linux-kernel - Re: [PATCH v3 1/2] hwmon: (jc42) Convert register access and caching to regmap/regcache

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Message-ID: <7de80d4f-7c25-6b6b-ebb0-e9e1254dac53@roeck-us.net>
Date:   Fri, 21 Oct 2022 21:04:14 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     linux-hwmon@...r.kernel.org, jdelvare@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] hwmon: (jc42) Convert register access and caching
 to regmap/regcache

On 10/21/22 10:51, Martin Blumenstingl wrote:
> Hi Guenter,
> 
> On Fri, Oct 21, 2022 at 7:11 PM Guenter Roeck <linux@...ck-us.net> wrote:
> [...]
>>> @@ -368,17 +361,14 @@ static int jc42_write(struct device *dev, enum hwmon_sensor_types type,
>>>                }
>>>                data->config = (data->config & ~JC42_CFG_HYST_MASK) |
>>>                                (hyst << JC42_CFG_HYST_SHIFT);
>>> -             ret = i2c_smbus_write_word_swapped(data->client,
>>> -                                                JC42_REG_CONFIG,
>>> -                                                data->config);
>>> +             ret = regmap_write(data->regmap, JC42_REG_CONFIG,
>>> +                                data->config);
>>>                break;
>>
>> This code sequence still requires a mutex since another thread could modify
>> the upper limit (and/or the hysteresis) while the hysteresis is in the process
>> of being written. Worst case there could be a mismatch between the value in
>> data->config and the value actually written into the chip. Granted, that is
>> unlikely to happen, but the race still exists.
> Thanks for spotting this - this is indeed a potential issue.
> Do you also want me to add locking for the data->config access (read)
> in jc42_read()? Without a lock there in theory jc42_write() may have
> already updated data->config with a new value while hardware still has
> the old value. So in the end the read output may show a hysteresis
> which was not programmed to the registers at that time.
> 

Thanks for noticing. I had missed that one. Yes, please.

Thanks,
Guenter