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

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Message-ID: <CAD=FV=XoqTeUg9mQLck-fwtFptSqn2eC1htmRkt_b-pih+wqug@mail.gmail.com>
Date:   Thu, 23 Jun 2022 16:11:48 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     "Vijaya Krishna Nivarthi (Temp)" <vnivarth@....qualcomm.com>
Cc:     "Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com>,
        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 Tue, Jun 21, 2022 at 10:58 AM Vijaya Krishna Nivarthi (Temp)
<vnivarth@....qualcomm.com> wrote:
>
> Hi,
>
> For desired_clk = 100 and clock rates like 1st from below, DIV_ROUND_UP seems to cause missing candidate solutions.
>
> static unsigned long clk_round_rate_test(struct clk *clk, unsigned long in_freq)
> {
>         //unsigned long root_freq[] = {301, 702, 1004};
>         //unsigned long root_freq[] = {301, 702, 1004, 2000, 3000};
>         //unsigned long root_freq[] = {50, 97, 99};
>         //unsigned long root_freq[] = {50, 97, 99, 200};
>         //unsigned long root_freq[] = {92, 110, 193, 230};
>         //unsigned long root_freq[] = {92, 110, 193, 230, 300, 401};
>         //unsigned long root_freq[] = {92, 110, 193, 230, 295, 296, 297, 401};
>         //unsigned long root_freq[] = {92, 110, 193, 230, 295, 296, 297, 300, 401};
>         //unsigned long root_freq[] = {197, 198, 199};
>         unsigned long root_freq[] = {197, 198, 199, 200};
>         int i;
>         size_t n = sizeof root_freq / sizeof *root_freq;
>
>         for (i = 0; i < n; i++) {
>                 if (root_freq[i] >= in_freq)
>                         return root_freq[i];
>         }
>         return root_freq[n-1];
> }
>
> I modified to handle such cases, optimised little and uploaded a patch.
> It seems to work for all the cases like above.

I think it would have been simpler to just change this little section:

/* Save the first (lowest freq) within 2% */
actual_mult = DIV_ROUND_CLOSEST(freq, desired_clk) * desired_clk;
if (!ser_clk && freq <= actual_mult + two_percent) {
  ser_clk = freq;
  *clk_div = div;
}

That was the only bug, right? Then you could keep the DIV_ROUND_UP() solution?

-Doug