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

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Message-ID: <058f8b9c-55db-4735-bf61-b1e0f3d05953@quicinc.com>
Date: Wed, 9 Apr 2025 15:30:02 +0530
From: Mukesh Kumar Savaliya <quic_msavaliy@...cinc.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>,
        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>,
        Chandana Kishori Chiluveru <cchiluve@....qualcomm.com>
Subject: Re: [PATCH] serial: msm: Configure correct working mode before
 starting earlycon



On 4/8/2025 10:52 PM, Stephan Gerhold wrote:
> 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.
> 
Looks surprising. I haven't seen pre-kernel UART console ever works in 
DMA mode. It was always fixed to work in FIFO/PIO mode. From what i know.
> 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.
> 
AFAIR, console UART was always configured in FIFO/PIO mode. For non 
Console application e.g. Bluetooth it works in DMA mode.
> 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,