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

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Date: Fri, 19 Jan 2024 22:26:58 +1030
From: Subhajit Ghosh <subhajit.ghosh@...aklogic.com>
To: Jonathan Cameron <jic23@...nel.org>,
 Matti Vaittinen <mazziesaccount@...il.com>
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 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.

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!

Regards,
Subhajit Ghosh