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Message-ID: <CAD=FV=XCgsyTRT-T5jKN6c7tJ=du8gbpkMccm2VZpz+TFWyLsw@mail.gmail.com>
Date: Fri, 1 Jul 2022 08:08:10 -0700
From: Doug Anderson <dianders@...omium.org>
To: "Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com>
Cc: Andy Gross <agross@...nel.org>,
"bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
Konrad Dybcio <konrad.dybcio@...ainline.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
linux-arm-msm <linux-arm-msm@...r.kernel.org>,
"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
"Mukesh Savaliya (QUIC)" <quic_msavaliy@...cinc.com>,
Matthias Kaehlcke <mka@...omium.org>,
Stephen Boyd <swboyd@...omium.org>
Subject: Re: [V2] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which
otherwise could return a sub-optimal clock rate.
Hi,
On Fri, Jul 1, 2022 at 4:04 AM Vijaya Krishna Nivarthi (Temp) (QUIC)
<quic_vnivarth@...cinc.com> wrote:
>
> Hi,
>
>
> > -----Original Message-----
> > From: Doug Anderson <dianders@...omium.org>
> > Sent: Thursday, June 30, 2022 4:45 AM
> > To: Vijaya Krishna Nivarthi (Temp) (QUIC) <quic_vnivarth@...cinc.com>
> > Cc: Andy Gross <agross@...nel.org>; bjorn.andersson@...aro.org; Konrad
> > Dybcio <konrad.dybcio@...ainline.org>; Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org>; Jiri Slaby <jirislaby@...nel.org>; linux-arm-
> > msm <linux-arm-msm@...r.kernel.org>; linux-serial@...r.kernel.org; LKML
> > <linux-kernel@...r.kernel.org>; Mukesh Savaliya (QUIC)
> > <quic_msavaliy@...cinc.com>; Matthias Kaehlcke <mka@...omium.org>;
> > Stephen Boyd <swboyd@...omium.org>
> > Subject: Re: [V2] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which
> > otherwise could return a sub-optimal clock rate.
> >
> >
> >
> > > + /* Save the first (lowest freq) within tolerance */
> > > + ser_clk = freq;
> > > + *clk_div = new_div;
> > > + /* no more search for exact match required in 2nd run */
> > > + if (!exact_match)
> > > + break;
> > > + }
> > > + }
> > >
> > > - prev = freq;
> > > + div = freq / desired_clk + 1;
> >
> > Can't you infinite loop now?
> >
> > Start with:
> >
> > desired_clk = 10000
> > div = 1
> > percent_tol = 2
> >
> >
> > Now:
> >
> > mult = 10000
> > offset = 200
> > test_freq = 9800
> > freq = 9800
> > div = 9800 / 10000 + 1 = 0 + 1 = 1
> >
> > ...and then you'll loop again with "div = 1", won't you? ...or did I get
> > something wrong in my analysis? This is the reason my proposed algorithm
> > had two loops.
> >
> >
>
> I went back to your proposed algorithm and made couple of simple changes, and it seemed like what we need.
>
> a) look only for exact match once a clock rate within tolerance is found
> b) swap test_freq and freq at end of while loops to make it run as desired
>
>
> maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
> div = 1;
>
> while (div < maxdiv) {
> mult = (unsigned long long)div * desired_clk;
> if (mult != (unsigned long)mult)
> break;
>
> if (ser_clk)
> offset = 0;
> ===================a=====================
> else
> offset = div_u64(mult * percent_tol, 100);
>
> /*
> * Loop requesting (freq - 2%) and possibly (freq).
> *
> * We'll keep track of the lowest freq inexact match we found
> * but always try to find a perfect match. NOTE: this algorithm
> * could miss a slightly better freq if there's more than one
> * freq between (freq - 2%) and (freq) but (freq) can't be made
> * exactly, but that's OK.
> *
> * This absolutely relies on the fact that the Qualcomm clock
> * driver always rounds up.
> */
> test_freq = mult - offset;
> while (test_freq <= mult) {
> freq = clk_round_rate(clk, test_freq);
>
> /*
> * A dead-on freq is an insta-win. This implicitly
> * handles when "freq == mult"
> */
> if (!(freq % desired_clk)) {
> *clk_div = freq / desired_clk;
> return freq;
> }
>
> /*
> * Only time clock framework doesn't round up is if
> * we're past the max clock rate. We're done searching
> * if that's the case.
> */
> if (freq < test_freq)
> return ser_clk;
>
> /* Save the first (lowest freq) within tolerance */
> if (!ser_clk && freq <= mult + offset) {
> ser_clk = freq;
> *clk_div = div;
> }
>
> /*
> * If we already rounded up past mult then this will
> * cause the loop to exit. If not then this will run
> * the loop a second time with exactly mult.
> */
> test_freq = max(test_freq + 1, mult);
> ====b====
> }
>
> /*
> * freq will always be bigger than mult by at least 1.
> * That means we can get the next divider with a DIV_ROUND_UP.
> * This has the advantage of skipping by a whole bunch of divs
> * If the clock framework already bypassed them.
> */
> div = DIV_ROUND_UP(freq, desired_clk);
> ===b==
> }
>
>
> Will also drop exact_match now.
>
> Will upload v3 after testing.
The more I've been thinking about it, the more I wonder if we even
need the special case of looking for an exact match at all. It feels
like we should choose one: we either look for the best match or we
look for the one with the lowest clock source rate. The weird
half-half approach that we have right now feels like over-engineering
and complicates things.
How about this (again, only lightly tested). Worst case if we _truly_
need a close-to-exact match we could pass a tolerance of 0 in and we'd
get something that's nearly exact, though I'm not suggesting we
actually do that. If we think 2% is good enough then we should just
accept the first (and lowest clock rate) 2% match we find.
abs_tol = div_u64((u64)desired_clk * percent_tol, 100);
maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
div = 1;
while (div <= maxdiv) {
mult = (u64)div * desired_clk;
if (mult != (unsigned long)mult)
break;
offset = div * abs_tol;
freq = clk_round_rate(clk, mult - offset);
/* Can only get lower if we're done */
if (freq < mult - offset)
break;
/*
* Re-calculate div in case rounding skipped rates but we
* ended up at a good one, then check for a match.
*/
div = DIV_ROUND_CLOSEST(freq, desired_clk);
achieved = DIV_ROUND_CLOSEST(freq, div);
if (achieved <= desired_clk + abs_tol &&
achieved >= desired_clk - abs_tol) {
*clk_div = div;
return freq;
}
/*
* Always increase div by at least one, but we'll go more than
* one if clk_round_rate() gave us something higher.
*/
div = DIV_ROUND_UP(max(freq, (unsigned long)mult) + 1, desired_clk);
}
return 0;
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