Date: Tue, 30 Jun 2015 06:22:14 -0700 From: Leonardo Fogel <leonardofogel@yahoo.com.br> To: freebsd-arm@freebsd.org Subject: Improvement to am335x_lcd_calc_divisor() Message-ID: <1435670534.33073.YahooMailBasic@web120806.mail.ne1.yahoo.com>
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Hello. Given a reference frequency (R) and a target frequency (F), am335x_lcd_calc_divisor(R,F) calculates the divisor (d) such that abs(R/d - F) is the minimum; ideally R/d == F. https://svnweb.freebsd.org/base/head/sys/arm/ti/am335x/am335x_lcd.c?revision=284534&view=markup#l234 The function does a brute-force search; it tests d=2,...,255. I would like to propose a simpler (and faster) implementation. Let r be the real number R/F, such that R/r == F. The integer d that produces the minimum abs(R/d - F) also produces the minimum abs(r - d). The trivial solution is d = lround(r) (rounds to the nearest integer). We do not need floating point arithmetic. Let f be the fractional part of r, namely, the real number (R%F)/F. If f >= 0.5, then lround(r) rounds up. Otherwise, it rounds down. We can test whether 10*f >= 5. Here is the proposal: if (freq == 0) /* there is a bug somewhere. */ return (255); /* The fractional part of the real number reference/freq, times 10. */ fraction_x10 = 10*(reference%freq)/freq; if (fraction_x10 >= 5) div = reference/freq + 1; /* rounds up */ else div = reference/freq ; /* rounds down */ /* Raster mode case: divisors are in range from 2 to 255 */ if (div < 2) div = 2; else if (div > 255) dir = 255; return (div); If you think it is worth, I'll be glad to submit a path. I just can not test it, though. Thank you. Leonardohome | help
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