linux-kernel - Re: [PATCH] iio: gts-helper: Fix division loop

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Date: Mon, 22 Jan 2024 20:28:22 +1030
From: Subhajit Ghosh <subhajit.ghosh@...aklogic.com>
To: Matti Vaittinen <mazziesaccount@...il.com>,
 Jonathan Cameron <jic23@...nel.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Lars-Peter Clausen <lars@...afoo.de>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: gts-helper: Fix division loop

On 22/1/24 17:20, Matti Vaittinen wrote:
> On 1/19/24 13:56, Subhajit Ghosh wrote:
>> On 8/1/24 02:52, Jonathan Cameron wrote:
>>> On Thu, 4 Jan 2024 11:34:28 +0200
>>> Matti Vaittinen <mazziesaccount@...il.com> wrote:
>>>
>>>> The loop based 64bit division may run for a long time when dividend is a
>>>> lot bigger than the divider. Replace the division loop by the
>>>> div64_u64() which implementation may be significantly faster.
>>>>
>>>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>>>> Fixes: 38416c28e168 ("iio: light: Add gain-time-scale helpers")
>>>
>>> Hmm. Fix or not perf improvement?  I'm going to take the middle ground
>>> and leave the fixes tag, but not rush this in.
>>>
>>> So applied to the togreg branch of iio.git and for now just pushed out
>>> as testing for 0-day etc to take a look before I rebase that tree after
>>> rc1.
>>>
>>>
>>>
>>>> ---
>>>>
>>>> I've implemented also a fixup series for supporting rounding of
>>>> gains/scales:
>>>> https://lore.kernel.org/lkml/37d3aa193e69577353d314e94463a08d488ddd8d.1701780964.git.mazziesaccount@gmail.com/
>>>>
>>>> That series does also remove the offending loop.
>>>>
>>>> We don't currently have any in-tree users of GTS helpers which would
>>>> need the rounding support so pushing the rounding is not urgent (and I
>>>> haven't heard of Subjahit whose driver required the rounding). Hence, we
>>>> may want to only take this loop fix in for now (?) and reconsider
>>>> rounding when someone need that.
>>>>
>>>> Jonathan, what's your take on this?
>>> Agreed - let us wait for the rounding to have a user, but makes sense
>>> to tidy this corner up in the meantime.
>>>
>>> Thanks,
>>>
>>> Jonathan
>>>
>>>>
>>>>   drivers/iio/industrialio-gts-helper.c | 5 ++---
>>>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/iio/industrialio-gts-helper.c b/drivers/iio/industrialio-gts-helper.c
>>>> index 7653261d2dc2..abcab2d38589 100644
>>>> --- a/drivers/iio/industrialio-gts-helper.c
>>>> +++ b/drivers/iio/industrialio-gts-helper.c
>>>> @@ -34,7 +34,7 @@
>>>>   static int iio_gts_get_gain(const u64 max, const u64 scale)
>>>>   {
>>>>       u64 full = max;
>>>> -    int tmp = 1;
>>>> +    int tmp = 0;
>>>>       if (scale > full || !scale)
>>>>           return -EINVAL;
>>>> @@ -48,8 +48,7 @@ static int iio_gts_get_gain(const u64 max, const u64 scale)
>>>>           tmp++;
>>>>       }
>>>> -    while (full > scale * (u64)tmp)
>>>> -        tmp++;
>>>> +    tmp += div64_u64(full, scale);
>>>>       return tmp;
>>>>   }
>>>>
>>>> base-commit: 2cc14f52aeb78ce3f29677c2de1f06c0e91471ab
>>>
>>>
>> Hi Matti,
>>
>> Your fix works beautifully with the latest version of apds9306 driver which I am working on.
>> All available scale values can be set without any errors. Thank you.
> 
> Thanks for testing Subhajit! Just to ensure we have no miscommunication - did you test just this division fix, or the rounding fix here:
> https://lore.kernel.org/lkml/37d3aa193e69577353d314e94463a08d488ddd8d.1701780964.git.mazziesaccount@gmail.com/
You are most welcome. I did not check the above rounding fix pointed out by the link. I will be happy to check it and let you know by the end of this month.
I checked this division fix.

> 
>> Moving to a new city with a new full time job with the assumption of getting more time
>> for my list of opensource projects and contributions proved to be utterly wrong!
> 
> Well, I can't blame you :) Being in a new work at new city sounds like you have a lot on your plate right now. Give it half a year and things will stabilize though :) Oh, and falsely assuming that "when XXX, I will have the time to do YYY" - been there done that :)
> 
> Good luck on the new work and city!
Thank you Matti.
> 
> Yours,
>      -- Matti
> 
Regards,
Subhajit Ghosh