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Message-ID: <CAD=FV=U20bzea7qzuMCgrAApFqBZ_sc9QsRdUZ_xX4ptRas-0w@mail.gmail.com>
Date: Mon, 11 Jul 2022 11:05:38 -0700
From: Doug Anderson <dianders@...omium.org>
To: Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
Cc: Andy Gross <agross@...nel.org>,
Bjorn Andersson <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: [V4] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which
otherwise could return a sub-optimal clock rate.
Hi,
On Mon, Jul 11, 2022 at 10:33 AM Vijaya Krishna Nivarthi
<quic_vnivarth@...cinc.com> wrote:
>
> In the logic around call to clk_round_rate(), for some corner conditions,
> 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>
> ---
> v4: replaced pr_dbg calls with dev_dbg
> v3: simplified algorithm further, fixed robot compile warnings
> v2: removed minor optimisations to make more readable
> v1: intial patch contained slightly complicated logic
> ---
> drivers/tty/serial/qcom_geni_serial.c | 89 +++++++++++++++++++++--------------
> 1 file changed, 54 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 2e23b65..f88b042 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -943,52 +943,71 @@ 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)
> {
> - unsigned long ser_clk;
> - unsigned long desired_clk;
> - unsigned long freq, prev;
> + unsigned long freq;
> unsigned long div, maxdiv;
> - int64_t mult;
> -
> - desired_clk = baud * sampling_rate;
> - if (!desired_clk) {
> - pr_err("%s: Invalid frequency\n", __func__);
> - return 0;
> - }
> + u64 mult;
> + unsigned long offset, abs_tol, achieved;
>
> + abs_tol = div_u64((u64)desired_clk * percent_tol, 100);
> maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
> - prev = 0;
> -
> - for (div = 1; div <= maxdiv; div++) {
> - mult = div * desired_clk;
> - if (mult > ULONG_MAX)
> + div = 1;
> + while (div <= maxdiv) {
> + mult = (u64)div * desired_clk;
> + if (mult != (unsigned long)mult)
> break;
>
> - freq = clk_round_rate(clk, (unsigned long)mult);
> - if (!(freq % desired_clk)) {
> - ser_clk = freq;
> - break;
> - }
> + offset = div * abs_tol;
> + freq = clk_round_rate(clk, mult - offset);
>
> - if (!prev)
> - ser_clk = freq;
> - else if (prev == freq)
> + /* Can only get lower if we're done */
> + if (freq < mult - offset)
> break;
>
> - prev = freq;
> + /*
> + * 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;
> + }
> +
> + div = DIV_ROUND_UP(freq, desired_clk);
> }
>
> - if (!ser_clk) {
> - pr_err("%s: Can't find matching DFS entry for baud %d\n",
> - __func__, baud);
> - return ser_clk;
> + return 0;
> +}
> +
> +static unsigned long get_clk_div_rate(struct clk *clk, struct device *dev,
> + unsigned int baud, unsigned int sampling_rate, unsigned int *clk_div)
> +{
> + unsigned long ser_clk;
> + unsigned long desired_clk;
> +
> + desired_clk = baud * sampling_rate;
> + if (!desired_clk) {
> + dev_dbg(dev, "Invalid frequency\n");
> + return 0;
> }
>
> - *clk_div = ser_clk / desired_clk;
> - if (!(*clk_div))
> - *clk_div = 1;
> + /*
> + * try to find a clock rate within 2% tolerance, then within
> + */
> + ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, 2);
> + if (!ser_clk)
> + ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, 5);
> +
> + if (!ser_clk)
> + dev_err(dev, "Couldn't find suitable clock rate for %d\n", desired_clk);
> + else
> + dev_dbg(dev, "desired_clk-%d, ser_clk-%d, clk_div-%d\n",
> + desired_clk, ser_clk, *clk_div);
Pretty sure the robot is going to yell at you again here. Here is my
analysis in detail:
desired_clk
* type: unsigned long
* proper format code: %lu
* marginally acceptable code: %ld
* format code you used in v3: %lu
* did robot yell at you about this in v3: no
* format code you used in v4: %d
* will robot yell at you in v4: YES
ser_clk:
* type: unsigned long
* proper format code: %lu
* marginally acceptable code: %ld
* format code you used in v3: %lu
* did robot yell at you about this in v3: no
* format code you used in v4: %d
* will robot yell at you in v4: YES
*clk_div:
* type: unsigned int
* proper format code: %u
* marginally acceptable code: %d
* format code you used in v3: %lu
* did robot yell at you about this in v3: YES
* format code you used in v4: %d
* will robot yell at you in v4: no
* should you change it to %u: yes
-Doug
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