linux-kernel - Re: [PATCH v5 16/18] thermal/drivers/tegra: Remove unneeded lock when setting a trip point

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Message-ID: <ZABwllXuTHbUhnue@orome>
Date:   Thu, 2 Mar 2023 10:47:02 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
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>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        "open list:TEGRA ARCHITECTURE SUPPORT" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v5 16/18] thermal/drivers/tegra: Remove unneeded lock
 when setting a trip point

On Wed, Mar 01, 2023 at 09:14:44PM +0100, Daniel Lezcano wrote:
> The function tegra_tsensor_enable_hw_channel() takes the thermal zone
> lock to prevent "a potential" race with a call to set_trips()
> callback.
> 
> The driver must not play with the thermal framework core code
> internals.
> 
> The tegra_tsensor_enable_hw_channel() is called by:
> 
>  - the suspend / resume callbacks
>  - the probe function after the thermal zones are registered
> 
> The thermal zone lock taken in this function is supposed to protect
> from a call to the set_trips() callback which writes in the same
> register.
> 
> The potential race is when suspend / resume are called at the same
> time as set_trips. This one is called only in
> thermal_zone_device_update().
> 
>  - At suspend time, the 'in_suspend' is set, thus the
>    thermal_zone_device_update() bails out immediately and set_trips is
>    not called during this moment.
> 
>  - At resume time, the thermal zone is updated at PM_POST_SUSPEND,
>    thus the driver has already set the TH2 temperature.
> 
>  - At probe time, we register the thermal zone and then we set the
>    TH2. The only scenario I can see so far is the interrupt fires, the
>    thermal_zone_update() is called exactly at the moment
>    tegra_tsensor_enable_hw_channel() a few lines after registering it.
> 
> Enable the channels before setting up the interrupt. We close the
> potential race window without using the thermal zone's lock.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> Suggested-by: Thierry Reding <thierry.reding@...il.com>
> ---
>  drivers/thermal/tegra/tegra30-tsensor.c | 25 ++++++++++++++-----------
>  1 file changed, 14 insertions(+), 11 deletions(-)

Acked-by: Thierry Reding <treding@...dia.com>

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