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

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Date: Mon, 22 Jan 2024 08:50:44 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Subhajit Ghosh <subhajit.ghosh@...aklogic.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 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/

> 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!

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~