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

[Guenter Roeck]

On 10/21/22 10:51, Martin Blumenstingl wrote:
> Hi Guenter,
>
> On Fri, Oct 21, 2022 at 7:11 PM Guenter Roeck <linux@xxxxxxxxxxxx> 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