linux-kernel - Re: [PATCH v2] Input: add i2c/smbus driver for elan touchpad

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Date:	Thu, 24 Jul 2014 11:05:16 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Duson Lin <dusonlin@....com.tw>
Cc:	linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
	bleung@...omium.org, agnescheng@...omium.org, phoenix@....com.tw,
	jeff.chuang@....com.tw
Subject: Re: [PATCH v2] Input: add i2c/smbus driver for elan touchpad

Hi,

On Tue, Jan 07, 2014 at 11:08:03AM +0800, Duson Lin wrote:
> +/*
> + ******************************************************************
> + * General functions
> + ******************************************************************
> + */
> +/*
> + * (value from firmware) * 10 + 790 = dpi
> + * we also have to convert dpi to dots/mm (*10/254 to avoid floating point)
> + */
> +static unsigned int elan_convert_res(char val)
> +{
> +	int res;
> +	if (val & 0x80) {
> +		val = ~val + 1;
> +		res = (790 - val * 10) * 10 / 254;
> +	} else
> +		res = (val * 10 + 790) * 10 / 254;
> +	return res;
> +}

Why isn't this simply:

	res = ((int)val * 10 + 790) * 10 / 254;

?

If high bit is 1 you basically do the 2 complement by hand to get to
positive and then subtract. Which shoudl be the same as adding positive
value.

Thanks.

-- 
Dmitry
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