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

[Tom Rini]

On Mon, Jun 28, 2021 at 10:40:09AM +0200, Stephan Gerhold 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>
> Reviewed-by: Ramon Fried <rfried.dev at gmail.com>

Applied to u-boot/master, thanks!

-- 
Tom
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