linux-kernel - Re: [RFC PATCH 2/2] iio: light: Add support for APDS9306 Light Sensor

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Message-ID: <8dd207d3-a9f0-2c06-795e-efed30656e35@tweaklogic.com>
Date:   Thu, 13 Apr 2023 11:33:57 +0800
From:   Subhajit Ghosh <subhajit.ghosh@...aklogic.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Shreeya Patel <shreeya.patel@...labora.com>,
        Paul Gazzillo <paul@...zz.com>,
        Zhigang Shi <Zhigang.Shi@...eon.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>
Subject: Re: [RFC PATCH 2/2] iio: light: Add support for APDS9306 Light Sensor

On 12/4/23 21:37, Andy Shevchenko wrote:
> On Wed, Apr 12, 2023 at 12:29:15PM +0800, Subhajit Ghosh wrote:
> 
> ...
> 
>>>> +static const struct regmap_config apds9306_regmap = {
>>>> +	.name = "apds9306_regmap",
>>>> +	.reg_bits = 8,
>>>> +	.val_bits = 8,
>>>> +	.rd_table = &apds9306_readable_table,
>>>> +	.wr_table = &apds9306_writable_table,
>>>> +	.volatile_table = &apds9306_volatile_table,
>>>> +	.precious_table = &apds9306_precious_table,
>>>> +	.max_register = APDS9306_ALS_THRES_VAR,
>>>> +	.cache_type = REGCACHE_RBTREE,
>>>
>>> Do you need an internal regmap lock? If so, why?
>> For event interface - interrupt enable, adaptive interrupt enable,
>> upper and lower threshold values, selection of clear or als
>> channels for interrupt, the mutex in the driver's private data structure
>> is not used.
>> I thought to use the regmap's internal locking mechanism for
>> mutual exclusion as the values are directly written to or read from
>> the device registers form the write_event(), read_event(),
>> write_event_config() and read_event_config().
>> What do you think?
> 
> I didn't get. If you have a sequence of registers to be read/write/modified/etc
> in IRQ handler and/or elsewhere and at the same time in IRQ or elsewhere you
> have even a single IO access to the hardware you have to be sure that the IO
> ordering has no side effects. regmap API does not guarantee that. It only works
> on a simple read/write/modify of a _single_ register, or a coupled group of
> registers (like bulk ops), if your case is sparse, you on your own and probably
> lucky enough not to have an issue during the testing. So, take your time and
> think more about what you are doing in the driver and what locking schema
> should take place.
> 
> ...
Agree. I have to rethink and re-implement the locking mechanism.

> 
>>>> +static int apds9306_power_state(struct apds9306_data *data,
>>>> +		enum apds9306_power_states state)
>>>> +{
>>>> +	int ret;
>>>> +
>>>> +	/* Reset not included as it causes ugly I2C bus error */
>>>> +	switch (state) {
>>>> +	case standby:
>>>> +		return regmap_field_write(data->regfield_en, 0);
>>>> +	case active:
>>>> +		ret = regmap_field_write(data->regfield_en, 1);
>>>> +		if (ret)
>>>> +			return ret;
>>>> +		/* 5ms wake up time */
>>>> +		usleep_range(5000, 10000);
>>>> +		break;
>>>> +	default:
>>>> +		return -EINVAL;
>>>> +	}
>>>
>>>> +	return 0;
>>>
>>> Move that to a single user of this line inside the switch-case.
>> Sorry, I did not get you. Can you please elaborate?
> 
> The user of this return is only one case in the switch. Instead of breaking
> the switch-case, just move this return statement to there.
> 
Ok. It will be done.

> ...
> 
>>>> +	struct device *dev = &data->client->dev;
>>>
>>> Why data contains I²C client pointer, what for?
>> I copied the implementation. It will be re-implemented.
> 
> I mean, how client pointer is used in comparison to the plain pointer to the
> generic device object.
> 
> ...
> 
>>>> +		while (retries--) {
>>>> +			ret = regmap_read(data->regmap, APDS9306_MAIN_STATUS,
>>>> +					&status);
>>>> +			if (ret) {
>>>> +				dev_err(dev, "read status failed: %d\n", ret);
>>>> +				return ret;
>>>> +			}
>>>> +			if (status & APDS9306_ALS_DATA_STAT_MASK)
>>>> +				break;
>>>> +			/*
>>>> +			 * In case of continuous one-shot read from userspace,
>>>> +			 * new data is available after sampling period.
>>>> +			 * Delays are in the range of 25ms to 2secs.
>>>> +			 */
>>>> +			fsleep(delay);
>>>> +		}
>>>
>>> regmap_read_poll_timeout().
>> According to the regmap_read_poll_timeout() documentation, the maximum time
>> to sleep between reads should be less than ~20ms as it uses usleep_range().
>>
>> If userspace is doing continuous reads, then data is available after sampling
>> period (25ms to 2sec) or integration time (3.125ms to 400ms) whichever is
>> greater.
>>
>> The runtime_suspend() function is called after 5 seconds, so the device is
>> still active and running.
>>
>> If the ALS data bit is not set in status reg, it is efficient to sleep for
>> one sampling period rather than continuously checking the status reg
>> within ~20ms if we use regmap_read_poll_timeout().
>>
>> Do you have any suggestions?
> 
> Yes, Use proposed API. It takes _two_ timeout parameters, one of which is the
> same as your delay. You may actually resplit it by multiplying retries and
> decreasing delay to satisfy the regmap_read_poll_timeout() recommendation.
> 
Yes, that can be done. I will re-write this function in the next patch.

Thanks once again Andy for the detailed review.

Regards,
Subhajit Ghosh