Messages in this thread | | | Date | Mon, 16 Apr 2007 23:35:18 +0400 | From | Anton Vorontsov <> | Subject | Re: [PATCH 6/7] [RFC] ds2760 battery driver |
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On Mon, Apr 16, 2007 at 12:14:27PM -0700, Matt Reimer wrote: > On 4/15/07, Pavel Machek <pavel@ucw.cz> wrote: > > > + di->update_time = jiffies; > > > + > > > + /* DS2760 reports voltage in units of 4.88mV, but the battery class > > > + * reports in units of mV, so convert by multiplying by 4.875. > > > + * We approximate because integer math is cheap, and close enough. > > */ > > > + di->voltage_raw = (di->raw[DS2760_VOLTAGE_MSB] << 3) | > > > + (di->raw[DS2760_VOLTAGE_LSB] >> 5); > > > + di->voltage_mV = (di->voltage_raw * 5) - (di->voltage_raw / 8); > > > > Hmm, not sure if such tricks re really worth it... should not compiler > > be doing this? > > The shifts (<< 3 and >> 5) are just to get the bits reassembled in the > right positions. The multiplication by 5 and subtracting 1/8 is > because (AFAIK) we can't do floating point multiplication in the > kernel. I'm open to suggestions.
Because we are in micro world now, divisions already replaced by multiplication. I.e.
/* DS2760 reports voltage in units of 4.88mV, but the battery class * reports in units of uV, so convert by multiplying by 4880. */ di->voltage_raw = (di->raw[DS2760_VOLTAGE_MSB] << 3) | (di->raw[DS2760_VOLTAGE_LSB] >> 5); di->voltage_uV = di->voltage_raw * 4880;
As a side effect, now we're not losing any precision. :-)
> > > + /* Calculate the empty level at the present temperature. */ > > > + scale[4] = di->raw[DS2760_ACTIVE_EMPTY + 4]; > > > + for (i = 3; i >= 0; i--) > > > + scale[i] = scale[i + 1] + di->raw[DS2760_ACTIVE_EMPTY + i]; > > > + > > > + di->empty_mAh = battery_interpolate(scale, di->temp_C / 10); > > > > Wow. > > Yeah, mea culpa. I don't like its obtuseness either, and haven't > revisited it since I got it working. Basically the battery has an > array stored in its EEPROM that represents the empty level at various > temperature thresholds. What makes this complicated is that the array > does not contain all absolute values; just the first is absolute, and > the others are relative to it, in reverse order. The above code > converts it to an array of absolute values from low to high. I'm open > to suggestions about this one too.
By the way. Matt, you're more familiar with ds2760 specs, could you enlighten me about "* 4" in this snippet?
> acr[0] = (di->full_active_mAh * 4) >> 8; ^^^ > acr[1] = (di->full_active_mAh * 4) & 0xff; ^^^ > if (w1_ds2760_write(di->w1_dev, acr, > DS2760_CURRENT_ACCUM_MSB, 2) < 2) > printk(KERN_ERR "ACR reset failed\n");
> Matt
Thanks!
-- Anton Vorontsov email: cbou@mail.ru backup email: ya-cbou@yandex.ru irc://irc.freenode.org/bd2 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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