drivers: spi: stm32: allow for using soft NSS in peripheral mode

The current stm32 drivers will only enable soft NSS support if
the device is using a Zephyr managed GPIO for CS, or if the
CONFIG_SPI_STM32_USE_HW_SS flag is set.  Neither of these are
actually appropriate for a peripheral, which doesn't control CS.

With this change, it is possible for the SPI interface to be
configured as a peripheral but still use soft NSS -- otherwise,
short of creating a "fake" GPIO configuration, the peripheral
will always use hard NSS, and for some boards/configurations,
that is unworkable.  In the case of a board where the CS
is not hooked up, we can use soft NSS to allow the peripheral
to assume CS and be driven by the SPI clock signal alone.

Our board has only three signals (clock, COPI, POCI) enabled
on one of the SPI interfaces (it is communicating with another
MCU on the same board), and works with this change.  Without
the change, we must enable a "fake" GPIO driver configuration
(which is never used for anything, and really makes little
sense for a peripheral, but it just happens to force the
driver to go down that code path).  We cannot disable the
CONFIG_SPI_STM32_USE_HW_SS mechanism, as that impacts ALL
interfaces, and we have a second interface in controller
mode that speaks to flash and requires the NSS_HARD codepath.

Signed-off-by: Rob Newberry <rob@zenomoto.com>

Author: rob-zeno

drivers/spi/spi_ll_stm32.c

@@ -935,7 +935,11 @@ static int spi_stm32_configure(const struct device *dev,
 		LL_SPI_SetNSSMode(spi, LL_SPI_NSS_SOFT);
 	} else {
 		if ((config->operation & SPI_OP_MODE_SLAVE) != 0U) {
-			LL_SPI_SetNSSMode(spi, LL_SPI_NSS_HARD_INPUT);
+			if (cfg->peripheral_use_soft_nss) {
+				LL_SPI_SetNSSMode(spi, LL_SPI_NSS_SOFT);
+			} else {
+				LL_SPI_SetNSSMode(spi, LL_SPI_NSS_HARD_INPUT);
+			}
 		} else {
 			LL_SPI_SetNSSMode(spi, LL_SPI_NSS_HARD_OUTPUT);
 		}

drivers/spi/spi_ll_stm32.h

@@ -39,6 +39,7 @@ struct spi_stm32_config {
 	const struct stm32_pclken *pclken;
 	bool fifo_enabled: 1;
 	bool ioswp: 1;
+	bool peripheral_use_soft_nss: 1;
 };
 
 #ifdef CONFIG_SPI_STM32_DMA

dts/bindings/spi/st,stm32-spi-common.yaml

@@ -21,3 +21,7 @@ properties:
   ioswp:
     type: boolean
     description: Swap SPI MOSI and MISO pins
+
+  periph-use-soft-nss:
+     type: boolean
+     description: use soft NSS for this interface when in peripheral mode