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Date: Mon, 8 Jan 2024 08:35:17 +0200
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Subject: Re: [PATCH] iio: gts-helper: Fix division loop
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To: Jonathan Cameron <jic23@kernel.org>
Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>,
 Lars-Peter Clausen <lars@metafoo.de>, linux-iio@vger.kernel.org,
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References: <ZZZ7pJBGkTdFFqiY@dc78bmyyyyyyyyyyyyydt-3.rev.dnainternet.fi>
 <20240107162253.66c1f0f1@jic23-huawei>
From: Matti Vaittinen <mazziesaccount@gmail.com>
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On 1/7/24 18:22, Jonathan Cameron wrote:
> On Thu, 4 Jan 2024 11:34:28 +0200
> Matti Vaittinen <mazziesaccount@gmail.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@gmail.com>
>> Fixes: 38416c28e168 ("iio: light: Add gain-time-scale helpers")
> 
> Hmm. Fix or not perf improvement?

Ha. I don't fancy calling my own doings as bugs - but when a code can 
keep kernel busy looping for several seconds... :|

>  I'm going to take the middle ground
> and leave the fixes tag, but not rush this in.

I think this is fine. It is nice to get this fix in stable if we have 
longer living stables with GTS helpers, because that's where many people 
do "real work" - and because it is likely some devices with new sensors 
are being built on products using these stable releases.

Still, the current users (ROHM light sensors) don't use that big 
dividends. Hence I don't think this needs any high speed fixup process :)

Thanks!


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

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