Re: [PATCH] serial: imx: keep the CPU out of deep idle while an RS485 port is open

[sashiko-bot@kernel.org]

> In RS485 mode the driver releases the transmitter (RTS/DE) from the TX
> complete interrupt. On i.MX6UL/ULL the imx6sx cpuidle driver offers an
> "ARM power off" state with a declared exit latency of 300us, measured at
> up to 2ms. When the CPU enters it while the last character is shifted
> out, DE is released that much later. Modbus RTU slaves may answer after
> 3.5 character times (1.75ms at >=19200 baud), so the head of their reply
> collides with our still-driven transmitter and is lost.
> 
> Add a CPU latency QoS request of 100us while an RS485 port is open. It
> excludes the power-off state but keeps WFI and WAIT (50us) available, so
> cpuidle can stay enabled and the constraint only applies while the port
> is in use.
> 
> The request is added on startup and removed on shutdown, and its value
> follows the RS485 state in imx_uart_rs485_config(), so RS485 enabled
> through TIOCSRS485 after the port is opened, as libmodbus does, is
> covered as well.
> 
> Tested-by: Rodrigo M. Duarte <rodrigo.duarte@ossystems.com.br>
> Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261015144627.2461915-1-otavio@ossystems.com.br?part=1