drivers: uart: increase timeout precision in uart_rx_enable

[nordic-krch]

Is your enhancement proposal related to a problem? Please describe.
Currently, uart_rx_enable gets timeout given in milliseconds. timeout refers to inactivity period which leads to UART_RX_RDY event. Millisecond resolution may significantly lower communication throughput if protocol requires ACK. For example, device transmits 10 bytes @1Mbaud, it takes ~100us but receiver has 1ms timeout so packet will be received after ~1100us. Another case is when hardware has inactivity detection.

Describe the solution you'd like
Change timeout to microseconds. Optionally, add inline function or macro for converting characters to microseconds.

** Alternative **
Change timeout to characters. It is baudrate independent and may be suitable for hardware inactivity detection.

[shlomow]

Maybe it is worth to add more definitions like SYS_FOREVER_MS to the api that have soft restrictions on the drivers implementation as well as the occurrence of the UART_RX_RDY signal. For example, we can add WAIT_INACTIVITY define that that may be implemented by drivers as 1 ms timeout, and can be implemented using some hardware advantages. In the define I described the UART_RX_RDY will be received when the driver find inactivity (very not formal definition) and the reception won't stop as the buffer is not full yet.

[mbolivar-nordic]

** Alternative **
Change timeout to characters. It is baudrate independent and may be suitable for hardware inactivity detection.

Why is k_timeout_t not listed as an alternative? If you did consider it, why did you reject it?

[nordic-krch]

k_timeout_t was rejected because it is to kernel bound. Some drivers may use hardware timeout or require higher precision than system clock and use other source for calculating timeout, e.g. counter device.