drivers: serial: nrfx_uarte: Rework driver to support new features

Rework driver to support new way of asynchronous RX handling.
Previously RX was handled in two modes: using RXDRDY interrupt for byte
counting or TIMER + PPI. Both modes had flaws. RXDRDY interrupt mode
could miscalculated amount of received bytes when interrupt was not
handled on time. Data was not lost but was not reported on time that
could lead to issues. PPI+TIMER mode requires additional resources
thus it was not the default mode. Often user was not aware of that
option and was expiriencing driver RX faults.

New RX mode is switching buffers when there is new data (RXDRDY event
not set for given amount of time). It does not require additional
resources to get precise byte counting. Additionally, this is in line
with new UARTE feature (RX frame timeout) which is present in nRF54X
devices. The behavior of the driver is the same for legacy devices
and new one. For legacy devices k_timer periodic interrupts are used
to check if there are any new bytes and it is not needed when RX frame
timeout is present.

Improved RX mode is enabled by default
(CONFIG_UART_NRFX_UARTE_ENHANCED_RX=y) but legacy modes are still
available though not recommended to be used.

Note that new RX mode (CONFIG_UART_NRFX_UARTE_ENHANCED_RX=y) behaves
a bit different because timeout always triggers switch of buffers
which means that there will be no UART_RX_RDY events with non-zero
offset. It also means that every UART_RX_RDY will be followed by
UART_RX_BUF_RELEASED.

After rework, driver is recommended to be used for all platforms as it
performs much better and takes much less code than the second UART shim
available for Nordic devices.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

drivers/serial/Kconfig.nrfx

@@ -37,6 +37,18 @@ config UART_NRFX_UARTE_LEGACY_SHIM
 	# New shim takes more ROM. Until it is fixed use legacy shim.
 	default y
 
+config UART_NRFX_UARTE_ENHANCED_RX
+	bool "Enhanced RX handling"
+	depends on UART_ASYNC_API
+	depends on UART_NRFX_UARTE_LEGACY_SHIM
+	default y if !(UART_0_NRF_HW_ASYNC || UART_1_NRF_HW_ASYNC || UART_2_NRF_HW_ASYNC)
+	help
+	  Enable RX handling mode which is switching buffers on timeout. This is an
+	  enhancement compared to other two modes (default and hardware assisted).
+	  Default mode could miscount bytes when interrupt was not handled on time
+	  and hardware assisted required TIMER peripheral instance and PPI channel
+	  for accurate byte counting.
+
 config UART_ASYNC_TX_CACHE_SIZE
 	int "TX cache buffer size"
 	depends on UART_ASYNC_API

drivers/serial/Kconfig.nrfx_uart_instance

@@ -68,6 +68,7 @@ config UART_$(nrfx_uart_num)_NRF_ASYNC_LOW_POWER
 	depends on HAS_HW_NRF_UARTE$(nrfx_uart_num)
 	depends on UART_ASYNC_API
 	depends on UART_NRFX_UARTE_LEGACY_SHIM
+	default y
 	help
 	  When enabled, UARTE is enabled before each TX or RX usage and disabled
 	  when not used. Disabling UARTE while in idle allows to achieve lowest