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Date: Wed, 29 Jun 2022 10:25:45 +0000
From: "Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com>
To: Stephen Boyd <swboyd@...omium.org>,
"Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com>,
"agross@...nel.org" <agross@...nel.org>,
"bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
"jirislaby@...nel.org" <jirislaby@...nel.org>,
"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
CC: "Mukesh Savaliya (QUIC)" <quic_msavaliy@...cinc.com>,
"dianders@...omium.org" <dianders@...omium.org>,
"mka@...omium.org" <mka@...omium.org>
Subject: RE: [PATCH] tty: serial: qcom-geni-serial: Fix get_clk_div_rate()
which otherwise could return a sub-optimal clock rate.
Hi,
> -----Original Message-----
> From: Stephen Boyd <swboyd@...omium.org>
> Sent: Thursday, June 23, 2022 4:36 AM
> To: Vijaya Krishna Nivarthi (Temp) (QUIC) <quic_vnivarth@...cinc.com>;
> agross@...nel.org; bjorn.andersson@...aro.org;
> gregkh@...uxfoundation.org; jirislaby@...nel.org; linux-arm-
> msm@...r.kernel.org; linux-kernel@...r.kernel.org; linux-
> serial@...r.kernel.org
> Cc: Mukesh Savaliya (QUIC) <quic_msavaliy@...cinc.com>;
> dianders@...omium.org; mka@...omium.org
> Subject: Re: [PATCH] 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.
>
> Quoting Vijaya Krishna Nivarthi (2022-06-21 10:57:19)
> > In the logic around call to clk_round_rate, for some corner
> > conditions,
>
> clk_round_rate(), not the parethesis to indicate it's a function.
Done.
>
> > get_clk_div_rate() could return an sub-optimal clock rate. Also, if an
> > exact clock rate was not found lowest clock was being returned.
> >
> > Search for suitable clock rate in 2 steps
> > a) exact match or within 2% tolerance
> > b) within 5% tolerance
> > This also takes care of corner conditions.
> >
> > Fixes: c2194bc999d4 ("tty: serial: qcom-geni-serial: Remove uart
> > frequency table. Instead, find suitable frequency with call to
> > clk_round_rate")
> > Signed-off-by: Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
> > ---
> > drivers/tty/serial/qcom_geni_serial.c | 134
> > ++++++++++++++++++++++++++--------
> > 1 file changed, 102 insertions(+), 32 deletions(-)
> >
> > diff --git a/drivers/tty/serial/qcom_geni_serial.c
> > b/drivers/tty/serial/qcom_geni_serial.c
> > index 2e23b65..8d247c1 100644
> > --- a/drivers/tty/serial/qcom_geni_serial.c
> > +++ b/drivers/tty/serial/qcom_geni_serial.c
> > @@ -943,52 +943,123 @@ static int qcom_geni_serial_startup(struct
> uart_port *uport)
> > return 0;
> > }
> >
> > -static unsigned long get_clk_div_rate(struct clk *clk, unsigned int baud,
> > - unsigned int sampling_rate, unsigned int *clk_div)
> > +static unsigned long find_clk_rate_in_tol(struct clk *clk, unsigned int
> desired_clk,
> > + unsigned int *clk_div, unsigned int
> > +percent_tol, bool *exact_match)
>
> Do we really need to pass in a bool pointer here for 'exact_match'?
> Can't we calculate the exact match value in the callsite and simply pass a bool
> (not pointer) to constrain the logic in this function?
>
Passing exact_match as pointer.
> > {
> > + unsigned long freq;
> > + unsigned long div, maxdiv, new_div;
> > + unsigned long long mult;
>
> I think u64 is used more often than unsigned long long.
Done.
>
> > unsigned long ser_clk;
> > - unsigned long desired_clk;
> > - unsigned long freq, prev;
> > - unsigned long div, maxdiv;
> > - int64_t mult;
> > -
> > - desired_clk = baud * sampling_rate;
> > - if (!desired_clk) {
> > - pr_err("%s: Invalid frequency\n", __func__);
> > - return 0;
> > - }
> > + unsigned long test_freq, offset, new_freq;
> >
> > + ser_clk = 0;
> > maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
> > - prev = 0;
> > + div = 1;
> >
> > - for (div = 1; div <= maxdiv; div++) {
> > - mult = div * desired_clk;
> > - if (mult > ULONG_MAX)
> > + while (div <= maxdiv) {
> > + mult = (unsigned long long)div * desired_clk;
>
> Cast to u64?
