linux-kernel - Re: [PATCH V4 1/3] irqchip/sifive-plic: Add thead,c900-plic support

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Message-ID: <8735oxuxlq.wl-maz@kernel.org>
Date:   Tue, 19 Oct 2021 11:18:09 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Samuel Holland <samuel@...lland.org>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atish.patra@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Palmer Dabbelt <palmer@...belt.com>,
        Heiko Stübner <heiko@...ech.de>,
        Rob Herring <robh@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V4 1/3] irqchip/sifive-plic: Add thead,c900-plic support

On Tue, 19 Oct 2021 10:33:49 +0100,
Guo Ren <guoren@...nel.org> wrote:

> > If you have an 'automask' behavior and yet the HW doesn't record this
> > in a separate bit, then you need to track this by yourself in the
> > irq_eoi() callback instead. I guess that you would skip the write to
> > the CLAIM register in this case, though I have no idea whether this
> > breaks
> > the HW interrupt state or not.
> The problem is when enable bit is 0 for that irq_number,
> "writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM)" wouldn't affect
> the hw state machine. Then this irq would enter in ack state and no
> continues irqs could come in.

Really? This means that you cannot mask an interrupt while it is being
handled? How great...

> >
> > There is an example of this in the Apple AIC driver.
> Thx for the tip, I think your suggestion is:
> +++ b/drivers/irqchip/irq-sifive-plic.c
> @@ -163,7 +163,12 @@ static void plic_irq_eoi(struct irq_data *d)
>  {
>         struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> 
> -       writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +       if (irqd_irq_masked(d)) {
> +               plic_irq_unmask(d);
> +               writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +               plic_irq_mask(d);

This looks pretty dodgy. You are relying on interrupts being globally
masked on the CPU, I guess. It probably works today, but man, what a
terrible HW implementation.

You'll definitely have to move this into a c900-specific callback.

	M.

-- 
Without deviation from the norm, progress is not possible.