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Date: Thu, 2 Jul 2020 09:11:04 +0200
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Guru Das Srinagesh <gurus@...eaurora.org>
Cc: linux-pwm@...r.kernel.org,
Thierry Reding <thierry.reding@...il.com>,
linux-arm-kernel@...ts.infradead.org,
Daniel Thompson <daniel.thompson@...aro.org>,
Fabio Estevam <festevam@...il.com>,
Arnd Bergmann <arnd@...db.de>,
David Collins <collinsd@...eaurora.org>,
Stephen Boyd <sboyd@...nel.org>,
Shawn Guo <shawnguo@...nel.org>,
Sascha Hauer <s.hauer@...gutronix.de>,
linux-kernel@...r.kernel.org,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Dan Carpenter <dan.carpenter@...cle.com>,
Pengutronix Kernel Team <kernel@...gutronix.de>,
Joe Perches <joe@...ches.com>,
Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
Lee Jones <lee.jones@...aro.org>,
Guenter Roeck <linux@...ck-us.net>,
NXP Linux Team <linux-imx@....com>
Subject: Re: [PATCH v16 06/11] pwm: imx27: Use 64-bit division macro and
function
Hello,
On Tue, Jun 02, 2020 at 03:31:10PM -0700, Guru Das Srinagesh wrote:
> Since the PWM framework is switching struct pwm_state.period's
> datatype to u64, prepare for this transition by using
> DIV_ROUND_UP_ULL to handle a 64-bit dividend, and div64_u64 to handle a
> 64-bit divisor.
>
> Also handle a possible overflow in the calculation of period_cycles when
> both clk_rate and period are large numbers. This logic was unit-tested
> out by calculating period_cycles using both the existing logic and the
> proposed one, and the results are as below.
>
> ----------------------------------------------------------------------------
> clk_rate period existing proposed
> ----------------------------------------------------------------------------
> 1000000000 18446744073709551615 18446744072 18446744073000000000
> (U64_MAX)
> ----------------------------------------------------------------------------
> 1000000000 4294967291 4294967291 4294967291
> ----------------------------------------------------------------------------
>
> Overflow occurs in the first case with the existing logic, whereas the
> proposed logic handles it better, sacrificing some precision in a best-effort
> attempt to handle overflow. As for the second case where there are
> more typical values of period, the proposed logic handles that correctly
> too.
>
> Cc: Shawn Guo <shawnguo@...nel.org>
> Cc: Sascha Hauer <s.hauer@...gutronix.de>
> Cc: Pengutronix Kernel Team <kernel@...gutronix.de>
> Cc: Fabio Estevam <festevam@...il.com>
> Cc: NXP Linux Team <linux-imx@....com>
> Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> ---
> drivers/pwm/pwm-imx27.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 43 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
> index 732a6f3..147729d 100644
> --- a/drivers/pwm/pwm-imx27.c
> +++ b/drivers/pwm/pwm-imx27.c
> @@ -202,7 +202,7 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
> sr = readl(imx->mmio_base + MX3_PWMSR);
> fifoav = FIELD_GET(MX3_PWMSR_FIFOAV, sr);
> if (fifoav == MX3_PWMSR_FIFOAV_4WORDS) {
> - period_ms = DIV_ROUND_UP(pwm_get_period(pwm),
> + period_ms = DIV_ROUND_UP_ULL(pwm_get_period(pwm),
> NSEC_PER_MSEC);
> msleep(period_ms);
>
> @@ -212,6 +212,45 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
> }
> }
>
> +static int pwm_imx27_calc_period_cycles(const struct pwm_state *state,
> + unsigned long clk_rate,
> + unsigned long *period_cycles)
> +{
> + u64 c = 0, c1, c2;
> +
> + c1 = clk_rate;
> + c2 = state->period;
> + if (c2 > c1) {
> + c2 = c1;
> + c1 = state->period;
> + }
> +
> + if (!c1 || !c2) {
> + pr_err("clk rate and period should be nonzero\n");
> + return -EINVAL;
> + }
> +
> + if (c2 <= div64_u64(U64_MAX, c1)) {
> + c = c1 * c2;
> + do_div(c, 1000000000);
> + } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000))) {
> + do_div(c1, 1000);
> + c = c1 * c2;
> + do_div(c, 1000000);
> + } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000000))) {
> + do_div(c1, 1000000);
> + c = c1 * c2;
> + do_div(c, 1000);
> + } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000000000))) {
> + do_div(c1, 1000000000);
> + c = c1 * c2;
} else {
???
> + }
> +
> + *period_cycles = c;
> +
> + return 0;
> +}
> +
> static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> const struct pwm_state *state)
> {
> @@ -226,10 +265,9 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> pwm_get_state(pwm, &cstate);
>
> clkrate = clk_get_rate(imx->clk_per);
> - c = clkrate * state->period;
> -
> - do_div(c, NSEC_PER_SEC);
> - period_cycles = c;
> + ret = pwm_imx27_calc_period_cycles(state, clkrate, &period_cycles);
> + if (ret)
> + return ret;
I would expect this problem to be generic enough that it justifies a
function in pwm-core (or even more generic?).
Something like:
/*
* calculates *result = a * b / c.
* Returns false on overflow (and doesn't assign result in this
* case); true otherwise.
*/
static inline bool multdiv(u64 *result, u64 a, u64 b, u64 c)
I'd split this out in a separate patch though? (Or is imx27 the only
driver in this series with this problem?)
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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