drivers: spi: siwx91x: Add helper to round GSPI freq to valid range

Add gspi_siwx91x_pick_lower_freq() to select the nearest lower
supported GSPI clock frequency based on the requested value.
The GSPI clock can operate only at discrete frequencies such as
80 MHz, 40 MHz, 26.6 MHz, 20 MHz, and so on (80 MHz divided by
integer factors). If the requested frequency does not match one
of these valid steps, it is rounded down to the nearest lower
supported frequency.

The driver now logs both the requested and the actual programmed
frequency along with the divider value, helping users verify the
effective SPI clock configuration.

Signed-off-by: Sai Santhosh Malae <Santhosh.Malae@silabs.com>

Author: smalae

drivers/spi/spi_silabs_siwx91x_gspi.c

@@ -76,13 +76,37 @@ static bool spi_siwx91x_is_dma_enabled_instance(const struct device *dev)
 #endif
 }
 
+void gspi_siwx91x_pick_lower_freq(uint32_t clock_hz, uint32_t requested_hz, uint32_t *actual_hz_out,
+				  uint32_t *div_out)
+{
+	/* Calculate divider that ensures freq <= requested */
+	uint32_t divider = DIV_ROUND_UP(clock_hz, 2 * requested_hz);
+	uint32_t actual_hz;
+
+	if (divider == 0U) {
+		divider = 1U;
+	}
+
+	/* Compute the actual achievable frequency */
+	actual_hz = clock_hz / (2U * divider);
+
+	if (actual_hz_out) {
+		*actual_hz_out = actual_hz;
+	}
+
+	if (div_out) {
+		*div_out = divider;
+	}
+}
+
 static int gspi_siwx91x_config(const struct device *dev, const struct spi_config *spi_cfg,
 			       spi_callback_t cb, void *userdata)
 {
 	__maybe_unused struct gspi_siwx91x_data *data = dev->data;
 	const struct gspi_siwx91x_config *cfg = dev->config;
 	uint32_t bit_rate = spi_cfg->frequency;
 	uint32_t clk_div_factor;
+	uint32_t actual_freq;
 	uint32_t clock_rate;
 	int ret;
 	__maybe_unused int channel_filter;
@@ -119,7 +143,13 @@ static int gspi_siwx91x_config(const struct device *dev, const struct spi_config
 		if (ret) {
 			return ret;
 		}
-		clk_div_factor = ((clock_rate / spi_cfg->frequency) / 2);
+
+		gspi_siwx91x_pick_lower_freq(clock_rate, spi_cfg->frequency, &actual_freq,
+					     &clk_div_factor);
+		if (spi_cfg->frequency != actual_freq) {
+			LOG_INF("Requested %u Hz, programmed %u Hz (divider=%u)",
+				spi_cfg->frequency, actual_freq, clk_div_factor);
+		}
 	}
 
 	if (clk_div_factor < 1) {