Re: [PATCH 2/2] drivers: thermal: tsens: add timeout to get_tem_tsens_valid

[Matthias Kaehlcke <mka@chromium.org>]

On Sun, Sep 05, 2021 at 07:47:08PM +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.
> 
> Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
> ---
>  drivers/thermal/qcom/tsens.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/thermal/qcom/tsens.c b/drivers/thermal/qcom/tsens.c
> index b1162e566a70..38afde1a599f 100644
> --- a/drivers/thermal/qcom/tsens.c
> +++ b/drivers/thermal/qcom/tsens.c
> @@ -599,6 +599,7 @@ 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;
>  
> @@ -607,13 +608,21 @@ int get_temp_tsens_valid(const struct tsens_sensor *s, int *temp)
>  		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 (!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 (valid || ret)
> +					break;
> +			} while (!(ret = time_after_eq(jiffies, timeout)));
> +
>  			if (ret)
>  				return ret;

With the overloading of 'ret' the return logic is getting a bit more
convoluted. Also the function should probably return -ETIMEDOUT or
some other meaningful error if the bit is never valid.

How about keeping the 'while (!valid)' condition, and adding

	if (time_after_eq(jiffies, timeout))
		return -ETIMEDOUT;

inside the loop?