[PATCH] serial: serial_msm: Ensure BAM/single character mode are disabled

[Ramon Fried]

On Mon, Jun 28, 2021 at 11:40 AM Stephan Gerhold <stephan at gerhold.net> wrote:
>
> At the moment, the U-Boot serial_msm driver does not initialize the
> UART_DM_DMEN register with the required value. Usually this does not
> cause any problems, because there is Qualcomm's LK bootloader running
> before U-Boot which initializes the register with the correct value.
>
> It's important that this register is initialized correctly, because
> the U-Boot driver does not make use of the BAM/DMA or single character
> mode functionality of the UART controller. A different bootloader
> before U-Boot might initialize the register differently.
>
> For example, on DragonBoard 410c U-Boot can also be installed to the
> "aboot" partition (replacing LK entirely). In this case U-Boot is
> loaded directly by SBL, which seems to use the single-character mode
> for some reason. In single character mode there is always just one
> char in the FIFO, instead of the 4 characters expected by
> msm_serial_fetch(). It also causes issues with "earlycon" later in
> the Linux kernel, which tries to output 4 chars at once,
> but only the first char will be written.
>
> This causes early UART log in Linux to be corrupted like this:
>
>     [ 00ano:ameoi .Q1B[ 00ac _idaM00080oo'ahani-lcle._20). 15NdNii 5 SPMSJ20:U2
>     [ 00rkoolmsamel
>     [ 00Fw ]elamletopsioble
>     [ 00ore
>
> instead of
>
>     [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd030]
>     [    0.000000] Machine model: Qualcomm Technologies, Inc. APQ 8016 SBC
>     [    0.000000] earlycon: msm_serial_dm0 at MMIO 0x00000000078b0000 (options '')
>     [    0.000000] printk: bootconsole [msm_serial_dm0] enabled
>
> Make sure to initialize UART_DM_DMEN correctly to fix this issue
> when loading U-Boot directly after SBL (instead of through LK).
>
> There is no functional difference when loading U-Boot through LK
> since LK also initializes UART_DM_DMEN to 0x0. [1]
>
> [1]: https://git.linaro.org/landing-teams/working/qualcomm/lk.git/tree/platform/msm_shared/uart_dm.c?h=dragonboard410c-LA.BR.1.2.7-03810-8x16.0-linaro3#n203
>
> Cc: Ramon Fried <rfried.dev at gmail.com>
> Signed-off-by: Stephan Gerhold <stephan at gerhold.net>
> ---
>
>  drivers/serial/serial_msm.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/serial/serial_msm.c b/drivers/serial/serial_msm.c
> index d8c6c2f6b5..d8dd5c1104 100644
> --- a/drivers/serial/serial_msm.c
> +++ b/drivers/serial/serial_msm.c
> @@ -23,6 +23,7 @@
>  /* Serial registers - this driver works in uartdm mode*/
>
>  #define UARTDM_DMRX             0x34 /* Max RX transfer length */
> +#define UARTDM_DMEN             0x3C /* DMA/data-packing mode */
>  #define UARTDM_NCF_TX           0x40 /* Number of chars to TX */
>
>  #define UARTDM_RXFS             0x50 /* RX channel status register */
> @@ -197,6 +198,9 @@ static void uart_dm_init(struct msm_serial_data *priv)
>         writel(MSM_BOOT_UART_DM_8_N_1_MODE, priv->base + UARTDM_MR2);
>         writel(MSM_BOOT_UART_DM_CMD_RESET_RX, priv->base + UARTDM_CR);
>         writel(MSM_BOOT_UART_DM_CMD_RESET_TX, priv->base + UARTDM_CR);
> +
> +       /* Make sure BAM/single character mode is disabled */
> +       writel(0x0, priv->base + UARTDM_DMEN);
>  }
>  static int msm_serial_probe(struct udevice *dev)
>  {
> --
> 2.32.0
>
Reviewed-by: Ramon Fried <rfried.dev at gmail.com>