linux-kernel - Re: [RFC][PATCH] hwmon: add support for Sensirion C1 sensor

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Date:	Fri, 20 Jul 2012 17:13:51 +0200
From:	Johannes Winkelmann <johannes.winkelmann@...il.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org,
	Jean Delvare <khali@...ux-fr.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Johannes Winkelmann <johannes.winkelmann@...sirion.com>
Subject: Re: [RFC][PATCH] hwmon: add support for Sensirion C1 sensor

On Fri, Jul 20, 2012 at 4:03 PM, Oliver Neukum <oneukum@...e.de> wrote:
> On Friday 20 July 2012 14:57:22 Johannes Winkelmann wrote:
>
>> +/* sysfs attributes */
>> +static struct shtc1_data *shtc1_update_client(struct device *dev)
>> +{
>> +     struct i2c_client *client = to_i2c_client(dev);
>> +     struct shtc1_data *data = i2c_get_clientdata(client);
>> +
>> +     char buf[SHTC1_RESPONSE_LENGTH];
>
> Is this used for DMA?

No

>
>> +     int val;
>> +     int ret;
>> +
>> +     mutex_lock(&data->update_lock);
>> +
>> +     /*
>> +      * initialize 'ret' in case we had a valid result before, but
>> +      * read too quickly in which case we return the last values
>> +      */
>> +     ret = !data->valid;
>> +
>> +     if (time_after(jiffies, data->last_updated + HZ / 10)
>> +         || !data->valid) {
>> +             ret = shtc1_update_values(client, data, buf, sizeof(buf));
>> +
>> +             if (ret)
>> +                     goto out;
>> +
>> +             /*
>> +              * From datasheet:
>> +              *   T = -45 + 175 * ST / 2^16
>> +              *   RH = -10 + 120 * SRH / 2^16
>> +              *
>> +              * Adapted for integer fixed point (3 digit) arithmetic
>> +              */
>> +             val = (buf[0] << 8) | buf[1];
>> +             data->temperature = ((21875 * val) >> 13) - 45000;
>> +             val = (buf[3] << 8) | buf[4];
>
> We have dedicated macros for conversion of endianness.

Like this:
		val = swab16p((__le16 *)buf);
		data->temperature = ((21875 * val) >> 13) - 45000;
		val = swab16p((__le16 *)(buf+2));
		data->humidity = ((15000 * val) >> 13) - 10000;


>
>> +             data->humidity = ((15000 * val) >> 13) - 10000;
>> +
>> +             data->last_updated = jiffies;
>> +             data->valid = 1;
>> +     }
>> +
>> +out:
>> +     mutex_unlock(&data->update_lock);
>> +
>> +     return ret == 0 ? data : NULL;
>> +}
>         Regards
>                 Oliver
>

Thanks, Johannes
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