Async UART API example not working on STM32U585 due to buffers not being in nocache memory region.

[Scrath1]

Describe the bug

The Async UART API causes an EFAULT error when used on the STM32U585 chip due to the buffers allegedly being in a cached memory region.

The same problem was reported in #75032 but the workaround of setting CONFIG_DCACHE=n does not work anymore as the config option is overwritten during the build process.

The only way I can get the example to work is by changing the default value of CONFIG_DCACHE to disabled directly in zephyr/soc/st/stm32/stm32u5x/Kconfig.defconfig.

Regression

• This is a regression.

Steps to reproduce

1. Create board overlay for Async UART API example
2. Build the project west build -p -b b_u585i_iot02a
3. Flash the project west flash
4. See the error in the debug log

Relevant log output

Debug Log:
============================================================================================
*** Booting Zephyr OS build v4.1.0-5189-g34f5f8d55646 ***
[00:00:05.000,000] <inf> sample: Loop 0: Sending 4 packets
[00:00:05.000,000] <err> uart_stm32: Tx buffer should be placed in a nocache memory region
[00:00:05.000,000] <err> sample: Unknown error (-14)
[00:00:05.000,000] <err> uart_stm32: Tx buffer should be placed in a nocache memory region
[00:00:05.000,000] <err> sample: Unknown error (-14)
[00:00:05.000,000] <err> uart_stm32: Tx buffer should be placed in a nocache memory region
[00:00:05.000,000] <err> sample: Unknown error (-14)
[00:00:05.000,000] <err> uart_stm32: Tx buffer should be placed in a nocache memory region
[00:00:05.000,000] <err> sample: Unknown error (-14)
[00:00:05.000,000] <err> uart_stm32: Rx buffer should be placed in a nocache memory region
[00:00:05.000,000] <inf> sample: RX is now enabled


Build Log warning about DCACHE setting being overwritten:
============================================================================================
warning: DCACHE (defined at
C:/Users/steitobi/zephyrproject/zephyr/soc/st/stm32\stm32h5x\Kconfig.defconfig:13,
C:/Users/steitobi/zephyrproject/zephyr/soc/st/stm32\stm32u5x\Kconfig.defconfig:16,
C:/Users/steitobi/zephyrproject/zephyr/soc/renesas/ra\ra8d1\Kconfig.defconfig:19,
C:/Users/steitobi/zephyrproject/zephyr/soc/nxp/imxrt\imxrt5xx\Kconfig.defconfig:96,
C:/Users/steitobi/zephyrproject/zephyr/soc/mediatek/mt8xxx/Kconfig.defconfig:36,
C:/Users/steitobi/zephyrproject/zephyr/soc/ambiq\apollo5x\Kconfig.defconfig:15, arch/Kconfig:1038)
was assigned the value 'n' but got the value 'y'. See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_DCACHE and/or look up DCACHE in the
menuconfig/guiconfig interface. The Application Development Primer, Setting Configuration Values,
and Kconfig - Tips and Best Practices sections of the manual might be helpful too.


Build Log warning about NOCACHE_MEMORY setting being overwritten:
============================================================================================
warning: NOCACHE_MEMORY (defined at
C:/Users/steitobi/zephyrproject/zephyr/soc/mediatek/mt8xxx/Kconfig.defconfig:43, arch/Kconfig:410)
was assigned the value 'y' but got the value 'n'. Check these unsatisfied dependencies:
(SOC_FAMILY_MTK || ARCH_HAS_NOCACHE_MEMORY_SUPPORT) (=n). See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_NOCACHE_MEMORY and/or look up
NOCACHE_MEMORY in the menuconfig/guiconfig interface. The Application Development Primer, Setting
Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful
too.

Impact

Functional Limitation – Some features not working as expected, but system usable.

Environment

• OS: Windows 11
• Toolchain: Zephyr SDK 0.17.4 (also occurred on 0.17.1), West version 1.4.0
• Board: B-U585I-IOT02A Discovery kit

Additional Context

Board overlay file:

/ {
	chosen {
		/* USART 1 is the debug USB port */
		zephyr,console = &usart1;
		zephyr,shell-uart = &usart1;
	};
};

&gpdma1 {
	status = "okay";
};

// Debug USB port UART
&usart1 {
	status = "okay";
	current-speed = <115200>;
	dmas = <&gpdma1 2 35 (STM32_DMA_MODE_NORMAL | STM32_DMA_PRIORITY_HIGH |
				STM32_DMA_PERIPH_8BITS | STM32_DMA_MEM_8BITS)>,
			<&gpdma1 3 34 (STM32_DMA_MODE_NORMAL | STM32_DMA_PRIORITY_HIGH |
				STM32_DMA_PERIPH_8BITS | STM32_DMA_MEM_8BITS)>;
			dma-names = "tx", "rx";
};

[Scrath1]

It seems like the nocache region errors were due to a borked zephyr installation on my side.

I had a colleague try to reproduce the example as I had listed it here and on his machine everything worked as intended. Then after taking his exact project and trying to build it again on my machine, the error was back.

Reinstalling zephyr fixed the buffer issue. The only "problem" that still remains is the inability to set CONFIG_DCACHE option, which is still being overwritten. That doesn't matter for me anymore though in terms of functionality.