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

Tomasz Figa
2016-08-16T09:33:37Z

2016-08-16 18:10 GMT+09:00 Krzysztof Kozlowski <k.kozlowski@samsung.com>:
> On 08/16/2016 11:00 AM, Tomasz Figa wrote:
>> 2016-08-16 17:25 GMT+09:00 Seung-Woo Kim <sw0312.kim@samsung.com>:
>>> Hi Krzysztof,
>>>
>>> On 2016년 08월 16일 16:37, Krzysztof Kozlowski wrote:
>>>> On 08/02/2016 12:16 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;
>>>>> + } else {
>>>>> + for (div = variant->div_base; div < 4; ++div)
>>>>> + if ((rate >> (variant->bits + div)) < freq)
>>>>> + break;
>>>>> + }
>>>>
>>>> I have trouble with understanding the idea behind initial code from
>>>> Tomasz (commit 11ad39ede24ee). The variant->bits for all SoC except
>>>> S3C24xx is 32. This means the shift:
>>>> if ((rate >> (variant->bits + div)) < freq)
>>>> will be always by 32 or more... In practice this will choose always a
>>>> "div" of 0 because in first iteration of this loop, the shift will be by 32.
>>>
>>> I also confused that part, but I figured out that the bit is used to
>>> consider modulation bit to generate pwm signal from the input clock.
>>>
>>> Only the old s3c2440 has 16 bit modulation timer for pwm, and all later
>>> soc has 32 bit modulation timer. So 32 bit timer cases, with the lowest
>>> div, it can generate all frequencies which can be assigned with 32bit
>>> variable.
>>> But I uses fls() to consider 64bit case also even though there is no
>>> really that kind of clock.
>>
>> The code may look complicated (in fact I had to think a bit to recall
>> what exactly it was supposed to do), but I'm not sure how it could be
>> simplified. It's generally intended to handle variant->bits < 32 cases
>> only and is effectively a no-op when variant->bits >= 32.
>
> Right, a comment for this behavior would be very useful. No need to
> waste time for re-thinking it later.
>
>> I would suggest just making rate an u64 and be done with the warning.
>> IMHO adding this kind of special cases only complicates the (already
>> complicated) code unnecessarily.
>
> u64 could solve the warning but then one would have to figure out
> whether the casts are safe or not. Unsigned long is assigned to rate and
> then returned.
>
> How about specific check (+comment) like:
> if (variant->bits < 32) {

Technically this still doesn't guarantee safe behavior, but in
practice (24xx has bits == 16) should be fine, assuming that it stops
UBSAN from grumbling.

> /* Only for s3c24xx */
> // the for loop as it was
> } else {
> /* For other variants just choose lowest divider always */

/*
* Other variants have enough counter bits to generate any requested rate,
* so no need to check higher divisors.
*/

> div = variant->div_base;
> }
>
> For me this is quite obvious and error-prone (explicit check for value
> to be used in shift).

I guess you meant error-proof. :) Sounds good to me.