Re: [PATCH] serial: qcom-geni: Set UPSTAT_AUTOCTS for hardware flow control

[sashiko-bot@kernel.org]

> qcom_geni_serial_set_termios() clears UART_CTS_MASK when CRTSCTS is set,
> enabling hardware CTS flow control. However, it never sets UPSTAT_AUTOCTS,
> so uart_softcts_mode() also selects software-assisted CTS flow control.
> 
> If CTS is deasserted when uart_change_line_settings() samples the modem
> status, serial core sets hw_stopped and stops transmission. The GENI
> driver does not handle CTS-change interrupts, so asserting CTS again does
> not clear hw_stopped or restart the transmitter.
> 
> On a Motorola Moto G10 with WCN3950, this can stall Bluetooth setup in
> tty_wait_until_sent() while qca_setup() closes the serial port. A captured
> stall had one byte left in the software transmit queue, hw_stopped set,
> and CTS asserted, with the UART powered and its interrupt enabled.
> 
> Set UPSTAT_AUTOCTS under the port lock when CRTSCTS is enabled, and clear
> it when CRTSCTS is disabled, to match the hardware configuration.
> 
> Fixes: 8a8a66a1a18a ("tty: serial: qcom_geni_serial: Add support for flow control")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Michael McAllister <me@michaelmcallister.org>

Sashiko has reviewed this patch and found no issues.

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