linux-kernel - Re: [PATCH V3] hwmon: driver for Sensirion SHT21 humidity and temperature sensor

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Date:	Mon, 3 Jan 2011 07:12:42 -0800
From:	Guenter Roeck <guenter.roeck@...csson.com>
To:	Urs Fleisch <urs.fleisch@...il.com>
CC:	Jonathan Cameron <jic23@....ac.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LM Sensors <lm-sensors@...sensors.org>,
	Jean Delvare <khali@...ux-fr.org>
Subject: Re: [PATCH V3] hwmon: driver for Sensirion SHT21 humidity and
 temperature sensor

On Mon, Jan 03, 2011 at 09:01:34AM -0500, Urs Fleisch wrote:
> Hi Jonathan,
> 
> > Is there actualy a use case that means these can't both be set
> > by platform data?  Also, if these are acceptable, should they have
> > some means of indicating which combination of values are available?
> 
> I can imagine a use case where an embedded device wants to operate in a
> low-power mode while still monitoring humidity and/or temperature. Measurement
> time and thus power consumption of the SHT21 are significantly smaller when
> using a low resolution. Or you can use a low resolution while waiting for a
> significant change in humidity and/or temperature and then switch to a higher
> resolution to actually report the measurement values.
> 
> > Rely on platform data to get this right rather than an adhoc test that
> > might well pass on some random devices.
> 
> I removed that probing stuff. As read and write addresses for the user
> register are different, it would probably not pass on another device, but it
> could disturb another device.
> 
> > Sorry to say I missed some error handling issues the first time around.
> > Please always provide userspace with the most detailed and correct error
> > possible. Normally this is the one comming up from the bus subsystem.
> 
> All error codes should now be passed up to userspace.
> 
> Thanks again,
> Urs
> 
> Signed-off-by: Urs Fleisch <urs.fleisch@...sirion.com>

All the above shows up in the commit log if the patch is applied. Just adds
additional work for the maintainers.

> ---
[ ...] 
> +
> +/**
> + * sht21_show_temperature() - show temperature measurement value in sysfs
> + * @dev: device
> + * @attr: device attribute
> + * @buf: sysfs buffer (PAGE_SIZE) where measurement values are written to
> + *
> + * Will be called on read access to temp1_input sysfs attribute.
> + * Returns number of bytes written into buffer, negative errno on error.
> + */
> +static ssize_t sht21_show_temperature(struct device *dev,
> +       struct device_attribute *attr,
> +       char *buf)
> +{
> +       struct i2c_client *client = to_i2c_client(dev);
> +       int result = sht21_update_measurements(client);
> +       if (result >= 0) {
> +               struct sht21 *sht21 = i2c_get_clientdata(client);
> +
> +               return sprintf(buf, "%d\n", sht21->temperature);
> +       }
> +       return result;

Highly unusual way of detecting and returning errors. Common would be

	if (result < 0)
		return result;
	/* other code */

ie keep the code flow intact. Makes the code much easier to read.

Guenter
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