if ((data->ctx.tx_count > 1) ||
+			    (data->ctx.rx_count > 1)) {
+				/* more buffers to transfer so
+				 * this isn't last
+				 */
+				last = false;
+			}
+		}
+
 		data->status_flags = 0;
 
 		ret = spi_mcux_dma_move_buffers(dev, dma_len, spi_cfg, last);