Re: [PATCH 09/25] serial: sh-sci: Avoid overflow in sci_baud_calc_hscif()

Laurent Pinchart
2015-11-19T20:37:12Z

Hi Geert,

Thank you for the patch.

On Thursday 19 November 2015 19:38:48 Geert Uytterhoeven wrote:
> If bps >= 1048576, the multiplication of "(sr * (1 << (2 * c + 1))" and
> "bps" will overflow, and both br and err will contain bogus values.
> Skip the current and all higher clock select predividers when overflow
> is detected. Simplify the calculations using intermediates while we're
> at it.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> ---
> drivers/tty/serial/sh-sci.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index fba1e1eea15dc3a1..97a0f8ef5adc55a2 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -1870,7 +1870,7 @@ static unsigned int sci_scbrr_calc(struct sci_port *s,
> unsigned int bps, static void sci_baud_calc_hscif(unsigned int bps,
> unsigned long freq, int *brr, unsigned int *srr, unsigned int *cks)
> {
> - unsigned int sr, br, c;
> + unsigned int sr, br, a, b, c;
> int err, recv_margin;
> int min_err = 1000; /* 100% */
> int recv_max_margin = 0;
> @@ -1880,12 +1880,14 @@ static void sci_baud_calc_hscif(unsigned int bps,
> unsigned long freq, int *brr, for (sr = 8; sr <= 32; sr++) {
> for (c = 0; c <= 3; c++) {
> /* integerized formulas from HSCIF documentation */
> - br = DIV_ROUND_CLOSEST(freq, (sr *
> - (1 << (2 * c + 1)) * bps));
> + a = sr * (1 << (2 * c + 1));
> + if (bps > UINT_MAX / a)
> + break;
> +
> + b = a * bps;

This is becoming unreadable. Could you please use proper variable names ? A
comment that explains the calculation would also be useful.

> + br = DIV_ROUND_CLOSEST(freq, b);
> br = clamp(br, 1U, 256U);
> - err = DIV_ROUND_CLOSEST(freq, (br * bps * sr *
> - (1 << (2 * c + 1)) / 1000)) -
> - 1000;
> + err = DIV_ROUND_CLOSEST(freq, (br * b) / 1000) - 1000;
> /* Calc recv margin
> * M: Receive margin (%)
> * N: Ratio of bit rate to clock (N = sampling rate)

--
Regards,

Laurent Pinchart