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Date: Tue, 12 Jul 2022 18:21:59 +0530
From: Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
To: Doug Anderson <dianders@...omium.org>
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.
On 7/11/2022 11:35 PM, Doug Anderson wrote:
> 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
Thank you very much.
For some reason robot seems to be more forgiving towards dev_funcs than
pr_funcs, at least it did not yell at v4.
Also I found usage of %d for uls at many locations.
Made the changes and uploaded V5.
-Vijay/
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