Re: [PATCH] MIPS: Loongson64: env: Fixup serial clock-frequency when using LEFI

[Jiaxun Yang]

On Mon, 2 Feb 2026, at 4:53 AM, Yao Zi wrote:
> When booting from LEFI firmware, the devicetree is chosen by matching
> bridge type and CPU PRID. However, serials on Loongson devices may not
> have the same clock frequency across different boards. For example,
> CPU UARTs found on Loongson 3A4000 is supplied by the system clock,
> which may be either 25MHz or 100MHz.
>
> Luckily, LEFI firmware interface provides information about UART
> address and corresponding clock frequency. Let's fixup clock-frequency
> properties for serials after FDT selection by matching FDT nodes with
> addresses provided by firmware.

Reviewed-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>

Well, it's hacky, but it works :-)

Thanks
Jiaxun

>
> Signed-off-by: Yao Zi <me@xxxxxxxx>
> ---
>
> This is tested on LS3A4000_7A1000_NUC_BOARD_V2.1, which utilizes a 25MHz
> oscillator as system clock input. Without the patch, serial output is
> completely broken after kernel initialization.
>
>  arch/mips/loongson64/env.c | 98 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 98 insertions(+)
>
> diff --git a/arch/mips/loongson64/env.c b/arch/mips/loongson64/env.c
> index be8d2ad10750..11ddf02d6a15 100644
> --- a/arch/mips/loongson64/env.c
> +++ b/arch/mips/loongson64/env.c
> @@ -16,6 +16,7 @@
> 
>  #include <linux/dma-map-ops.h>
>  #include <linux/export.h>
> +#include <linux/libfdt.h>
>  #include <linux/pci_ids.h>
>  #include <linux/string_choices.h>
>  #include <asm/bootinfo.h>
> @@ -57,6 +58,101 @@ void __init prom_dtb_init_env(void)
>  		loongson_fdt_blob = (void *)fw_arg2;
>  }
> 
> +static int __init lefi_fixup_fdt_serial(void *fdt, u64 uart_addr, u32 uart_clk)
> +{
> +	int node, len, depth = -1;
> +	const fdt64_t *reg;
> +	fdt32_t *clk;
> +
> +	for (node = fdt_next_node(fdt, -1, &depth);
> +	     node >= 0 && depth >= 0;
> +	     node = fdt_next_node(fdt, node, &depth)) {
> +		reg = fdt_getprop(fdt, node, "reg", &len);
> +		if (!reg || len <= 8 || fdt64_ld(reg) != uart_addr)
> +			continue;
> +
> +		clk = fdt_getprop_w(fdt, node, "clock-frequency", &len);
> +		if (!clk) {
> +			pr_warn("UART 0x%llx misses clock-frequency property\n",
> +				uart_addr);
> +			return -ENOENT;
> +		} else if (len != 4) {
> +			pr_warn("UART 0x%llx has invalid clock-frequency property\n",
> +				uart_addr);
> +			return -EINVAL;
> +		}
> +
> +		fdt32_st(clk, uart_clk);
> +
> +		return 0;
> +	}
> +
> +	return -ENODEV;
> +}
> +
> +static void __init lefi_fixup_fdt(struct system_loongson *system)
> +{
> +	static unsigned char fdt_buf[16 << 10] __initdata;
> +	struct uart_device *uartdev;
> +	bool is_loongson64g;
> +	u64 uart_base;
> +	int ret, i;
> +
> +	ret = fdt_open_into(loongson_fdt_blob, fdt_buf, sizeof(fdt_buf));
> +	if (ret) {
> +		pr_err("Failed to open FDT to fix up\n");
> +		return;
> +	}
> +
> +	is_loongson64g = (read_c0_prid() & PRID_IMP_MASK) == PRID_IMP_LOONGSON_64G;
> +
> +	for (i = 0; i < system->nr_uarts; i++) {
> +		uartdev = &system->uarts[i];
> +
> +		ret = lefi_fixup_fdt_serial(fdt_buf, uartdev->uart_base,
> +					    uartdev->uartclk);
> +		/*
> +		 * LOONGSON64G's CPU serials are mapped to two different
> +		 * addresses, one full-featured but differs from
> +		 * previous generations, one fully compatible with them.
> +		 *
> +		 * It's unspecified that which mapping should uart_base refer
> +		 * to, thus we should try fixing up with both.
> +		 */
> +		if (ret == -ENODEV && is_loongson64g) {
> +			switch (uartdev->uart_base) {
> +			case 0x1fe00100:
> +				uart_base = 0x1fe001e0;
> +				break;
> +			case 0x1fe00110:
> +				uart_base = 0x1fe001e8;
> +				break;
> +			case 0x1fe001e0:
> +				uart_base = 0x1fe00100;
> +				break;
> +			case 0x1fe001e8:
> +				uart_base = 0x1fe00110;
> +				break;
> +			default:
> +				pr_err("Unexpected UART address 0x%llx passed by firmware\n",
> +				       uartdev->uart_base);
> +				ret = -EINVAL;
> +				goto err_fixup;
> +			}
> +
> +			ret = lefi_fixup_fdt_serial(fdt_buf, uart_base,
> +						    uartdev->uartclk);
> +		}
> +
> +err_fixup:
> +		if (ret)
> +			pr_err("Couldn't fix up FDT node for UART 0x%llx\n",
> +			       uartdev->uart_base);
> +	}
> +
> +	loongson_fdt_blob = fdt_buf;
> +}
> +
>  void __init prom_lefi_init_env(void)
>  {
>  	struct boot_params *boot_p;
> @@ -237,4 +333,6 @@ void __init prom_lefi_init_env(void)
> 
>  	if (!loongson_fdt_blob)
>  		pr_err("Failed to determine built-in Loongson64 dtb\n");
> +	else
> +		lefi_fixup_fdt(esys);
>  }
> -- 
> 2.52.0

-- 
- Jiaxun