linux-kernel - Re: [PATCH v2] irqchip/loongson-pch-lpc: Use legacy domain for PCH-LPC IRQ controller

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Message-ID: <61dff58f-b425-4af5-b5b9-76d9981ccbd5@gmail.com>
Date: Thu, 11 Sep 2025 17:45:38 +0800
From: Robin <robin.toplinux@...il.com>
To: Ming Wang <wangming01@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
 Jiaxun Yang <jiaxun.yang@...goat.com>, Thomas Gleixner <tglx@...utronix.de>,
 linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] irqchip/loongson-pch-lpc: Use legacy domain for
 PCH-LPC IRQ controller



On 9/11/25 16:35, Ming Wang wrote:
> On certain Loongson platforms, drivers attempting to request a legacy
> ISA IRQ directly via request_irq() (e.g., IRQ 4) may fail. The
> virtual IRQ descriptor is not fully initialized and lacks a valid irqchip.
> 
> This issue does not affect ACPI-enumerated devices described in DSDT,
> as their interrupts are properly mapped via the GSI translation path.
> This indicates the LPC irqdomain itself is functional but is not correctly
> handling direct VIRQ-to-HWIRQ mappings.
> 
> The root cause is the use of irq_domain_create_linear(). This API sets
> up a domain for dynamic, on-demand mapping, typically triggered by a GSI
> request. It does not pre-populate the mappings for the legacy VIRQ range
> (0-15). Consequently, if no ACPI device claims a specific GSI
> (e.g., GSI 4), the corresponding VIRQ (e.g., VIRQ 4) is never mapped to
> the LPC domain. A direct call to request_irq(4, ...) then fails because
> the kernel cannot resolve this VIRQ to a hardware interrupt managed by
> the LPC controller.
> 
> The PCH-LPC interrupt controller is an i8259-compatible legacy device
> that requires a deterministic, static 1-to-1 mapping for IRQs 0-15 to
> support legacy drivers.
> 
> Fix this by replacing irq_domain_create_linear() with
> irq_domain_create_legacy(). This API is specifically designed for such
> controllers. It establishes the required static 1-to-1 VIRQ-to-HWIRQ
> mapping for the entire legacy range (0-15) immediately upon domain
> creation. This ensures that any VIRQ in this range is always resolvable,
> making direct calls to request_irq() for legacy IRQs function correctly.
> 
> Signed-off-by: Ming Wang <wangming01@...ngson.cn>

I have tested this patch on a Loongson 3A6000 platform.

Tested-by: Robin <robin.toplinux@...il.com>

> 
> ---
> Changes in v2:
>    - Address review comments from Huacai Chen.
>    - Fix function call indentation style.
> ---
>   drivers/irqchip/irq-loongson-pch-lpc.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-loongson-pch-lpc.c b/drivers/irqchip/irq-loongson-pch-lpc.c
> index 2d4c3ec128b8..01fe4325ff84 100644
> --- a/drivers/irqchip/irq-loongson-pch-lpc.c
> +++ b/drivers/irqchip/irq-loongson-pch-lpc.c
> @@ -200,8 +200,13 @@ int __init pch_lpc_acpi_init(struct irq_domain *parent,
>   		goto iounmap_base;
>   	}
>   
> -	priv->lpc_domain = irq_domain_create_linear(irq_handle, LPC_COUNT,
> -					&pch_lpc_domain_ops, priv);
> +	/*
> +	 * The LPC interrupt controller is a legacy i8259-compatible device,
> +	 * which requires a static 1:1 mapping for IRQs 0-15.
> +	 * Use irq_domain_create_legacy() to establish this static mapping early.
> +	 */
> +	priv->lpc_domain = irq_domain_create_legacy(irq_handle, LPC_COUNT, 0, 0,
> +						    &pch_lpc_domain_ops, priv);
>   	if (!priv->lpc_domain) {
>   		pr_err("Failed to create IRQ domain\n");
>   		goto free_irq_handle;