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Date:   Tue, 19 Sep 2023 15:37:52 +0100
Message-ID: <86y1h2cjpb.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Biju Das <biju.das.jz@bp.renesas.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>,
        Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Biju Das <biju.das.au@gmail.com>, linux-kernel@vger.kernel.org,
        linux-renesas-soc@vger.kernel.org
Subject: Re: [PATCH 3/3] irqchip: renesas-rzg2l: Fix irq storm with edge trigger detection for TINT
In-Reply-To: <20230918122411.237635-4-biju.das.jz@bp.renesas.com>
References: <20230918122411.237635-1-biju.das.jz@bp.renesas.com>
        <20230918122411.237635-4-biju.das.jz@bp.renesas.com>
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On Mon, 18 Sep 2023 13:24:11 +0100,
Biju Das <biju.das.jz@bp.renesas.com> wrote:
> 
> In case of edge trigger detection, enabling the TINT source causes a
> phantum interrupt that leads to irq storm. So clear the phantum interrupt
> in rzg2l_irqc_irq_enable().
> 
> This issue is observed when the irq handler disables the interrupts using
> disable_irq_nosync() and scheduling a work queue and in the work queue,
> re-enabling the interrupt with enable_irq().
> 
> Fixes: 3fed09559cd8 ("irqchip: Add RZ/G2L IA55 Interrupt Controller driver")
> Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
> Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> ---
>  drivers/irqchip/irq-renesas-rzg2l.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/irqchip/irq-renesas-rzg2l.c b/drivers/irqchip/irq-renesas-rzg2l.c
> index 33a22bafedcd..78a9e90512a6 100644
> --- a/drivers/irqchip/irq-renesas-rzg2l.c
> +++ b/drivers/irqchip/irq-renesas-rzg2l.c
> @@ -144,6 +144,12 @@ static void rzg2l_irqc_irq_enable(struct irq_data *d)
>  		reg = readl_relaxed(priv->base + TSSR(tssr_index));
>  		reg |= (TIEN | tint) << TSSEL_SHIFT(tssr_offset);
>  		writel_relaxed(reg, priv->base + TSSR(tssr_index));
> +		/*
> +		 * In case of edge trigger detection, enabling the TINT source
> +		 * cause a phantum interrupt that leads to irq storm. So clear
> +		 * the phantum interrupt.
> +		 */
> +		rzg2l_tint_eoi(d);

This looks incredibly unsafe. disable_irq()+enable_irq() with an
interrupt being made pending in the middle, and you've lost that
interrupt.

What prevents this scenario?

	M.

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