spi: spi-cadence: Fix SPI CS gets toggling sporadically

As part of unprepare_transfer_hardware, SPI controller will be disabled
which will indirectly deassert the CS line. This will create a problem
in some of the devices where message will be transferred with
cs_change flag set(CS should not be deasserted).
As per SPI controller implementation, if SPI controller is disabled then
all output enables are inactive and all pins are set to input mode which
means CS will go to default state high(deassert). This leads to an issue
when core explicitly ask not to deassert the CS (cs_change = 1). This
patch fix the above issue by checking the Slave select status bits from
configuration register before disabling the SPI.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20220606062525.18447-1-amit.kumar-mahapatra@xilinx.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-cadence.c

@@ -69,6 +69,7 @@
 #define CDNS_SPI_BAUD_DIV_SHIFT		3 /* Baud rate divisor shift in CR */
 #define CDNS_SPI_SS_SHIFT		10 /* Slave Select field shift in CR */
 #define CDNS_SPI_SS0			0x1 /* Slave Select zero */
+#define CDNS_SPI_NOSS			0x3C /* No Slave select */
 
 /*
  * SPI Interrupt Registers bit Masks
@@ -450,15 +451,20 @@ static int cdns_prepare_transfer_hardware(struct spi_master *master)
  * @master:	Pointer to the spi_master structure which provides
  *		information about the controller.
  *
- * This function disables the SPI master controller.
+ * This function disables the SPI master controller when no slave selected.
  *
  * Return:	0 always
  */
 static int cdns_unprepare_transfer_hardware(struct spi_master *master)
 {
 	struct cdns_spi *xspi = spi_master_get_devdata(master);
+	u32 ctrl_reg;
 
-	cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);
+	/* Disable the SPI if slave is deselected */
+	ctrl_reg = cdns_spi_read(xspi, CDNS_SPI_CR);
+	ctrl_reg = (ctrl_reg & CDNS_SPI_CR_SSCTRL) >>  CDNS_SPI_SS_SHIFT;
+	if (ctrl_reg == CDNS_SPI_NOSS)
+		cdns_spi_write(xspi, CDNS_SPI_ER, CDNS_SPI_ER_DISABLE);
 
 	return 0;
 }