linux-kernel - Re: [PATCH 2/3] iio: test: test gain-time-scale helpers

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Date: Mon, 15 Jan 2024 15:01:32 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Lars-Peter Clausen <lars@...afoo.de>, linux-kernel@...r.kernel.org,
 linux-iio@...r.kernel.org
Subject: Re: [PATCH 2/3] iio: test: test gain-time-scale helpers

On 1/13/24 18:12, Jonathan Cameron wrote:
> On Wed, 10 Jan 2024 12:12:55 +0200
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> Some light sensors can adjust both the HW-gain and integration time.
>> There are cases where adjusting the integration time has similar impact
>> to the scale of the reported values as gain setting has.
>>
>> IIO users do typically expect to handle scale by a single writable 'scale'
>> entry. Driver should then adjust the gain/time accordingly.
>>
>> It however is difficult for a driver to know whether it should change
>> gain or integration time to meet the requested scale. Usually it is
>> preferred to have longer integration time which usually improves
>> accuracy, but there may be use-cases where long measurement times can be
>> an issue. Thus it can be preferable to allow also changing the
>> integration time - but mitigate the scale impact by also changing the gain
>> underneath. Eg, if integration time change doubles the measured values,
>> the driver can reduce the HW-gain to half.
>>
>> The theory of the computations of gain-time-scale is simple. However,
>> some people (undersigned) got that implemented wrong for more than once.
>> Hence some gain-time-scale helpers were introduced.
>>
>> Add some simple tests to verify the most hairy functions.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>>

..

>> +static void test_iio_gts_chk_scales_all(struct kunit *test, struct iio_gts *gts,
>> +					const int *vals, int len)
>> +{
>> +	static const int gains[] = {1, 2, 4, 8, 16, 32, 64, 128, 256, 512,
>> +				    1024, 2048, 4096, 4096 * 2, 4096 * 4,
>> +				    4096 * 8};
>> +
>> +	int expected[ARRAY_SIZE(gains) * 2];
>> +	int i, ret;
>> +	int exp_len = ARRAY_SIZE(gains) * 2;
> 
> Use this for expected[*] just above?

Doing:
const int exp_len = ARRAY_SIZE(gains) * 2;
int expected[exp_len];

gives me:
warning: ISO C90 forbids variable length array ‘expected’ [-Wvla]

I could drop the whole exp_len variable, but I prefer test code which is 
as obvious as it gets if any of the checks fails. For me the check:

>> +	KUNIT_EXPECT_EQ(test, exp_len, len);
>> +	if (len != exp_len)
>> +		return;

is (very slightly) more obvious than:
	KUNIT_EXPECT_EQ(test, ARRAY_SIZE(gains) * 2, len);
	if (len != ARRAY_SIZE(gains) * 2)
		return;

I guess I'll leave this one as it is. Just kick me in v2 if I 
misunderstood you :)

Yours,
	-- Matti

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

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