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

[Stephan Gerhold <stephan@gerhold.net>]

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@gmail.com>
Signed-off-by: Stephan Gerhold <stephan@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