linux-kernel - Re: [PATCH] Support for the pressure sensor BMP085 from Bosch Sensortec.

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Message-Id: <201006230126.48452.christoph.mair@gmail.com>
Date:	Wed, 23 Jun 2010 01:26:47 +0200
From:	Christoph Mair <christoph.mair@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jonathan Cameron <kernel@...23.retrosnub.co.uk>,
	linux-kernel@...r.kernel.org,
	Stefan Schmidt <stefan@...enfreihafen.org>,
	Datta Shubhrajyoti <shubhrajyoti@...com>
Subject: Re: [PATCH] Support for the pressure sensor BMP085 from Bosch Sensortec.

Am Montag 21 Juni 2010, 23:49:58 schrieben Sie:
> On Sun, 20 Jun 2010 22:50:58 +0200
> 
> Christoph Mair <christoph.mair@...il.com> wrote:
> > This driver adds support for the BMP085 digital pressure sensor
> > from Bosch Sensortec.
> 
> If there's a datasheet available, it would be nice to provide a
> reference to that in the driver.
I will send a patch to fix this.

> Please do send a full description of the proposed API so we can review
> that.  What are the contents of these files?  What are the units of
> those contents, etc?
There is a short description before the methods bmp085_get_temperature() and 
bmp085_get_pressure(), but anyway.

Here is a Description of the current sysfs user interface:

The BMP085 digital pressure sensor can measure ambient air pressure and 
temperature. Both values can be obtained from sysfs files. The pressure is 
measured by reading from pressure0_input. Valid values range from 30000 to 
110000 pascal with a resolution of 1 pascal (=0.01 millibar).

temp0_input holds the current temperature in degree celsius, multiplied by 10. 
This results in a resolution of a tenth degree celsius. Values range from -400 
to 850.

To increase the accuracy, this chip can calculate the average of 1, 2, 4 or 8 
samples. This behavior is controlled through the oversampling sysfs file. Two 
to the power of the value written to that file specifies how many samples will 
be used. Valid values: 0..3.

> Are there any similar drivers in the tree (I don't think so) and if so
> does this new driver offer the same interface?
I did not find one.

> Bear in mind that if new drivers for similar devices _do_ come along
> then we'd prefer that those drivers implement the same interface as
> this one.  So is this driver's interface well-designed from that point
> of view?
The pressure0_input and temp0_input should follow the naming convention of 
hwmon devices. I think that would be ok for other sensors too. The 
oversampling is special to this device and others may not support this.

A more general problem is the obtainable resolution. I work on a humidity 
sensor driver for the SHT21. It measures temperatures with a resolution of 
0.01 celsius, ten times higher than the bmp085.
Maybe we need an additional sysfs file which contains the current measurement 
resolution.

I think it would be nice to have a general sensor api which for example 
specifies the sensor type, the units used for this type and the resolution of 
each result.
Any thoughts on this?

> Also, we're supposed to docuemnt these things formally in
> Documentation/ABI/.
I will send a patch for this one too.

> 
> > ...
> > 
> > + * The chip will use 2^oversampling samples for interlan averaging.
> 
> I assumed that was supposed to read "internal".
True.

Best Regards,
  Christoph
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