STM32H7a3ZI-q SPI bus speed is half of what is set

[CodingGhost]

when using SPI (Im using SPI4) on the stm32H7a3ZI-q, the actual SCK Frequency is half of what is set in the configuration.
Im testing this on current master, this is my configuration:

static struct spi_config spi_cfg = {
	.operation = SPI_WORD_SET(8) | SPI_TRANSFER_MSB | SPI_OP_MODE_MASTER | SPI_LINES_SINGLE |  SPI_HOLD_ON_CS,
	.frequency = 8000000,
	.slave = 0,
	.cs = 0,
};

&spi4 {
	status = "okay";
	label="ARDUCAM_MEGA";
	pinctrl-0 = <&spi4_sck_pe12 &spi4_miso_pe13 &spi4_mosi_pe14>;
	pinctrl-names = "default";
	cs-gpios = <&gpiob 6 (GPIO_ACTIVE_LOW | GPIO_PULL_UP)>;
	clock-frequency=<8000000>;
	
	arducam_mega: arducam_mega@0 {
		compatible = "masjao,arducam_mega";
		reg = <0>;
		spi-max-frequency=<8000000>;
	};
	
};

Expected behavior
I Expect the Bus to operate at the frequency that I have configured.

Impact
It seems like just configuring the Bus to 16MHZ when I actually need 8MHZ works as a workaround. But this was very confusing and costs some time to figure out as I didnt know why I had massive performance problems.

Logs and console output
SPI debug logging is too much for the uart to handle, but the messages that I do see on init are no problem.

[djiatsaf-st]

After testing on my side, I was able to observe the same behavior that you pointed out on an oscilloscope. After investigation, it is not surprising that we need to modify the clock to achieve the desired frequency of the SCK.

You must first understand that there are three frequencies involved here:

• The frequency of the system clock source that supplies the SPI bus
• The frequency of the SPI bus (FreqSPI)
• The baud rate or the SCK frequency (Fsck).

When you configure:
clock-frequency=<8000000>; and .frequency = 8000000,

in reality, you are configuring the frequency of the SPI bus and not the baud rate as mentionned here https://docs.zephyrproject.org/apidoc/latest/structspi__config.html#aa1ec6933fe66f91653c5be488e4c9b2a and here https://docs.zephyrproject.org/latest/build/dts/api/bindings/spi/st%2Cstm32-spi.html#dtbinding-st-stm32-spi .

The baud rate = FreqSPI * prescaler (½ by default). We have a finite range of prescalers (½, ¼, …, 1/256) already defined in STM32 SPI driver.

So, if you configure an SPI bus frequency of 8 MHz with the prescaler of ½ by default, you get a baud rate or Fsck of 4 MHz. To achieve 8 MHz, you must double the frequency of the SPI bus.

I think the SPI driver will approximate the baud rate based on the frequency of the SPI bus and the prescalers to have a value that is as close as possible to the desired baud rate. Therefore, it may sometimes be necessary to change the clock source of the SPI bus to have a frequency that allows us to obtain the desired baud rate.

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