Re: [V2] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which otherwise could return a sub-optimal clock rate.

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, Jul 6, 2022 at 10:44 AM Vijaya Krishna Nivarthi
<quic_vnivarth@quicinc.com> wrote:
>
> Hi,
>
>
> On 7/6/2022 8:56 PM, Doug Anderson wrote:
> > Hi,
> >
> > On Mon, Jul 4, 2022 at 11:57 AM Vijaya Krishna Nivarthi (Temp)
> > <vnivarth@qti.qualcomm.com> wrote:
> >> Hi,
> >>
> >>
> >>> -----Original Message-----
> >>> From: Doug Anderson <dianders@chromium.org>
> >>> Sent: Friday, July 1, 2022 8:38 PM
> >>> To: Vijaya Krishna Nivarthi (Temp) (QUIC) <quic_vnivarth@quicinc.com>
> >>> Cc: Andy Gross <agross@kernel.org>; bjorn.andersson@linaro.org; Konrad
> >>> Dybcio <konrad.dybcio@somainline.org>; Greg Kroah-Hartman
> >>> <gregkh@linuxfoundation.org>; Jiri Slaby <jirislaby@kernel.org>; linux-arm-
> >>> msm <linux-arm-msm@vger.kernel.org>; linux-serial@vger.kernel.org; LKML
> >>> <linux-kernel@vger.kernel.org>; Mukesh Savaliya (QUIC)
> >>> <quic_msavaliy@quicinc.com>; Matthias Kaehlcke <mka@chromium.org>;
> >>> Stephen Boyd <swboyd@chromium.org>
> >>> Subject: Re: [V2] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which
> >>> otherwise could return a sub-optimal clock rate.
> >>>
> >>> WARNING: This email originated from outside of Qualcomm. Please be wary
> >>> of any links or attachments, and do not enable macros.
> >>>
> >>> Hi,
> >>>
> >>> On Fri, Jul 1, 2022 at 4:04 AM Vijaya Krishna Nivarthi (Temp) (QUIC)
> >>> <quic_vnivarth@quicinc.com> wrote:
> >>>> Hi,
> >>>>
> >>>>
> >>>>> -----Original Message-----
> >>>>> From: Doug Anderson <dianders@chromium.org>
> >>>>> Sent: Thursday, June 30, 2022 4:45 AM
> >>>>> To: Vijaya Krishna Nivarthi (Temp) (QUIC)
> >>>>> <quic_vnivarth@quicinc.com>
> >>>>> Cc: Andy Gross <agross@kernel.org>; bjorn.andersson@linaro.org;
> >>>>> Konrad Dybcio <konrad.dybcio@somainline.org>; Greg Kroah-Hartman
> >>>>> <gregkh@linuxfoundation.org>; Jiri Slaby <jirislaby@kernel.org>;
> >>>>> linux-arm- msm <linux-arm-msm@vger.kernel.org>;
> >>>>> linux-serial@vger.kernel.org; LKML <linux-kernel@vger.kernel.org>;
> >>>>> Mukesh Savaliya (QUIC) <quic_msavaliy@quicinc.com>; Matthias
> >>>>> Kaehlcke <mka@chromium.org>; Stephen Boyd
> >>> <swboyd@chromium.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);
> >> Wouldn’t DIV_ROUND_UP(freq, desired_clk) suffice here?
> >> freq >= mult-offset, else we would have hit break.
> > No. As you say, freq >= "mult-offset". That means that freq could be
> > == "mult-offset", right? If offset > 0 then freq could be < mult. Then
> > your DIV_ROUND_UP() would just take you right back to where you
> > started the loop with and you'd end up with an infinite loop, wouldn't
> > you?
> >
> Probably No.
>
> ( (freq >= mult-offset) && (freq <= mult) ) =>
>
> ( (freq >= mult-offset) && (freq <= mult+offset) )
>
> would mean that
>
> div = DIV_ROUND_CLOSEST(freq, desired_clk);
> evaluates to original div and we are within tolerance and hence we can return and hence don't even reach DIV_ROUND_UP?
>
> Please note that freq can skip any multiples and land up anywhere.
>
> As long as it has not gone beyond clock rate table, either it lands
> within tolerance of nearest multiple of desired_clk (in which case we
> return)
>
> OR
>
> We move on to next div = (freq/desired_clk + 1)

Ah, you are correct. So just:

div = DIV_ROUND_UP(freq, desired_clk);

...because freq _has_ to be greater than mult. If it was < "mult -
offset" we would have ended the loop. If it was between "mult -
offset" and "mult + offset" (inclusive) then we would have success. So
freq must be > "mult + offset" at the end of the loop.

-Doug