FreeBsd I2C utility gives same value as MSB and LSB from DS75 sensor 16bit register.

[Ian Lepore]

On Wed, 2019-05-08 at 14:06 -0500, Karl Denninger wrote:
> On 5/3/2019 06:43, Prasad M wrote:
> > Hi Folks,
> > 
> > I am trying to access a DS75 I2C temperature sensor. It kind of works with
> > the default i2c utility. But it returns twice the high byte instead of the
> > high and low byte for the temperature. Probably because it does 1 byte
> > reads on the I2C bus, sending a stop condition after every byte.
> > 
> > The device expects continuous reads and no stop/start in between the 2
> > bytes. Trying all options i2c, most notably the -m mode switch, yields the
> > same results all the time. Looking at the code in i2c.c
> > 
> > Could any one help to solve this problem with working I2CRDWR utility.
> 
> I don't know if you can do it with that utility, but coding up a simple
> (~10-15 lines of "C") that reads two bytes is trivially easy.
> 
> You simply stuff a message buffer requesting 2 bytes in response and
> call ioctl; you get back two bytes.
> 

This should now be fixed as of r348120 in 13-current, which I will MFC
to 12-stable after it gets some testing.  The fix adds a new "-m tr"
mode.  'tr' is for "transfer", a mode in which the complete set of i2c
operations are all handed to the driver at once using I2CRDWR, rather
than doing a set of individual START/REPEAT/STOP commands from
userland.  Some drivers (such as rpi) only work with complete
transfers.

So, to read the temperature from a DS75, it should now work like:

 i2c -m tr -a 0x40 -d r -o 0 -w 8 -c 2

-- Ian