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Message-ID: <4118d15a-a70c-457d-8f62-d4f61f1157de@collabora.com>
Date:   Thu, 16 Nov 2023 15:20:20 +0100
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Subject: Re: [PATCH] cpuidle: idle exit_latency overflow
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To:     Bo Ye <bo.ye@mediatek.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>
Cc:     yongdong.zhang@mediatek.com, mtk24676 <C.Cheng@mediatek.com>,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
References: <20231116132619.69500-1-bo.ye@mediatek.com>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <20231116132619.69500-1-bo.ye@mediatek.com>
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Il 16/11/23 14:26, Bo Ye ha scritto:
> From: mtk24676 <C.Cheng@mediatek.com>
> 
> In detail:
> In kernel-6.1, in the __cpuidle_driver_init function in
> driver/cpuidle/driver.c, there is a line of code that causes
> an overflow. The line is s->exit_latency_ns = s->exit_latency
> * NSEC_PER_USEC. The overflow occurs because the product of an
> int type and a constant exceeds the range of the int type.
> 
> In C language, when you perform a multiplication operation, if
> both operands are of int type, the multiplication operation is
> performed on the int type, and then the result is converted to
> the target type. This means that if the product of int type
> multiplication exceeds the range that int type can represent,
> an overflow will occur even if you store the result in a
> variable of int64_t type.
> 
> Signed-off-by: mtk24676 <C.Cheng@mediatek.com>

It all makes sense, but "mtk24676" is not the name of a person, what's going on?

> Signed-off-by: bo.ye <bo.ye@mediatek.com>

That's the same, you perhaps wanted to write "Bo Ye", not "bo.ye".

Regards,
Angelo

> ---
> 
> diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> index d9cda7f..631ca16 100644
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -187,7 +187,7 @@
>   			s->target_residency = div_u64(s->target_residency_ns, NSEC_PER_USEC);
>   
>   		if (s->exit_latency > 0)
> -			s->exit_latency_ns = s->exit_latency * NSEC_PER_USEC;
> +			s->exit_latency_ns = (u64)s->exit_latency * NSEC_PER_USEC;
>   		else if (s->exit_latency_ns < 0)
>   			s->exit_latency_ns =  0;
>   		else