linux-kernel - RE: [PATCH 7/7] leds-lm3530: enhanced arithmetic operation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <B567DBAB974C0544994013492B949F8E3812CD356C@EXMAIL03.scwf.nsc.com>
Date:	Fri, 20 Jan 2012 03:20:11 -0800
From:	"Kim, Milo" <Milo.Kim@...com>
To:	"Lars-Peter Clausen" <lars@...afoo.de>
cc:	"Linus Walleij" <linus.walleij@...aro.org>,
	"shreshthakumar.sahu@...ricsson.com" 
	<shreshthakumar.sahu@...ricsson.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rpurdie@...ys.net" <rpurdie@...ys.net>
Subject: RE: [PATCH 7/7] leds-lm3530: enhanced arithmetic operation

The reason why dividing by two is only 7-bits of brightness value in the LM3530 register.

Bit7 | bit6 | bit5 | bit4 | bit3 | bit2 | bit1 | bit0
------------------------------------------------------
 N/A |        LED Brightness Data (Bits[6:0])
------------------------------------------------------

So maximum value is limited to 127.

And we can think the alternative is '(brt_val & 0x7F)' - taking off the MSB to make it 7bits.
But the result is unexpected because value 128 and 255 has same brightness.
Brightness should be changed step by step, so appropriate method is dividing by two.

For the bit arithmetic and division, is it safe to predict same results ?
As you told, gcc makes same operation for that.(The objdump shows the same assembly code.)
But I think that using bit arithmetic is good habit.

Thanks,
Milo -

-----Original Message-----
From: Lars-Peter Clausen [mailto:lars@...afoo.de] 
Sent: Friday, January 20, 2012 5:32 PM
To: Kim, Milo
Cc: Linus Walleij; shreshthakumar.sahu@...ricsson.com; linux-kernel@...r.kernel.org; rpurdie@...ys.net
Subject: Re: [PATCH 7/7] leds-lm3530: enhanced arithmetic operation

On 01/20/2012 02:52 AM, Kim, Milo wrote:
> Use shift operation rather than 'divide-by-2'.

The compiler will already take of this for you.

But why is the divide by two necessary anyway? This sort of looks like
max_brightness is not set properly and the code compensates for it by
ignoring the lsb.

> 
> Signed-off-by: Milo(Woogyom) Kim <milo.kim@...com>
> 
> diff --git a/drivers/leds/leds-lm3530.c b/drivers/leds/leds-lm3530.c
> index 51c1f6c..e0b1ba8 100644
> --- a/drivers/leds/leds-lm3530.c
> +++ b/drivers/leds/leds-lm3530.c
> @@ -249,12 +249,12 @@ static void lm3530_brightness_set(struct led_classdev *led_cdev,
>  
>  		/* set the brightness in brightness control register*/
>  		err = i2c_smbus_write_byte_data(drvdata->client,
> -				LM3530_BRT_CTRL_REG, brt_val / 2);
> +				LM3530_BRT_CTRL_REG, brt_val >> 1);
>  		if (err)
>  			dev_err(&drvdata->client->dev,
>  				"Unable to set brightness: %d\n", err);
>  		else
> -			drvdata->brightness = brt_val / 2;
> +			drvdata->brightness = brt_val >> 1;
>  
>  		if (brt_val == 0 && !pdata->is_vin_always_on) {
>  			err = regulator_disable(drvdata->regulator);


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/