linux-kernel - Re: [PATCH] tty: serial: qcom-geni-serial: minor fixes to get_clk_div

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Date:   Fri, 10 Jun 2022 10:20:30 -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>,
        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: [PATCH] tty: serial: qcom-geni-serial: minor fixes to get_clk_div_rate()

Hi,

On Fri, Jun 10, 2022 at 2:33 AM Vijaya Krishna Nivarthi (Temp) (QUIC)
<quic_vnivarth@...cinc.com> wrote:
>
> Hi,
>
> Re-sending (2nd attempt) as emails are bouncing...
>
>
> > >
> > > But then once again, we would likely need 2 loops because while we are
> > > ok with giving up on search for best_div on finding something within
> > > 2% tolerance, we may not want to give up on exact match (freq %
> > > desired_clk == 0 )
> >
> > Ah, it took me a while to understand why two loops. It's because in one case
> > you're trying multiplies and in the other you're bumping up to the next
> > closest clock rate. I don't think you really need to do that. Just test the (rate -
> > 2%) and the rate. How about this (only lightly tested):
> >
> >     ser_clk = 0;
> >     maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
> >     div = 1;
> >     while (div < maxdiv) {
>
>
> div <= maxdiv ?

Ah, sure.


> >         mult = (unsigned long long)div * desired_clk;
> >         if (mult != (unsigned long)mult)
> >             break;
> >
> >         two_percent = mult / 50;
> >
> >         /*
> >          * 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 - two_percent;
> >         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 2% */
> >             if (!ser_clk && freq <= mult + two_percent) {
> >                 ser_clk = freq;
> >                 *clk_div = div;
> >             }
>
> My last concern is with search happening only within 2% tolerance.
> Do we fail otherwise?
>
> This real case has best tolerance of 1.9% and seems close.
>
> [   17.963672] 20220530 desired_clk-51200000
> [   21.193550] 20220530 returning ser_clk-52174000, div-1, diff-974000
>
> Perhaps we can fallback on 1st clock rate?

I don't feel super comfortable just blindly falling back on the 1st
clock rate. It could be wildly (more than 5%) wrong, can't it?

IMO:
* If you're not comfortable with 2%, you could always pick 3% or 4%.
As I said, my random web search seemed to indicate that up to 5% was
perhaps OK.
* It's probably overkill, but you could abstract the whole search out
and try searching once for 2% and then try 4%?


-Doug