Messages in this thread | | | Date | Wed, 14 Jul 2021 13:38:22 +0300 | From | Andy Shevchenko <> | Subject | Re: [RFC PATCH] clk: fractional-divider: Correct max_{m,n} handed over to rational_best_approximation() |
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On Wed, Jul 14, 2021 at 06:10:46PM +0800, Liu Ying wrote: > On Wed, 2021-07-14 at 12:12 +0300, Andy Shevchenko wrote: > > On Wed, Jul 14, 2021 at 02:41:29PM +0800, Liu Ying wrote:
...
> > > /* > > > * Get rate closer to *parent_rate to guarantee there is no overflow > > > * for m and n. In the result it will be the nearest rate left shifted > > > * by (scale - fd->nwidth) bits. > > > */ > > > > I don't know how to rephrase above comment better. > > > > > scale = fls_long(*parent_rate / rate - 1); > > > if (scale > fd->nwidth) > > > rate <<= scale - fd->nwidth; > > > > This takes an advantage of the numbers be in a form of > > > > n = k * 2^m, (1) > > > > where m will be scale in the snippet above. Thus, if n can be represented by > > (1), we opportunistically reduce amount of bits needed for it by shifting right > > by m bits. > > > > Does it make sense? > > Thanks for your explaination. > But, sorry, Jacky and I still don't understand this.
Okay, We have two values in question: r_o (original rate of the parent clock) r_u (the rate user asked for)
We have a pre-scaler block which asks for m (denominator) n (nominator) values to be provided to satisfy the (2)
r_u ~= r_o * m / n, (2)
where we try our best to make it "=" instead of "~=".
Now, m and n have the limitation by a range, e.g.
n >= 1, n < N_lim, where N_lim = 2^nlim. (3)
Hence, from (2) and (3), assuming the worst case m = 1,
ln2(r_o / r_u) <= nlim. (4)
The above code tries to satisfy (4).
Have you got it now?
> > The code looks good to me, btw, although I dunno if you need to call the newly > > introduced function before or after the above mentioned snippet. > > Assuming that snippet is fully orthogonal to this patch, then it > doesn't matter if it's before or after.
Please, double check this. Because you play with limits, while we expect them to satisfy (3).
-- With Best Regards, Andy Shevchenko
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