linux-kernel - Re: [PATCH] power: supply: bq27xxx_battery: Do not return ENODEV when busy

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Message-ID: <20240913212715.gmchsmmaqrhksmhx@pali>
Date: Fri, 13 Sep 2024 23:27:15 +0200
From: Pali Rohár <pali@...nel.org>
To: Jerry Lv <Jerry.Lv@...s.com>
Cc: Sebastian Reichel <sre@...nel.org>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...s.com
Subject: Re: [PATCH] power: supply: bq27xxx_battery: Do not return ENODEV
 when busy

On Friday 13 September 2024 16:45:37 Jerry Lv wrote:
> Multiple applications may access the device gauge at the same time, so the
> gauge may be busy and EBUSY will be returned. The driver will set a flag to
> record the EBUSY state, and this flag will be kept until the next periodic
> update. When this flag is set, bq27xxx_battery_get_property() will just
> return ENODEV until the flag is updated.

I did not find any evidence of EBUSY. Which function and to which caller
it returns? Do you mean that bq27xxx_read() returns -EBUSY?

> Even if the gauge was busy during the last accessing attempt, returning
> ENODEV is not ideal, and can cause confusion in the applications layer.

It would be better to either propagate correct error or return old value
from cache...

> Instead, retry accessing the gauge to update the properties is as expected.
> The gauge typically recovers from busy state within a few milliseconds, and
> the cached flag will not cause issues while updating the properties.
> 
> Signed-off-by: Jerry Lv <Jerry.Lv@...s.com>
> ---
>  drivers/power/supply/bq27xxx_battery.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/power/supply/bq27xxx_battery.c b/drivers/power/supply/bq27xxx_battery.c
> index 750fda543308..eefbb5029a3b 100644
> --- a/drivers/power/supply/bq27xxx_battery.c
> +++ b/drivers/power/supply/bq27xxx_battery.c
> @@ -2029,7 +2029,7 @@ static int bq27xxx_battery_get_property(struct power_supply *psy,
>  		bq27xxx_battery_update_unlocked(di);
>  	mutex_unlock(&di->lock);
>  
> -	if (psp != POWER_SUPPLY_PROP_PRESENT && di->cache.flags < 0)
> +	if (psp != POWER_SUPPLY_PROP_PRESENT && di->cache.flags < 0 && di->cache.flags != -EBUSY)
>  		return -ENODEV;

... but ignoring error and re-using the error return value as flags in
code later in this function is bad idea.

>  
>  	switch (psp) {
> 
> ---
> base-commit: da3ea35007d0af457a0afc87e84fddaebc4e0b63
> change-id: 20240913-foo-fix2-a0d79db86a0b
> 
> Best regards,
> -- 
> Jerry Lv <Jerry.Lv@...s.com>
>