Done.
>
> > + if (mult != (unsigned long)mult)
>
> What is this checking for? Do we expect the rate to be larger than 32-bits on
> 32-bit machines?
>
Since we are multiplying rate with divider this is safety check?
> > break;
> >
> > - freq = clk_round_rate(clk, (unsigned long)mult);
> > - if (!(freq % desired_clk)) {
> > - ser_clk = freq;
> > - break;
> > + /*
> > + * Loop requesting a freq within tolerance and possibly exact 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 - offset) 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.
> > + * We make use of exact_match as an I/O param.
> > + */
> > +
> > + /* look only for exact match if within tolerance is already found */
> > + if (ser_clk)
> > + offset = 0;
> > + else
> > + offset = (mult * percent_tol) / 100;
>
> This needs to use div_u64() to be compatible with 32-bit machines.
>
Done. Thank you.
> > +
> > + test_freq = mult - offset;
> > + freq = clk_round_rate(clk, test_freq);
> > +
> > + /*
> > + * A dead-on freq is an insta-win, look for it only in 1st run
> > + */
> > + if (*exact_match) {
> > + if (!(freq % desired_clk)) {
> > + ser_clk = freq;
> > + *clk_div = freq / desired_clk;
> > + return ser_clk;
> > + }
> > + }
> > +
> > + if (!ser_clk) {
> > + new_div = DIV_ROUND_CLOSEST(freq, desired_clk);
> > + new_freq = new_div * desired_clk;
> > + offset = (new_freq * percent_tol) / 100;
> > +
> > + if (new_freq - offset <= freq && freq <= new_freq + offset) {
> > + /* 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;
> > + }
> > }
> >
> > - if (!prev)
> > - ser_clk = freq;
> > - else if (prev == freq)
> > + div = freq / desired_clk + 1;
> > +
> > + /*
> > + * 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)
> > break;
> > + }
> > +
> > + *exact_match = false;
> > + return ser_clk;
> > +}
> > +
> > +static unsigned long get_clk_div_rate(struct clk *clk, unsigned int baud,
> > + unsigned int sampling_rate, unsigned int
> > +*clk_div) {
> > + unsigned long ser_clk;
> > + unsigned long desired_clk;
> > + unsigned long desired_tol;
> > + bool exact_match;
> >
> > - prev = freq;
> > + desired_clk = baud * sampling_rate;
> > + if (!desired_clk) {
> > + pr_err("%s: Invalid frequency\n", __func__);
> > + return 0;
> > }
> >
> > - if (!ser_clk) {
> > - pr_err("%s: Can't find matching DFS entry for baud %d\n",
> > - __func__, baud);
> > + /* try to find exact clock rate or within 2% tolerance */
> > + ser_clk = 0;
> > + exact_match = true;
> > + desired_tol = 2;
> > +
> > + ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, desired_tol,
> &exact_match);
> > + if (ser_clk) {
> > + if (!exact_match)
> > + pr_warn("Cannot find exact match clk_rate,
> > + using one within 2 percent tolerance\n");
>
> Should this be a pr_warn_once()? Because otherwise users are going to see
> this error potentially quite often if tolerances can't be achieved.
>
Removed the message and implemented as per Doug's suggestion.
> > return ser_clk;
> > }
> >
> > - *clk_div = ser_clk / desired_clk;
> > - if (!(*clk_div))
> > - *clk_div = 1;
> > + /* try within 5% tolerance now, no need to look for exact match */
> > + exact_match = false;
> > + desired_tol = 5;
> > +
> > + ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, desired_tol,
> &exact_match);
> > + if (ser_clk)
> > + pr_warn("Cannot find exact match clk_rate, using one
> > + within 5 percent tolerance\n");
>
> This is a debug print?
>
Removed the message and implemented as per Doug's suggestion.
> > + else
> > + pr_err("Cannot find suitable clk_rate, giving up\n");
> >
> > return ser_clk;
> > }
Thank you.
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