[PATCH v2 2/4] xen/arm64: add early printk for the classic i.MX UART

[Wig Cheng <onlywig@gmail.com>]

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@gmail.com>
---
 xen/arch/arm/Kconfig.debug            | 12 +++++++++
 xen/arch/arm/arm64/debug-imx-uart.inc | 37 +++++++++++++++++++++++++++
 2 files changed, 49 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..19b1679020
--- /dev/null
+++ b/xen/arch/arm/arm64/debug-imx-uart.inc
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Early printk for the classic i.MX UART IP (i.MX8M family).
+ * The UART is expected to be fully initialized by the bootloader.
+ *
+ * Copyright 2026 Open-EP (E-Paper) Community
+ */
+
+#include <asm/imx-uart.h>
+
+/*
+ * 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 */
+        b.ne  1b                      /* Wait until there is room */
+.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:
+ */
-- 
2.43.0