Re: PMBus driver for FSP/3Y Power device with non-standard VOUT values (LINEAR11 vs LINEAR16)

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Mar 15, 2019 at 10:19:33AM +0000, Grönke, Christian wrote:
> Hello Guenter,
> 
> > -----Ursprüngliche Nachricht-----
> > Von: Guenter Roeck <groeck7@gmail.com> Im Auftrag von Guenter Roeck
> > Gesendet: Donnerstag, 14. März 2019 17:53
> > An: Grönke, Christian <C.Groenke@infodas.de>
> > Cc: linux-hwmon@vger.kernel.org
> > Betreff: Re: PMBus driver for FSP/3Y Power device with non-standard VOUT
> > values (LINEAR11 vs LINEAR16)
> > 
> > Hi Christian,
> > On Thu, Mar 14, 2019 at 04:08:32PM +0000, Grönke, Christian wrote:
> > >
> > > The framework code seemed to work fine. I used your code for the
> > conversion:
> > >     linear11 -> 'scaled integer' -> ieee754 It provided a way to test
> > > the code and was easy for me as my tries to do some other bit magic
> > > weren't successful. That means I partly tested the code from
> > > pmbus_data2reg_ieee754 as my read_word function uses this for the
> > > conversion. Of course not the module local function...
> > >
> > 
> > Wondering ... I would have thought that it should be possible to
> > implement a simplified linear11 <--> ieee754 conversion, without
> > converting to a scaled integer first. Have you tried that ?
> 
> There might be a more efficient and elegant way. I toyed around with
> the conversion for a moment but didn't figure out a working way 
> quickly.

This should work.

u16 lin2ieee(u16 lin11)
{
	s16 mantissa = ((s16)((lin11 & 0x7ff) << 5)) >> 5;
	int exponent = (((s16)lin11) >> 11) + 25;
	u16 sign = 0;

	/* simple case */
	if (mantissa == 0)
		return 0;

	if (mantissa < 0) {
		sign = 0x8000;
		mantissa = -mantissa;
	}

	while (mantissa < 0x400 && exponent > 1) {
		mantissa <<= 1;
		exponent--;
	}

	/* boundary checks */
	if (exponent > 30)
		return sign | 0x7bff;
	if (mantissa < 0x400)
		return sign | 0x0400;

	return sign | (exponent << 10) | (mantissa & 0x3ff);
}

Guenter