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

Thierry Reding
2023-03-01T14:19:45Z

[-- Attachment #1: Type: text/plain, Size: 3797 bytes --]

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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]