linux-kernel - Re: [PATCH v2 2/2] drivers: thermal: tsens: add timeout to get_tem_tsens

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Date:   Tue, 7 Sep 2021 14:57:18 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ansuel Smith <ansuelsmth@...il.com>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Amit Kucheria <amitk@...nel.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] drivers: thermal: tsens: add timeout to
 get_tem_tsens_valid

On Tue, Sep 07, 2021 at 11:25:43PM +0200, Ansuel Smith wrote:
> The function can loop and lock the system if for whatever reason the bit
> for the target sensor is NEVER valid. This is the case if a sensor is
> disabled by the factory and the valid bit is never reported as actually
> valid. Add a timeout check and exit if a timeout occurs. As this is
> a very rare condition, handle the timeout only if the first read fails.
> While at it also rework the function to improve readability.
> 
> Signed-off-by: Ansuel Smith <ansuelsmth@...il.com>
> ---
>  drivers/thermal/qcom/tsens.c | 40 +++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/thermal/qcom/tsens.c b/drivers/thermal/qcom/tsens.c
> index b1162e566a70..1ff244176beb 100644
> --- a/drivers/thermal/qcom/tsens.c
> +++ b/drivers/thermal/qcom/tsens.c
> @@ -599,26 +599,38 @@ int get_temp_tsens_valid(const struct tsens_sensor *s, int *temp)
>  	int hw_id = s->hw_id;
>  	u32 temp_idx = LAST_TEMP_0 + hw_id;
>  	u32 valid_idx = VALID_0 + hw_id;
> +	unsigned long timeout;
>  	u32 valid;
>  	int ret;
>  
>  	/* VER_0 doesn't have VALID bit */
> -	if (tsens_version(priv) >= VER_0_1) {
> +	if (tsens_version(priv) == VER_0)
> +		goto get_temp;
> +
> +	ret = regmap_field_read(priv->rf[valid_idx], &valid);
> +	if (ret || valid)
> +		goto check_valid;
> +
> +	timeout = jiffies + msecs_to_jiffies(20);
> +	do {
> +		/* Valid bit is 0 for 6 AHB clock cycles.
> +		 * At 19.2MHz, 1 AHB clock is ~60ns.
> +		 * We should enter this loop very, very rarely.
> +		 */
> +		ndelay(400);
>  		ret = regmap_field_read(priv->rf[valid_idx], &valid);
> -		if (ret)
> -			return ret;
> -		while (!valid) {
> -			/* Valid bit is 0 for 6 AHB clock cycles.
> -			 * At 19.2MHz, 1 AHB clock is ~60ns.
> -			 * We should enter this loop very, very rarely.
> -			 */
> -			ndelay(400);
> -			ret = regmap_field_read(priv->rf[valid_idx], &valid);
> -			if (ret)
> -				return ret;
> -		}
> -	}
> +		if (ret || valid)
> +			goto check_valid;
> +	} while (time_before(jiffies, timeout));
> +
> +	return -ETIMEDOUT;
> +
> +check_valid:
> +	/* Check ret of valid bit read */
> +	if (ret)
> +		return ret;
>  
> +get_temp:
>  	/* Valid bit is set, OK to read the temperature */
>  	*temp = tsens_hw_to_mC(s, temp_idx);

I still think that something like this would be clearer than the
multiple jumps to 'check_valid':


	ret = regmap_field_read(priv->rf[valid_idx], &valid);
	if (ret)
		return ret;

	timeout = jiffies + msecs_to_jiffies(20);

	while (!valid) {
		/* Valid bit is 0 for 6 AHB clock cycles.
		 * At 19.2MHz, 1 AHB clock is ~60ns.
		 * We should enter this loop very, very rarely.
		 */
		ndelay(400);
		ret = regmap_field_read(priv->rf[valid_idx], &valid);
		if (ret)
			return ret;

		if (time_after(jiffies, timeout))
			return -ETIMEDOUT;
	};

That said, I'm just a random dude, so my opinion isn't really important,
as long as the maintainers are happy with the change :)