The Radxa E24C has its rtc battery attached to its 6th i2c controller,
which we're currently lacking a clock definition for.  With the diff below,
derived from linux code, since I still can't find an RK3528 TRM, the
rtc starts working:

pcxrtc0 at iic2 addr 0x51: battery ok

(it's iic2 because only i2c1, i2c2 and i2c5 are enabled in the device tree)

ok?


Index: rkclock.c
===================================================================
RCS file: /cvs/src/sys/dev/fdt/rkclock.c,v
diff -u -p -r1.99 rkclock.c
--- rkclock.c	5 May 2026 10:23:27 -0000	1.99
+++ rkclock.c	16 Sep 2026 04:26:14 -0000
@@ -3255,6 +3255,12 @@ const struct rkclock rk3528_clocks[] = {
 		{ RK3528_XIN24M }
 	},
 	{
+		RK3528_CLK_I2C5, RK3528_CRU_CLKSEL_CON(63),
+		SEL(15, 14), 0,
+		{ RK3528_CLK_MATRIX_200M_SRC, RK3528_CLK_MATRIX_100M_SRC,
+		  RK3528_CLK_MATRIX_50M_SRC, RK3528_XIN24M }
+	},
+	{
 		RK3528_CLK_GMAC1_SRC_VPU, 0, 0, 0,
 		{ RK3528_CLK_PPLL_125M_MATRIX }
 	},
Index: rkclock_clocks.h
===================================================================
RCS file: /cvs/src/sys/dev/fdt/rkclock_clocks.h,v
diff -u -p -r1.71 rkclock_clocks.h
--- rkclock_clocks.h	5 May 2026 10:23:27 -0000	1.71
+++ rkclock_clocks.h	16 Sep 2026 04:26:14 -0000
@@ -303,6 +303,7 @@
 #define RK3528_CCLK_SRC_EMMC		140
 #define RK3528_BCLK_EMMC		143
 #define RK3528_TCLK_EMMC		144
+#define RK3528_CLK_I2C5			167
 #define RK3528_CLK_GMAC1_SRC_VPU	173
 #define RK3528_CLK_I2C1			221
 #define RK3528_CCLK_SRC_SDMMC0		295