linux-kernel - Re: [PATCH] power_supply: add CHARGE_COUNTER property and olpc

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Message-ID: <20080513122330.4c81e289@ephemeral>
Date:	Tue, 13 May 2008 12:23:30 -0400
From:	Andres Salomon <dilinger@...ued.net>
To:	richard@...top.org
Cc:	avorontsov@...mvista.com,
	Andrew Morton <akpm@...ux-foundation.org>, cbou@...l.ru,
	linux-kernel@...r.kernel.org, dwmw2@...radead.org
Subject: Re: [PATCH] power_supply: add CHARGE_COUNTER property and
 olpc_battery support for it

On Tue, 13 May 2008 10:20:15 -0400
"Richard A. Smith" <richard@...top.org> wrote:

> Andres Salomon wrote:
> 
> > +		ec_word = be16_to_cpu(ec_word);
> > +		val->intval = ec_word * 4167 / 10;
> 
> Whats wrong with:
> 
>  > +		val->intval = ((int)ec_word) * 6250 / 15;
> 
> Which does not have overflow problems and keeps more precision.
> 

You're right, of course.  Anton, can you please apply the following?



Subject: [PATCH] power_supply: fix up CHARGE_COUNTER output to be more precise

As Richard Smith pointed out, ACR * 6250 / 15  provides for less precision
loss than ACR * 4167 / 10, _and_ it doesn't overflow.  Switch to using
that equation for CHARGE_COUNTER.

Signed-off-by: Andres Salomon <dilinger@...ian.org>
---
 drivers/power/olpc_battery.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/power/olpc_battery.c b/drivers/power/olpc_battery.c
index a928165..f5d712a 100644
--- a/drivers/power/olpc_battery.c
+++ b/drivers/power/olpc_battery.c
@@ -19,7 +19,7 @@
 
 #define EC_BAT_VOLTAGE	0x10	/* uint16_t,	*9.76/32,    mV   */
 #define EC_BAT_CURRENT	0x11	/* int16_t,	*15.625/120, mA   */
-#define EC_BAT_ACR	0x12	/* int16_t,	*416.7,      µAh  */
+#define EC_BAT_ACR	0x12	/* int16_t,	*6250/15,    µAh  */
 #define EC_BAT_TEMP	0x13	/* uint16_t,	*100/256,   °C  */
 #define EC_AMB_TEMP	0x14	/* uint16_t,	*100/256,   °C  */
 #define EC_BAT_STATUS	0x15	/* uint8_t,	bitmask */
@@ -295,7 +295,7 @@ static int olpc_bat_get_property(struct power_supply *psy,
 			return ret;
 
 		ec_word = be16_to_cpu(ec_word);
-		val->intval = ec_word * 4167 / 10;
+		val->intval = ec_word * 6250 / 15;
 		break;
 	case POWER_SUPPLY_PROP_SERIAL_NUMBER:
 		ret = olpc_ec_cmd(EC_BAT_SERIAL, NULL, 0, (void *)&ser_buf, 8);
-- 
1.5.5.1

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