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

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Feb 28, 2023 at 12:22:35PM +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.
> 
> Disable the interrupt before setting up the hw channels and then
> enable it. We close the potential race window without using the
> thermal zone's lock.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> ---
>  drivers/thermal/tegra/tegra30-tsensor.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/thermal/tegra/tegra30-tsensor.c b/drivers/thermal/tegra/tegra30-tsensor.c
> index 4b2ea17910cd..3506c3f3c474 100644
> --- a/drivers/thermal/tegra/tegra30-tsensor.c
> +++ b/drivers/thermal/tegra/tegra30-tsensor.c
> @@ -359,9 +359,6 @@ static int tegra_tsensor_enable_hw_channel(const struct tegra_tsensor *ts,
>  
>  	tegra_tsensor_get_hw_channel_trips(tzd, &hot_trip, &crit_trip);
>  
> -	/* prevent potential racing with tegra_tsensor_set_trips() */
> -	mutex_lock(&tzd->lock);
> -
>  	dev_info_once(ts->dev, "ch%u: PMC emergency shutdown trip set to %dC\n",
>  		      id, DIV_ROUND_CLOSEST(crit_trip, 1000));
>  
> @@ -404,8 +401,6 @@ static int tegra_tsensor_enable_hw_channel(const struct tegra_tsensor *ts,
>  	val |= FIELD_PREP(TSENSOR_SENSOR0_CONFIG0_INTR_THERMAL_RST_EN, 1);
>  	writel_relaxed(val, tsc->regs + TSENSOR_SENSOR0_CONFIG0);
>  
> -	mutex_unlock(&tzd->lock);
> -
>  	err = thermal_zone_device_enable(tzd);
>  	if (err) {
>  		dev_err(ts->dev, "ch%u: failed to enable zone: %d\n", id, err);
> @@ -592,12 +587,24 @@ static int tegra_tsensor_probe(struct platform_device *pdev)
>  		return dev_err_probe(&pdev->dev, err,
>  				     "failed to request interrupt\n");
>  
> +	/*
> +	 * 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.

Thierry

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