linux-kernel - Re: [PATCH] hwmon: sparx5: Fix initial reading of temperature

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Date:   Fri, 4 Sep 2020 10:33:11 +0200
From:   Lars Povlsen <lars.povlsen@...rochip.com>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Lars Povlsen <lars.povlsen@...rochip.com>,
        Jean Delvare <jdelvare@...e.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        <linux-hwmon@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, "Zhang Rui" <rui.zhang@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Amit Kucheria" <amitk@...nel.org>
Subject: Re: [PATCH] hwmon: sparx5: Fix initial reading of temperature


Guenter Roeck writes:

> On 9/3/20 6:47 AM, Lars Povlsen wrote:
>> If the temperature is read before the internal calibration is
>> completed, the driver returns -EIO. Instead it should return -EAGAIN
>> to encourage repeating the operation.
>>
>> Signed-off-by: Lars Povlsen <lars.povlsen@...rochip.com>
>> ---
>>  drivers/hwmon/sparx5-temp.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/hwmon/sparx5-temp.c b/drivers/hwmon/sparx5-temp.c
>> index 1a2b1026b026..98be48e3a22a 100644
>> --- a/drivers/hwmon/sparx5-temp.c
>> +++ b/drivers/hwmon/sparx5-temp.c
>> @@ -56,7 +56,7 @@ static int s5_read(struct device *dev, enum hwmon_sensor_types type,
>>       case hwmon_temp_input:
>>               stat = readl_relaxed(hwmon->base + TEMP_STAT);
>>               if (!(stat & TEMP_STAT_VALID))
>> -                     return -EIO;
>> +                     return -EAGAIN;
>
> The problem is that this may result in a hard loop and effectively hang the system,
> or at least the calling process. It may be better to return -ENODATA in this situation.
>

Guenther,

Thank you for your response.

The motivation for the patch was as I tried to use thermal support to
control a fan, and I noticed an initial quip from
thermal_core.c:update_temperature() - it will throw a dev_warn() for
anything else than an "EAGAIN". The EAGAIN also seems to be used by
several other hwmon drivers in similar situations.

The bottom line is that I would like to get rid of the warning
message. Changing the error code to ENODATA does not fix what I am
trying to accomplish.

Do you have any suggestions on how to achieve the objective?

Sincerely,

---Lars

> Guenter
>
>>               value = stat & TEMP_STAT_TEMP;
>>               /*
>>                * From register documentation:
>> --
>> 2.27.0
>>

-- 
Lars Povlsen,
Microchip