On Tue, Mar 24, 2015 at 05:43:22PM +0200, Andy Shevchenko wrote:
> The Quark SoC data sheet describes the baud rate setting using fractional
> divider. The subset of possible values represented by a table suggests that the
> divisor has one block that could divide by 5. This explains a satan number in
> some cases in the table Thus, in this particular case the divisor can be
> evaluated as
What is a "satan number"?
> + /* Quark SoC has 200MHz xtal */
> + unsigned long xtal = 200000000, fref = xtal / 2;
> + /* Define two reference frequencies */
> + unsigned long fref1 = fref / 2, fref2 = fref * 2 / 5;
The Linux coding style generally frowns on multiple initializaitons on a
single line for legibility reasons.
> + /* Choose the best between two */
> + if (r2 >= r1 || q2 > 256) {
> + r = r1;
> + q = q1;
> + mul = mul1;
> + } else {
> + r = r2;
> + q = q2;
> + mul = (1 << 24) * 2 / 5; /* 0x666666 */
> + }
Making the comments on this a bit more explicit might be helpful - a
mention of why it's better to pick pick one of the settings or the other
would help.
> + /* In case the divisor is big enough */
> + if (fref / rate >= 80) {
> + u64 fssp;
> +
> + /* Calculate initial quot */
> + q1 = DIV_ROUND_CLOSEST(fref, rate);
> + mul1 = (1 << 24) / q1;
> +
> + /* Get the remainder */
> + fssp = (u64)fref * mul1;
> + do_div(fssp, 1 << 24);
> + r1 = abs(fssp - rate);
> +
> + /* Choose this one if it suits better */
> + if (r1 < r) {
> + q = 1;
> + mul = mul1;
> + }
> + }
So what do we do if the divisor is not big enough? I'm not sure that
one could reasonably expect someone to follow this code without doing
detective work. I think the biggest thing is that the comments aren't
saying why the code is doing what it's doing.
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