serial: msm: Configure correct working mode before starting earlycon

The MSM UART DM controller supports different working modes, e.g. DMA or
the "single-character mode", where all reads/writes operate on a single
character rather than 4 chars (32-bit) at once. When using earlycon,
__msm_console_write() always writes 4 characters at a time, but we don't
know which mode the bootloader was using and we don't set the mode either.

This causes garbled output if the bootloader was using the single-character
mode, because only every 4th character appears in the serial console, e.g.

  "[ 00oni pi  000xf0[ 00i s 5rm9(l)l s 1  1 SPMTA 7:C 5[ 00A ade k d[
   00ano:ameoi .Q1B[ 00ac _idaM00080oo'"

If the bootloader was using the DMA ("DM") mode, output would likely fail
entirely. Later, when the full serial driver probes, the port is
re-initialized and output works as expected.

Fix this also for earlycon by clearing the DMEN register and
reset+re-enable the transmitter to apply the change. This ensures the
transmitter is in the expected state before writing any output.

Cc: stable <stable@kernel.org>
Fixes: 0efe729 ("tty: serial: msm: Add earlycon support")
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250408-msm-serial-earlycon-v1-1-429080127530@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: stephan-gh

drivers/tty/serial/msm_serial.c

@@ -1746,6 +1746,12 @@ msm_serial_early_console_setup_dm(struct earlycon_device *device,
 	if (!device->port.membase)
 		return -ENODEV;
 
+	/* Disable DM / single-character modes */
+	msm_write(&device->port, 0, UARTDM_DMEN);
+	msm_write(&device->port, MSM_UART_CR_CMD_RESET_RX, MSM_UART_CR);
+	msm_write(&device->port, MSM_UART_CR_CMD_RESET_TX, MSM_UART_CR);
+	msm_write(&device->port, MSM_UART_CR_TX_ENABLE, MSM_UART_CR);
+
 	device->con->write = msm_serial_early_write_dm;
 	return 0;
 }