Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value

Uwe Kleine-König
2020-06-22T07:36:10Z

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On Sat, Jun 20, 2020 at 02:17:47PM +0200, Hans de Goede wrote:
> When the user requests a high enough period ns value, then the
> calculations in pwm_lpss_prepare() might result in a base_unit value of 0.
>
> But according to the data-sheet the way the PWM controller works is that
> each input clock-cycle the base_unit gets added to a N bit counter and
> that counter overflowing determines the PWM output frequency. Adding 0
> to the counter is a no-op. The data-sheet even explicitly states that
> writing 0 to the base_unit bits will result in the PWM outputting a
> continuous 0 signal.
>
> When the user requestes a low enough period ns value, then the
> calculations in pwm_lpss_prepare() might result in a base_unit value
> which is bigger then base_unit_range - 1. Currently the codes for this
> deals with this by applying a mask:
>
> base_unit &= (base_unit_range - 1);
>
> But this means that we let the value overflow the range, we throw away the
> higher bits and store whatever value is left in the lower bits into the
> register leading to a random output frequency, rather then clamping the
> output frequency to the highest frequency which the hardware can do.
>
> This commit fixes both issues by clamping the base_unit value to be
> between 1 and (base_unit_range - 1).
>
> Fixes: 684309e5043e ("pwm: lpss: Avoid potential overflow of base_unit")
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v3:
> - Change upper limit of clamp to (base_unit_range - 1)
> - Add Fixes tag
> ---
> drivers/pwm/pwm-lpss.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
> index 43b1fc634af1..80d0f9c64f9d 100644
> --- a/drivers/pwm/pwm-lpss.c
> +++ b/drivers/pwm/pwm-lpss.c
> @@ -97,6 +97,9 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
> freq *= base_unit_range;
>
> base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);

DIV_ROUND_CLOSEST_ULL is most probably wrong, too. But I didn't spend
the time to actually confirm that.

> + /* base_unit must not be 0 and we also want to avoid overflowing it */
> + base_unit = clamp_t(unsigned long long, base_unit, 1,
> + base_unit_range - 1);

.get_state seems to handle base_unit == 0 just fine?! Though this
doesn't look right either ...

Best regards
Uwe

--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |

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