spi: axi-spi-engine: add support for cs_off

This adds support for the spi_transfer::cs_off flag to the AXI SPI
Engine driver.

The logic is copied from the generic spi_transfer_one_message() in
spi.c.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-13-cc59db999b87@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: dlech

drivers/spi/spi-axi-spi-engine.c

@@ -216,14 +216,17 @@ static int spi_engine_compile_message(struct spi_engine *spi_engine,
 	struct spi_device *spi = msg->spi;
 	struct spi_transfer *xfer;
 	int clk_div, new_clk_div;
-	bool cs_change = true;
+	bool keep_cs = false;
 
 	clk_div = -1;
 
 	spi_engine_program_add_cmd(p, dry,
 		SPI_ENGINE_CMD_WRITE(SPI_ENGINE_CMD_REG_CONFIG,
 			spi_engine_get_config(spi)));
 
+	xfer = list_first_entry(&msg->transfers, struct spi_transfer, transfer_list);
+	spi_engine_gen_cs(p, dry, spi, !xfer->cs_off);
+
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
 		new_clk_div = spi_engine_get_clk_div(spi_engine, spi, xfer);
 		if (new_clk_div != clk_div) {
@@ -233,20 +236,28 @@ static int spi_engine_compile_message(struct spi_engine *spi_engine,
 					clk_div));
 		}
 
-		if (cs_change)
-			spi_engine_gen_cs(p, dry, spi, true);
-
 		spi_engine_gen_xfer(p, dry, xfer);
 		spi_engine_gen_sleep(p, dry, spi_engine, clk_div, xfer);
 
-		cs_change = xfer->cs_change;
-		if (list_is_last(&xfer->transfer_list, &msg->transfers))
-			cs_change = !cs_change;
-
-		if (cs_change)
-			spi_engine_gen_cs(p, dry, spi, false);
+		if (xfer->cs_change) {
+			if (list_is_last(&xfer->transfer_list, &msg->transfers)) {
+				keep_cs = true;
+			} else {
+				if (!xfer->cs_off)
+					spi_engine_gen_cs(p, dry, spi, false);
+
+				if (!list_next_entry(xfer, transfer_list)->cs_off)
+					spi_engine_gen_cs(p, dry, spi, true);
+			}
+		} else if (!list_is_last(&xfer->transfer_list, &msg->transfers) &&
+			   xfer->cs_off != list_next_entry(xfer, transfer_list)->cs_off) {
+			spi_engine_gen_cs(p, dry, spi, xfer->cs_off);
+		}
 	}
 
+	if (!keep_cs)
+		spi_engine_gen_cs(p, dry, spi, false);
+
 	return 0;
 }