Re: [Qemu-devel] [PATCH] tmp105: read temperature in milli-celsius

[Paolo Bonzini <pbonzini@redhat.com>]

> > @@ -78,7 +80,7 @@ static void tmp105_set_temperature(Object *obj, Visitor
> > *v, void *opaque,
> >          return;
> >      }
> >  
> > -    s->temperature = ((int16_t) (temp * 0x800 / 128000)) << 4;
> > +    s->temperature = (int16_t) (temp * 256 / 1000);
> 
> Did you check whether those magic 4 bits shift were for some other
> purpose such as flags possibly? CC'ing Alex Horn.

>From the data sheet, the temperature register is 9-12 bits, depending
on the configuration.  So the low 4 bits will always read back as zero.
However, they are already masked away when reading the temperature in
tmp105_read:

        s->buf[s->len ++] = (((uint16_t) s->temperature) >> 8);
        s->buf[s->len ++] = (((uint16_t) s->temperature) >> 0) &
                (0xf0 << ((~s->config >> 5) & 3));              /* R */

The equivalence of the formula can be proved as follows:

   ((int16_t) (temp * 0x800 / 128000)) << 4
          = (int16_t) (temp * 0x8000 / 128000) & ~15
          = (int16_t) (temp * (0x100 * 0x80) / (1000 * 0x80)) & ~15
          = (int16_t) (temp * 0x100 / 1000) & ~15

and the AND can be removed as mentioned above.

> Since we do have a tmp105-test, we should also add a regression test for
> the getter bug.

Yeah, if only tmp105-test already tested the setter. :)

Paolo