drivers: memc: compute prescaler automatically for stm32 xspi

For STM32 XSPI PSRAM driver, compute and set the prescaler automatically
according to the kernel clock and the max frequency of the PSRAM.
Copied from what is done in the STM32 XSPI Flash driver.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>

Author: gautierg-st

drivers/memc/memc_stm32_xspi_psram.c

@@ -40,12 +40,17 @@ LOG_MODULE_REGISTER(memc_stm32_xspi_psram, CONFIG_MEMC_LOG_LEVEL);
 #define DUMMY_CLK_CYCLES_READ	6U
 #define DUMMY_CLK_CYCLES_WRITE	6U
 
+#define STM32_XSPI_CLOCK_PRESCALER_MIN  0U
+#define STM32_XSPI_CLOCK_PRESCALER_MAX  255U
+#define STM32_XSPI_CLOCK_COMPUTE(bus_freq, prescaler) ((bus_freq) / ((prescaler) + 1U))
+
 struct memc_stm32_xspi_psram_config {
 	const struct pinctrl_dev_config *pcfg;
 	const struct stm32_pclken pclken;
 	const struct stm32_pclken pclken_ker;
 	const struct stm32_pclken pclken_mgr;
 	size_t memory_size;
+	uint32_t max_frequency;
 };
 
 struct memc_stm32_xspi_psram_data {
@@ -206,6 +211,7 @@ static int memc_stm32_xspi_psram_init(const struct device *dev)
 	XSPIM_CfgTypeDef cfg = {0};
 	XSPI_RegularCmdTypeDef cmd = {0};
 	XSPI_MemoryMappedTypeDef mem_mapped_cfg = {0};
+	uint32_t prescaler = STM32_XSPI_CLOCK_PRESCALER_MIN;
 	int ret;
 
 	/* Signals configuration */
@@ -259,6 +265,20 @@ static int memc_stm32_xspi_psram_init(const struct device *dev)
 	}
 #endif
 
+	for (; prescaler <= STM32_XSPI_CLOCK_PRESCALER_MAX; prescaler++) {
+		uint32_t clk = STM32_XSPI_CLOCK_COMPUTE(ahb_clock_freq, prescaler);
+
+		if (clk <= dev_cfg->max_frequency) {
+			break;
+		}
+	}
+
+	if (prescaler > STM32_XSPI_CLOCK_PRESCALER_MAX) {
+		LOG_ERR("XSPI could not find valid prescaler value");
+		return -EINVAL;
+	}
+
+	hxspi.Init.ClockPrescaler = prescaler;
 	hxspi.Init.MemorySize = find_msb_set(dev_cfg->memory_size) - 2;
 
 	if (HAL_XSPI_Init(&hxspi) != HAL_OK) {
@@ -339,6 +359,7 @@ static const struct memc_stm32_xspi_psram_config memc_stm32_xspi_cfg = {
 		       .enr = DT_CLOCKS_CELL_BY_NAME(STM32_XSPI_NODE, xspi_mgr, bits)},
 #endif
 	.memory_size = DT_INST_PROP(0, size) / 8, /* In Bytes */
+	.max_frequency = DT_INST_PROP(0, max_frequency),
 };
 
 static struct memc_stm32_xspi_psram_data memc_stm32_xspi_data = {
@@ -354,7 +375,6 @@ static struct memc_stm32_xspi_psram_data memc_stm32_xspi_data = {
 			.FreeRunningClock = HAL_XSPI_FREERUNCLK_DISABLE,
 			.ClockMode = HAL_XSPI_CLOCK_MODE_0,
 			.WrapSize = HAL_XSPI_WRAP_NOT_SUPPORTED,
-			.ClockPrescaler = 3U,
 			.SampleShifting = HAL_XSPI_SAMPLE_SHIFT_NONE,
 			.DelayHoldQuarterCycle = HAL_XSPI_DHQC_ENABLE,
 			.ChipSelectBoundary = HAL_XSPI_BONDARYOF_16KB,