Re: [PATCH] tty: serial: qcom-geni-serial: minor fixes to get_clk_div_rate()

[Vijaya Krishna Nivarthi]

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@xxxxxxxxxxx> wrote:
> > Add missing initialisation and correct type casting
> >
> > Signed-off-by: Vijaya Krishna Nivarthi <quic_vnivarth@xxxxxxxxxxx>
> > ---
> >   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/