[PATCH] mach-snapdragon: formalize APQ8016 debug UART init

[Stephan Gerhold]

On Wed, Jun 03, 2026 at 08:56:21AM +1000, Sam Day via B4 Relay wrote:
> From: Sam Day <me at samcday.com>
> 
> The dragonboard410c (and likely other SB-off devices) can run U-Boot
> early enough in the boot chain such that UART clock isn't started, and
> pins aren't configured.
> 
> APQ8016 has two UART peripherals, each with its own clock and TLMM pins.
> 
> Two new debug-apq8016-uartN.config fragments are provided. Each
> configures the appropriate UARTDM aperture. The newly introduced
> DEBUG_UART_APQ8016 Kconfig symbol pulls in MSM debug UART, and enables a
> board-level debug_uart_init function to start the right clock and mux
> the right TX pin.
> 
> Signed-off-by: Sam Day <me at samcday.com>
> ---
>  arch/arm/mach-snapdragon/Kconfig              |  9 +++++++
>  arch/arm/mach-snapdragon/Makefile             |  1 +
>  arch/arm/mach-snapdragon/debug-uart-apq8016.c | 37 +++++++++++++++++++++++++++
>  board/qualcomm/debug-apq8016-uart1.config     |  4 +++
>  board/qualcomm/debug-apq8016-uart2.config     |  4 +++
>  drivers/serial/Kconfig                        |  1 +
>  drivers/serial/serial_msm.c                   | 10 --------
>  7 files changed, 56 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm/mach-snapdragon/Kconfig b/arch/arm/mach-snapdragon/Kconfig
> index d3de8693b5a..43e6fd1bc42 100644
> --- a/arch/arm/mach-snapdragon/Kconfig
> +++ b/arch/arm/mach-snapdragon/Kconfig
> @@ -1,5 +1,14 @@
>  if ARCH_SNAPDRAGON
>  
> +config DEBUG_UART_APQ8016
> +	bool "APQ8016 early debug initialization"
> +	select CLK_QCOM_APQ8016
> +	select CONFIG_DEBUG_UART_MSM
> +	select DEBUG_UART_BOARD_INIT
> +	help
> +	  Ensures BLSP UART clocks+pins are setup. Useful when U-Boot is running from
> +	  aboot (such as on the dragonboard410c).
> +
>  config SYS_SOC
>  	default "snapdragon"
>  
> diff --git a/arch/arm/mach-snapdragon/Makefile b/arch/arm/mach-snapdragon/Makefile
> index 343e825c6fd..824bb4d5daf 100644
> --- a/arch/arm/mach-snapdragon/Makefile
> +++ b/arch/arm/mach-snapdragon/Makefile
> @@ -5,3 +5,4 @@
>  obj-y += board.o
>  obj-$(CONFIG_EFI_HAVE_CAPSULE_SUPPORT) += capsule_update.o
>  obj-$(CONFIG_OF_LIVE) += of_fixup.o
> +obj-$(CONFIG_DEBUG_UART_APQ8016) += debug-uart-apq8016.o
> diff --git a/arch/arm/mach-snapdragon/debug-uart-apq8016.c b/arch/arm/mach-snapdragon/debug-uart-apq8016.c
> new file mode 100644
> index 00000000000..2de9d99bed5
> --- /dev/null
> +++ b/arch/arm/mach-snapdragon/debug-uart-apq8016.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * SoC-specific debug UART initialization (clocks/pins) lives here
> + */
> +
> +#include <asm/io.h>
> +#include <dt-bindings/clock/qcom,gcc-msm8916.h>
> +
> +extern int apq8016_clk_init_uart(phys_addr_t gcc_base, unsigned long id);
> +
> +#define APQ8016_GCC_BASE		((phys_addr_t)0x01800000)
> +
> +#define APQ8016_TLMM_BASE		((phys_addr_t)0x01000000)
> +#define APQ8016_TLMM_FUNC_BLSP_UART	(2 << 2)
> +#define APQ8016_TLMM_DRV_8MA		(3 << 6)
> +#define APQ8016_TLMM_GPIO_ENABLE	BIT(9)
> +#define APQ8016_GPIO_CFG(gpio)		(APQ8016_TLMM_BASE + (phys_addr_t)(gpio) * 0x1000)
> +#define APQ8016_DEBUG_UART_PINCFG	(APQ8016_TLMM_FUNC_BLSP_UART | \
> +					 APQ8016_TLMM_DRV_8MA | \
> +					 APQ8016_TLMM_GPIO_ENABLE)

APQ8016_TLMM_GPIO_ENABLE is actually GPIO_OE ("output enable"). I guess
it makes sense to set this for a TX pin, but I think this is typically
not set for special-function pins like BLSP_UART. LK doesn't set it
AFAICT.

Thanks,
Stephan