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

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Message-ID: <5d950007-7a92-a41b-e569-79e806adb06a@quicinc.com>
Date:   Wed, 1 Jun 2022 16:15:59 +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>,
        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>, <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 6/1/2022 12:58 AM, Doug Anderson wrote:
> Hi,
>
> On Tue, May 31, 2022 at 11:18 AM Vijaya Krishna Nivarthi
> <quic_vnivarth@...cinc.com> wrote:
>> Add missing initialisation and correct type casting
>>
>> Signed-off-by: Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
>> ---
>>   drivers/tty/serial/qcom_geni_serial.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
>> index 4733a23..08f3ad4 100644
>> --- a/drivers/tty/serial/qcom_geni_serial.c
>> +++ b/drivers/tty/serial/qcom_geni_serial.c
>> @@ -943,11 +943,11 @@ static int qcom_geni_serial_startup(struct uart_port *uport)
>>   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 ser_clk = 0;
> In this patch it's not at all obvious why you'd need to init to 0. I
> think the "for loop" is guaranteed to run at least once because
> "max_div" is known at compile time. ...and currently each time through
> the "for" loop you'll always set "ser_clk".

Ok, I realised we will never break out of for loop exceeding ULONG_MAX 
in 1st pass, so yes ser_clk will always be set.

> I think in a future patch you'll want to _remove_ this from the for loop:
>
> if (!prev)
>    ser_clk = freq;

Intent is to save (and use) 1st freq if we cannot find an exact divider.

Isn't it ok?

For example please find debug output for a required frequency of 51.2MHz.

We try dividers 1, 2, 3 and end up with 52.1MHz the first result.

[   18.815432] 20220509 get_clk_div_rate desired_clk:51200000
[   18.821081] 20220509 get_clk_div_rate maxdiv:4095
[   18.825924] 20220509 get_clk_div_rate div:1
[   18.830239] 20220509 get_clk_div_rate freq:52174000
[   18.835288] 20220509 get_clk_div_rate div:2
[   18.839628] 20220509 get_clk_div_rate freq:100000000
[   18.844794] 20220509 get_clk_div_rate div:3
[   18.849119] 20220509 get_clk_div_rate freq:100000000
[   18.854254] 20220509 get_clk_div_rate reached max frequency breaking...
[   18.861072] 20220509 get_clk_div_rate clk_div=1, ser_clk=52174000

The behaviour was same earlier too when root_freq table was present.

The table did contain 51.2MHz and we would exit with same but on call to 
clk_set_rate(51.2MHz) we were ending up with 52.1MHz

>
> ...and _that's_ when you should init "ser_clk" to 0. Until then I'd
> leave it as uninitialized...
>
> Honestly, I'd throw all the fixes into one series, too.

My concern was if there would be a requirement to split the changes.

Will put in all in 1 series with Fixes tag.

>
>
>>          unsigned long desired_clk;
>>          unsigned long freq, prev;
>>          unsigned long div, maxdiv;
>> -       int64_t mult;
>> +       unsigned long long mult;
>>
>>          desired_clk = baud * sampling_rate;
>>          if (!desired_clk) {
>> @@ -959,8 +959,8 @@ static unsigned long get_clk_div_rate(struct clk *clk, unsigned int baud,
>>          prev = 0;
>>
>>          for (div = 1; div <= maxdiv; div++) {
>> -               mult = div * desired_clk;
>> -               if (mult > ULONG_MAX)
>> +               mult = (unsigned long long)div * (unsigned long long)desired_clk;
> I think you only need to cast one of the two. The other will be
> up-cast automatically.
Will change.
>
>
>> +               if (mult > (unsigned long long)ULONG_MAX)
> I don't think you need this cast. As far as I know the C language will
> "upcast" to the larger of the two types.
Will change.
>
>
> -Doug

Thank you.

-Vijay/