linux-kernel - [PATCH] serial: msm: Configure correct working mode before starting earlycon

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Message-Id: <20250408-msm-serial-earlycon-v1-1-429080127530@linaro.org>
Date: Tue, 08 Apr 2025 19:22:47 +0200
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Jiri Slaby <jirislaby@...nel.org>
Cc: Stephen Boyd <sboyd@...nel.org>, linux-arm-msm@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, 
 Sam Day <me@...cday.com>
Subject: [PATCH] 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@...r.kernel.org
Fixes: 0efe72963409 ("tty: serial: msm: Add earlycon support")
Signed-off-by: Stephan Gerhold <stephan.gerhold@...aro.org>
---
 drivers/tty/serial/msm_serial.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
index 1b137e06844425584afe5d3f647e9537c6e2d658..3449945493ceb42369d2acafca925350fccc4f82 100644
--- a/drivers/tty/serial/msm_serial.c
+++ b/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;
 }

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250408-msm-serial-earlycon-4c5dfa782496

Best regards,
-- 
Stephan Gerhold <stephan.gerhold@...aro.org>