linux-kernel - Re: [PATCH v4 16/19] thermal/drivers/tegra: Remove unneeded lock when setting a trip point

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Date:   Thu, 2 Mar 2023 11:11:26 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        "open list:TEGRA ARCHITECTURE SUPPORT" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v4 16/19] thermal/drivers/tegra: Remove unneeded lock when
 setting a trip point

On 02/03/2023 10:45, Thierry Reding wrote:

[ ... ]

>>>> +	/*
>>>> +	 * Disable the interrupt so set_trips() can not be called
>>>> +	 * while we are setting up the register
>>>> +	 * TSENSOR_SENSOR0_CONFIG1. With this we close a potential
>>>> +	 * race window where we are setting up the TH2 and the
>>>> +	 * temperature hits TH1 resulting to an update of the
>>>> +	 * TSENSOR_SENSOR0_CONFIG1 register in the ISR.
>>>> +	 */
>>>> +	disable_irq(irq);
>>>> +
>>>>    	for (i = 0; i < ARRAY_SIZE(ts->ch); i++) {
>>>>    		err = tegra_tsensor_enable_hw_channel(ts, i);
>>>>    		if (err)
>>>>    			return err;
>>>>    	}
>>>> +	enable_irq(irq);
>>>
>>> Instead of disabling and reenabling the interrupt, could we simply move
>>> the channel enabling code a couple of lines above, before the IRQ
>>> request call? If enabling the channels were to trigger an interrupt, it
>>> should get triggered right after requesting the IRQ.
>>
>> Won't we have a spurious interrupt if that situation happen ?
> 
> It wouldn't be a spurious interrupt, but rather something that we want
> to react to. It's ultimately the same result as your patch. In your
> variant we basically request the IRQ (which automatically enables it),
> then immediately disable it, enable the HW channels and then reenable
> the interrupt. If enabling the HW channels were to trigger an interrupt,
> that interrupt would be raised immediately after enable_irq() as well,
> as far as I can tell.

I see, thanks for the clarification

   -- Daniel

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