[PATCH v2 0/6] Fix UART autoboot issues on DragonBoard 410c

[Alexey Minnekhanov]

On 08.09.2025 14:31, Stephan Gerhold via groups.io wrote:
> There have been issues with autoboot on DB410c for years, where autoboot
> gets interrupted by spurious input on the UART console. I've tried fixing
> this back in 2021, but it turned out this fix was not enough. Looking into
> it further, there are two separate issues to fix:
> 
>   - Spurious input with UART disconnected: The RX pin is floating in this
>     case, resulting in garbage or a UART break event (0x00 byte). This is
>     interpreted as input in the autoboot prompt. Fix this by adding
>     bias-pull-up for the RX pin. I've made this change upstream in Linux
>     6.16 for all MSM8916/MSM8939 devices and it has already landed in U-Boot
>     through the recent subtree update of dts/upstream.
> 
>   - Spurious input when querying console size with UART connected: It's
>     unclear why this happens, but it seems to be some issue in the overly
>     complex implementation of 4-character packing mode in the serial_msm
>     driver. Fix this by switching the driver to use the more simple
>     single-character mode, which doesn't have this issue.
> 
> With these patches, autoboot is finally working reliably on DragonBoard
> 410c (and hopefully will remain so for the future :-)).
> 
FWIW I've tested this series using serial UART DM interface on SDM660 
device. It does not fix *all* the issues needed for it to function 
there, but I can confirm it does not break anything and makes things 
much better than before, now with proper resetting and single-character 
mode it's possible to not rely on configuration from previous boot 
loader, for example.

Tested-by: Alexey Minnekhanov <alexeymin at minlexx.ru>

--
Regards,
Alexey Minnekhanov