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

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Message-ID: <CAAhV-H4NDsrvuPH2o1D=UzTTH13Q62YxcHCZLN-QvZ2Dc5qEqw@mail.gmail.com>
Date: Thu, 11 Sep 2025 12:49:56 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Ming Wang <wangming01@...ngson.cn>
Cc: Jiaxun Yang <jiaxun.yang@...goat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip/loongson-pch-lpc: Use legacy domain for PCH-LPC
 IRQ controller

Hi, Wangming,

On Tue, Sep 9, 2025 at 8:58 PM Ming Wang <wangming01@...ngson.cn> 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.
If we create a legacy domain, the VIRQ0~15 are pre-allocated, then an
ACPI device claims GSI 4, what will happen?

>
> 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>
> ---
>  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..68b09cc8c400 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.
irq_domain_create_legacy()

> +        */
> +       priv->lpc_domain = irq_domain_create_legacy(irq_handle, LPC_COUNT, 0, 0,
> +                       &pch_lpc_domain_ops, priv);
The recommended indentation is:
       priv->lpc_domain = irq_domain_create_legacy(irq_handle, LPC_COUNT, 0, 0,

                   &pch_lpc_domain_ops, priv);

which means "&" is below "irq_handle".

Huacai

>         if (!priv->lpc_domain) {
>                 pr_err("Failed to create IRQ domain\n");
>                 goto free_irq_handle;
> --
> 2.43.0
>