The initial driver assumed that the controller's second chip select was
vestigial because none of the devices examined at the time used it. This
does not hold for all hardware: there are devices with a peripheral
connected to CS1. Since the controller only advertises one chip select
and keeps CS1 permanently deasserted, those peripherals cannot be used.

Advertise both native chip selects and select the appropriate CSBx bit in
->set_cs(). Build the initial SFCSR control value from zero. The
datasheet specifies CHIP_SEL for MMIO mode only, so keep it clear for
this PIO driver and set only the two active-low CSB bits to leave both
lines deasserted.

Rename the callback argument from "active" to "level" while touching this
code. ->set_cs() receives the physical line level, not a logical assertion
state: after applying SPI_CS_HIGH, the core passes false to drive the line
low and true to drive it high. The CSBx bits directly encode that level,
and the native outputs are active-low, so clear the selected bit for a
low/asserted line and set it for a high/deasserted line.

Signed-off-by: Jonas Jelonek <jonas@jonasjelonek.de>
---
 drivers/spi/spi-realtek-rtl.c | 36 +++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/drivers/spi/spi-realtek-rtl.c b/drivers/spi/spi-realtek-rtl.c
index e1c40ff2e49d..7b250b447abf 100644
--- a/drivers/spi/spi-realtek-rtl.c
+++ b/drivers/spi/spi-realtek-rtl.c
@@ -18,7 +18,6 @@ struct rtspi {
 #define RTL_SPI_SFCSR_CSB0		BIT(31)
 #define RTL_SPI_SFCSR_CSB1		BIT(30)
 #define RTL_SPI_SFCSR_RDY		BIT(27)
-#define RTL_SPI_SFCSR_CS		BIT(24)
 #define RTL_SPI_SFCSR_LEN_MASK		~(0x03 << 28)
 #define RTL_SPI_SFCSR_LEN1		(0x00 << 28)
 #define RTL_SPI_SFCSR_LEN4		(0x03 << 28)
@@ -29,17 +28,31 @@ struct rtspi {
 #define REG(x)		(rtspi->base + x)
 
 
-static void rt_set_cs(struct spi_device *spi, bool active)
+static void rt_set_cs(struct spi_device *spi, bool level)
 {
 	struct rtspi *rtspi = spi_controller_get_devdata(spi->controller);
-	u32 value;
+	unsigned int cs = spi_get_chipselect(spi, 0);
+	u32 cs_mask, value;
+
+	switch (cs) {
+	case 0:
+		cs_mask = RTL_SPI_SFCSR_CSB0;
+		break;
+	case 1:
+		cs_mask = RTL_SPI_SFCSR_CSB1;
+		break;
+	default:
+		return;
+	}
 
-	/* CS0 bit is active low */
 	value = __raw_readl(REG(RTL_SPI_SFCSR));
-	if (active)
-		value |= RTL_SPI_SFCSR_CSB0;
+
+	/* CSBx is active low */
+	if (level)
+		value |= cs_mask;
 	else
-		value &= ~RTL_SPI_SFCSR_CSB0;
+		value &= ~cs_mask;
+
 	__raw_writel(value, REG(RTL_SPI_SFCSR));
 }
 
@@ -138,11 +151,9 @@ static void init_hw(struct rtspi *rtspi)
 	value |= RTL_SPI_SFCR_RBO | RTL_SPI_SFCR_WBO;
 	__raw_writel(value, REG(RTL_SPI_SFCR));
 
-	value = __raw_readl(REG(RTL_SPI_SFCSR));
-	/* Permanently disable CS1, since it's never used */
-	value |= RTL_SPI_SFCSR_CSB1;
-	/* Select CS0 for use */
-	value &= RTL_SPI_SFCSR_CS;
+	/* CHIP_SEL is only used in MMIO mode; CSB0/CSB1 are active low. */
+	value = 0;
+	value |= RTL_SPI_SFCSR_CSB0 | RTL_SPI_SFCSR_CSB1;
 	__raw_writel(value, REG(RTL_SPI_SFCSR));
 }
 
@@ -171,6 +182,7 @@ static int realtek_rtl_spi_probe(struct platform_device *pdev)
 	ctrl->flags = SPI_CONTROLLER_HALF_DUPLEX;
 	ctrl->set_cs = rt_set_cs;
 	ctrl->transfer_one = transfer_one;
+	ctrl->num_chipselect = 2;
 
 	err = devm_spi_register_controller(&pdev->dev, ctrl);
 	if (err) {

base-commit: 531c719eaeff3ee12af85ccd259f60846214074c
-- 
2.53.0

On Mon, 21 Sep 2026 21:31:45 +0000, Jonas Jelonek wrote:
> spi: realtek-rtl: add support for second CS

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-7.4

Thanks!

[1/1] spi: realtek-rtl: add support for second CS
      https://git.kernel.org/broonie/spi/c/1f82e6956faa

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark