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

[Sumit Garg]

On Mon, Sep 08, 2025 at 01:31:20PM +0200, Stephan Gerhold 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 :-)).
> 
> Signed-off-by: Stephan Gerhold <stephan.gerhold at linaro.org>
> ---
> Changes in v2:
> - Rebase on top of U-Boot master and fix conflicts
> - Drop backported upstream DT changes (already landed through recent
>   subtree merge of DT changes in Linux 6.16)
> - Link to v1: https://lore.kernel.org/r/20250624-db410c-autoboot-fixes-v1-0-d96c97ae41d1@linaro.org

Nice fixes and cleanup, FWIW:

Acked-by: Sumit Garg <sumit.garg at oss.qualcomm.com>

-Sumit

> 
> ---
> Stephan Gerhold (6):
>       board: dragonboard410c: Drop now unneeded bootph-all for console
>       Revert "serial: serial_msm: Delay initialization to let pins stabilize"
>       serial: msm: Cleanup register naming
>       serial: msm: Reset after writing to DMEN
>       serial: msm: Re-enable after resetting
>       serial: msm: Use single character mode
> 
>  arch/arm/dts/apq8016-sbc-u-boot.dtsi |   9 ---
>  drivers/serial/serial_msm.c          | 127 ++++++++---------------------------
>  2 files changed, 29 insertions(+), 107 deletions(-)
> ---
> base-commit: 328747974a0620d0996113fe2b3906b06e96de00
> change-id: 20250419-db410c-autoboot-fixes-ad7f4f5973a8
> 
> Best regards,
> -- 
> Stephan Gerhold <stephan.gerhold at linaro.org>
>