Re: [PATCH v2] Input: add i2c/smbus driver for elan touchpad
flat view
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Date: 2014-07-24 18:05:23
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Hi, On Tue, Jan 07, 2014 at 11:08:03AM +0800, Duson Lin wrote:
quoted hunk ↗ jump to hunk
+/* + ****************************************************************** + * 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