include: spi.h: Add HW timing parameters

Expand the struct spi_cs_control to be able to specify hardware CS
timing parameters in addition to GPIO CS, without adding memory usage
and without breaking existing API usage.

Also add a property for delay between data frames to struct spi_config.

The purpose of these changes is to be able to better support hardware
which can configure specific fine tuned delays in the nanosecond range,
which is the range that most spi device datasheets are specified in.
Microseconds chip select control is not that useful except for the slow
control caused by using a GPIO.

The expectation is that this will be used by specifying peripheral
timing parameters for the devices on the spi bus in DT, and then those
drivers can call into the spi bus driver API by specifying these parameters.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

include/zephyr/drivers/spi.h

@@ -152,24 +152,47 @@ extern "C" {
 /**
  * @brief SPI Chip Select control structure
  *
- * This can be used to control a CS line via a GPIO line, instead of
- * using the controller inner CS logic.
+ * This describes how the SPI CS line should be controlled, including whether
+ * to use GPIO or native hardware CS, and timing parameters.
  *
  */
 struct spi_cs_control {
 	/**
 	 * GPIO devicetree specification of CS GPIO.
-	 * The device pointer can be set to NULL to fully inhibit CS control if
-	 * necessary. The GPIO flags GPIO_ACTIVE_LOW/GPIO_ACTIVE_HIGH should be
+	 *
+	 * If set to NULL, hardware driver can attempt to natively control CS
+	 * automatically if it has that capability and the correct pinmux has
+	 * been applied. Otherwise, NULL will inhibit CS control.
+	 *
+	 * The GPIO flags GPIO_ACTIVE_LOW/GPIO_ACTIVE_HIGH should be
 	 * equivalent to SPI_CS_ACTIVE_HIGH/SPI_CS_ACTIVE_LOW options in struct
 	 * spi_config.
 	 */
 	struct gpio_dt_spec gpio;
+
 	/**
-	 * Delay in microseconds to wait before starting the
-	 * transmission and before releasing the CS line.
+	 * Timing configuration.
+	 * For GPIO CS, delay is microseconds to wait before starting transmission and releasing CS.
+	 * For hardware CS, pcs_to_sck_delay_ns and sck_to_pcs_delay_ns control assertion/deassertion timing.
 	 */
-	uint32_t delay;
+	union {
+		/**
+		 * In the case of GPIO CS, this is delay in microseconds to wait
+		 * before starting the transmission and before releasing the CS line.
+		 */
+		uint32_t delay;
+
+		struct {
+			/** @brief Delay from PCS assertion to first SCK edge in nanoseconds.
+			 * If set to 0, hardware default will be used.
+			 */
+			uint8_t pcs_to_sck_delay_ns;
+			/** @brief Delay from last SCK edge to PCS deassertion in nanoseconds.
+			 * If set to 0, hardware default will be used.
+			 */
+			uint8_t sck_to_pcs_delay_ns;
+		};
+	};
 };
 
 /**
@@ -329,8 +352,10 @@ struct spi_config {
 	 * if not used).
 	 */
 	struct spi_cs_control cs;
-};
 
+	/** @brief Delay between data frames in nanoseconds (0 means 50% of frequency) */
+	uint16_t dfs_delay_ns;
+};
 /**
  * @brief Structure initializer for spi_config from devicetree
  *