Re: [PATCH 2/3] xen/arm64: add early printk for the classic i.MX UART

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 14-Aug-26 18:25, Wig Cheng wrote:
> Add an early printk implementation for the classic i.MX UART IP,
> selectable via EARLY_UART_CHOICE_IMX_UART.  The UART is expected to be
> fully initialized by the bootloader.
> 
> Signed-off-by: Wig Cheng <onlywig@xxxxxxxxx>
> ---
>  xen/arch/arm/Kconfig.debug            | 12 +++++++++
>  xen/arch/arm/arm64/debug-imx-uart.inc | 39 +++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
>  create mode 100644 xen/arch/arm/arm64/debug-imx-uart.inc
> 
> diff --git a/xen/arch/arm/Kconfig.debug b/xen/arch/arm/Kconfig.debug
> index 5a03b220ac..63a34b813a 100644
> --- a/xen/arch/arm/Kconfig.debug
> +++ b/xen/arch/arm/Kconfig.debug
> @@ -44,6 +44,14 @@ choice
>                 Say Y here if you wish the early printk to direct their
>                 output to a i.MX LPUART.
>  
> +     config EARLY_UART_CHOICE_IMX_UART
> +             select EARLY_UART_IMX_UART
> +             depends on ARM_64
> +             bool "Early printk via i.MX UART"
> +             help
> +               Say Y here if you wish the early printk to direct their
> +               output to the classic i.MX UART (i.MX8M family).
> +
>       config EARLY_UART_CHOICE_LINFLEX
>               select EARLY_UART_LINFLEX
>               depends on ARM_64
> @@ -97,6 +105,9 @@ config EARLY_UART_EXYNOS4210
>  config EARLY_UART_IMX_LPUART
>       select EARLY_PRINTK
>       bool
> +config EARLY_UART_IMX_UART
> +     select EARLY_PRINTK
> +     bool
>  config EARLY_UART_LINFLEX
>       select EARLY_PRINTK
>       bool
> @@ -185,6 +196,7 @@ config EARLY_PRINTK_INC
>       default "debug-cadence.inc" if EARLY_UART_CADENCE
>       default "debug-exynos4210.inc" if EARLY_UART_EXYNOS4210
>       default "debug-imx-lpuart.inc" if EARLY_UART_IMX_LPUART
> +     default "debug-imx-uart.inc" if EARLY_UART_IMX_UART
>       default "debug-linflex.inc" if EARLY_UART_LINFLEX
>       default "debug-meson.inc" if EARLY_UART_MESON
>       default "debug-mvebu.inc" if EARLY_UART_MVEBU
> diff --git a/xen/arch/arm/arm64/debug-imx-uart.inc 
> b/xen/arch/arm/arm64/debug-imx-uart.inc
> new file mode 100644
> index 0000000000..80ac647f5e
> --- /dev/null
> +++ b/xen/arch/arm/arm64/debug-imx-uart.inc
> @@ -0,0 +1,39 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * xen/arch/arm/arm64/debug-imx-uart.inc
This can go stale. Please drop.

> + *
> + * Early printk for the classic i.MX UART IP (i.MX6/7/8M families).
Don't mention 6 and 7 in a arm64 driver.

> + * The UART is expected to be fully initialized by the bootloader.
> + *
> + * Copyright 2026 Open-EP (E-Paper) Community
> + */
> +
> +#include <asm/imx-uart.h>
This header contains non-asm compatible macros. Please fix.

> +
> +/*
> + * Wait for the UART to be ready to transmit
> + * xb: register which contains the UART base address
> + * c: scratch register
> + */
> +.macro early_uart_ready xb, c
> +1:
> +        ldr   w\c, [\xb, #UTS]        /* <- Test register */
> +        tst   w\c, #UTS_TXFULL        /* Check TX FIFO full bit */
> +        bne   1b                      /* Wait until there is room */
b.ne please

> +.endm
> +
> +/*
> + * UART transmit character
> + * xb: register which contains the UART base address
> + * wt: register which contains the character to transmit
> + */
> +.macro early_uart_transmit xb, wt
> +        str   \wt, [\xb, #URTX0]      /* -> Transmitter register */
> +.endm
> +
> +/*
> + * Local variables:
> + * mode: ASM
> + * indent-tabs-mode: nil
> + * End:
> + */

Other than that, it looks good.

~Michal