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

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Date:   Tue, 31 May 2022 12:28:16 -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>,
        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 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".

I think in a future patch you'll want to _remove_ this from the for loop:

if (!prev)
  ser_clk = freq;

...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.


>         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.


> +               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.


-Doug