spi: rzv2h-rspi: add support for loopback mode

Add support for loopback mode for debugging purposes, allowing us to
test the SPI controller at the maximum SPI transfer clock without being
limited by external wiring.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://patch.msgid.link/20251119161434.595677-10-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Demon000

drivers/spi/spi-rzv2h-rspi.c

@@ -24,6 +24,7 @@
 /* Registers */
 #define RSPI_SPDR		0x00
 #define RSPI_SPCR		0x08
+#define RSPI_SPPCR		0x0e
 #define RSPI_SSLP		0x10
 #define RSPI_SPBR		0x11
 #define RSPI_SPSCR		0x13
@@ -40,6 +41,9 @@
 #define RSPI_SPCR_SCKASE	BIT(12)
 #define RSPI_SPCR_SPE		BIT(0)
 
+/* Register SPPCR */
+#define RSPI_SPPCR_SPLP2	BIT(1)
+
 /* Register SPBR */
 #define RSPI_SPBR_SPR_MIN	0
 #define RSPI_SPBR_SPR_PCLK_MIN	1
@@ -345,6 +349,7 @@ static int rzv2h_rspi_prepare_message(struct spi_controller *ctlr,
 	u8 bits_per_word;
 	u32 conf32;
 	u16 conf16;
+	u8 conf8;
 
 	/* Make sure SPCR.SPE is 0 before amending the configuration */
 	rzv2h_rspi_spe_disable(rspi);
@@ -389,6 +394,10 @@ static int rzv2h_rspi_prepare_message(struct spi_controller *ctlr,
 	/* Use SPCMD0 only */
 	writeb(0x0, rspi->base + RSPI_SPSCR);
 
+	/* Setup loopback */
+	conf8 = FIELD_PREP(RSPI_SPPCR_SPLP2, !!(spi->mode & SPI_LOOP));
+	writeb(conf8, rspi->base + RSPI_SPPCR);
+
 	/* Setup mode */
 	conf32 = FIELD_PREP(RSPI_SPCMD_CPOL, !!(spi->mode & SPI_CPOL));
 	conf32 |= FIELD_PREP(RSPI_SPCMD_CPHA, !!(spi->mode & SPI_CPHA));
@@ -490,7 +499,7 @@ static int rzv2h_rspi_probe(struct platform_device *pdev)
 	}
 
 	controller->mode_bits = SPI_CPHA | SPI_CPOL | SPI_CS_HIGH |
-				SPI_LSB_FIRST;
+				SPI_LSB_FIRST | SPI_LOOP;
 	controller->bits_per_word_mask = SPI_BPW_RANGE_MASK(4, 32);
 	controller->prepare_message = rzv2h_rspi_prepare_message;
 	controller->unprepare_message = rzv2h_rspi_unprepare_message;