Re: [PATCH] pwm: samsung: fix to use lowest div for large enough modulation bits

Joonyoung Shim
2016-08-03T01:58:34Z

Hi,

On 08/02/2016 07:16 PM, Seung-Woo Kim wrote:
>>From pwm_samsung_calc_tin(), there is routine to find the lowest
> divider possible to generate lower frequency than requested one.
> But it is always possible to generate requested frequency with
> large enough modulation bits, so this patch fixes to use lowest
> div for the case. This patch removes following UBSAN warning:
>
> UBSAN: Undefined behaviour in drivers/pwm/pwm-samsung.c:197:13
> shift exponent 32 is too large for 32-bit type 'long unsigned int'
> [...]
> [<c0670248>] (ubsan_epilogue) from [<c06707b4>] (__ubsan_handle_shift_out_of_bounds+0xd8/0x120)
> [<c06707b4>] (__ubsan_handle_shift_out_of_bounds) from [<c0694b28>] (pwm_samsung_config+0x508/0x6a4)
> [<c0694b28>] (pwm_samsung_config) from [<c069286c>] (pwm_apply_state+0x174/0x40c)
> [<c069286c>] (pwm_apply_state) from [<c0b2e070>] (pwm_fan_probe+0xc8/0x488)
> [<c0b2e070>] (pwm_fan_probe) from [<c07ba8b0>] (platform_drv_probe+0x70/0x150)
> [...]
>
> Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
> ---
> The UBSAN warning from ARM is reported with the patch in following link:
> https://patchwork.kernel.org/patch/9189575/
> ---
> drivers/pwm/pwm-samsung.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
> index ada2d32..ff0def6 100644
> --- a/drivers/pwm/pwm-samsung.c
> +++ b/drivers/pwm/pwm-samsung.c
> @@ -193,9 +193,13 @@ static unsigned long pwm_samsung_calc_tin(struct samsung_pwm_chip *chip,
> * divider settings and choose the lowest divisor that can generate
> * frequencies lower than requested.
> */
> - for (div = variant->div_base; div < 4; ++div)
> - if ((rate >> (variant->bits + div)) < freq)
> - break;
> + if (fls(rate) <= variant->bits) {
> + div = variant->div_base;

It's reasonable to me not to check to decide div at above case.

> + } else {
> + for (div = variant->div_base; div < 4; ++div)
> + if ((rate >> (variant->bits + div)) < freq)
> + break;
> + }
>

Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>

Thanks.