How does TF-M select UART1 pins?

[Jayant Tang]

Hello DevZone,

I have a confusing question about TF-M. Need  help from a TF-M expert.

In this document, it says that we must disable uart1 in non-secure application, because TF-M use it as secure peripheral.

My question is: how does TF-M select the pins used by uart1_secure?

My board selection is `nrf9160dk/nrf9160/ns`, which is a non-secure board.

I use AI to read the build progress and all the Ninja things. And it told me that TF-M just read the non-secure `zephyr.dts` and use the pin-ctrl of that.

I think this is not reasonable, because uart1 is disabled in my application. There is no reason for TF-M to use the pin-ctrl from a disabled uart.

More general question: how can I choose a peripheral as secure or non-secure in NCS? Should I configure this in Devicetree or Kconfig?

Best regards,

Jayant

[Hakon]

Hello,

My question is: how does TF-M select the pins used by uart1_secure?

I would assume the pins used are the ones you find in nrf9160dk_nrf9160_common-pinctrl.dtsi file.

More general question: how can I choose a peripheral as secure or non-secure in NCS? Should I configure this in Devicetree or Kconfig?

You should use kconfig for this. Check out this document; https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/tfm/tfm_secure_peripheral/README.html#enabling_secure_peripheral 

[Jayant Tang]

Hello,

Hakon said:

I would assume the pins used are the ones you find in nrf9160dk_nrf9160_common-pinctrl.dtsi file.

Yes. But I want to get it more precisely.

Since `nrf9160dk_nrf9160_common-pinctrl.dtsi` is included both in 9160 secure dts and 9160 non-secure dts, which one is the original dts file that be selected by TF-M?
If it use the same dts as the application, does that means the TF-M only refer to the pin-ctrl part of non-secure `zephyr.dts` ? 

[Hakon]

Jayant Tang said:

In this case, when building a non-secure firmware with TF-M, which device tree file will TF-M use?

TFM itself will be built with the secure version of the dts file. For the application, the non-secure dts file will be used.

Jayant Tang said:

If TF-M, as a secure firmware, relies on pin configurations from a non-secure device tree, that seems inconsistent. Isn’t that correct?

Yeah, but it does not. TFM does not use a non-secure device tree file at all.

[Jayant Tang]

Wow, that is more clear for me.

You mean when I select `nrf9160dk/nrf9160/ns` for my application, the TF-M will use `nrf9160dk/nrf9160` automatically. Because they are in the same board folder.

---

But when I choose the ns for my application, it seems that there is no way to set a `.overlay` devicetree file in sysbuild subfolder to modify the pins of TF-M. Is that correct?

I remember when in NCS v2.2.0, I can set overlay of SPM in "child_image" subfolder.

[Hakon]

I believe the way to do it now is to add an overlay file for it to a sysbuild directory; https://docs.nordicsemi.com/bundle/ncs-latest/page/zephyr/build/sysbuild/index.html#zephyr_application_configuration 

[Jayant Tang]

I understand that sysbuild is used for multi-image building.

But TF-M seems to be special. There is no way in sysbuild to configure the TF-M overlay.

More preciously, TF-M is more like a part of application, rather than an independent image:

---

After many times searching, I finally found that TF-M never use Zephyr serial driver.

All the answers are in these header files:

In `device_cfg.h` we have:

In `RTE_device.h` we have:

In `tfm_peripherals_config.h`, we have:

So, that means TF-M can directly read the non-secure Application's final `zephyr.dts`, and read the pin-ctrl of uart1, even if uart1 is disabled.

And this question:

Jayant Tang said:

how can I choose a peripheral as secure or non-secure in NCS? Should I configure this in Devicetree or Kconfig?

The answer is to set CONFIG_NRF_XXXX_SECURE.

So everything seems to be clear now. And this 

Jayant Tang said:

You mean when I select `nrf9160dk/nrf9160/ns` for my application, the TF-M will use `nrf9160dk/nrf9160` automatically. Because they are in the same board folder.

seems not to be correct.

---

I will test this tomorrow and get back here.

[Jayant Tang]

I have confirmed that:

1. TF-M doesn't have a stand alone sysbuild configuration, it is a part of the Application

2. TF-M will use application's non-secure uart1 pinctrl, even if uart1 is disabled.

---

CONFIG_SERIAL=y
CONFIG_TFM_LOG_LEVEL_SILENCE=n

 

&uart1 {
    status = "disabled";
};

&pinctrl{
	uart1_default: uart1_default {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 16)>,
				<NRF_PSEL(UART_RTS, 0, 14)>;
		};
		group2 {
			psels = <NRF_PSEL(UART_RX, 0, 17)>,
				<NRF_PSEL(UART_CTS, 0, 15)>;
			bias-pull-up;
		};
	};

	uart1_sleep: uart1_sleep {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 16)>,
				<NRF_PSEL(UART_RX, 0, 17)>,
				<NRF_PSEL(UART_RTS, 0, 14)>,
				<NRF_PSEL(UART_CTS, 0, 15)>;
			low-power-enable;
		};
	};
};

Cheers!