Subject: Re: [PATCH 1/5] max44000: Initial commit
To: Jonathan Cameron <jic23@kernel.org>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>
References: <cover.1460045763.git.leonard.crestez@intel.com>
 <5dbbd82290c575f595ae0907aaf8e03117a6d017.1460045763.git.leonard.crestez@intel.com>
 <alpine.DEB.2.02.1604072142510.26969@pmeerw.net>
 <570A513A.4020106@kernel.org>
Cc: linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        Hartmut Knaack <knaack.h@gmx.de>, Lars-Peter Clausen <lars@metafoo.de>,
        Daniel Baluta <daniel.baluta@intel.com>
From: Crestez Dan Leonard <leonard.crestez@intel.com>
Message-ID: <570BBDFC.6010601@intel.com>
Date: Mon, 11 Apr 2016 18:08:44 +0300
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On 04/10/2016 04:12 PM, Jonathan Cameron wrote:
> On 07/04/16 20:48, Peter Meerwald-Stadler wrote:
>>
>>> This just adds support for reporting illuminance with default settings.
>>>
>>> All default registers are written on probe because the device otherwise
>>> lacks a reset function.
>>
>> comments below
> Mostly fine, but a few corners need cleaning up.
>
> Also, I'm not keep on the brute force write everything.  The driver should
> cope with any values in those registers and deal with refreshing the
> cache etc so that it can do so.  Writing a bunch of defaults is rather a
> brittle approach.

But if the hardware is not in the expected state the driver won't report 
correct values, at least not without reading scaling factors.

It's not clear what you mean by brittle?

>>> +struct max44000_data {
>>> +	struct mutex lock;
>>> +	struct i2c_client *client;
> This client pointer isn't used outside probe and remove where it is easily
> available anyway.  Hence don't keep a copy in here.

Ok, will remove

>>> +static bool max44000_readable_reg(struct device *dev, unsigned int reg)
>>> +{
>>> +	return (1 << reg) & MAX44000_REGMASK_READABLE;
> See above.  This is a really nasty and hard to review way of doing this.
> switch (reg) {
>         REG1:
>         REG2:
>         REG3:
> 	  return true;
>         default:
>            return false;
>
> may be more code, but it's easy to tell if it is right.

Won't a switch result in larger executable code? Would it be acceptable 
to just expand the REGMASK_* into a large or-ing of (1 << 
MAX44000_REG_*)? Then it would be clear in the source what's going on 
but binary will be the same.

>>> +static const struct reg_default max44000_reg_defaults[] = {
>>> +	{ MAX44000_REG_CFG_MAIN,	0x24 },
>>> +	/* Upper 4 bits are not documented but start as 1 on powerup
> Multiline comment syntax please.

Ok, I will fix this in all the patches.

>>> +	.use_single_rw	= 1,
>>> +	.cache_type	= REGCACHE_FLAT,
> This always seems like a good idea, but tends to cause issues.
> FLAT is really only meant for very high performance devices, you
> are probably better with something else here.  If you are doing this
> deliberately to make the below writes actually occur, then please
> add a comment here.

I used REGCACHE_FLAT because my device has a very small number of 
registers and I assume it uses less memory. Honestly it would make sense 
for regmap to include a REGCACHE_AUTO cache_type and pick the cache 
implementation automatically based on number of registers.

>>> +static int max44000_force_write_defaults(struct max44000_data *data)
>>> +{
>>> +	int i, ret;
>>> +
>>> +	for (i = 0; i < ARRAY_SIZE(max44000_reg_defaults); ++i) {
>>> +		ret = regmap_write(data->regmap,
>>> +				   max44000_reg_defaults[i].reg,
>>> +				   max44000_reg_defaults[i].def);
> Silly question, but if the cached value matches the values you are trying
> to write here will this work?

Yes. It would not work otherwise since the regmap cache is explicitly 
initialized with my listed defaults.

As far as I can tell regmap_write will always write to the hardware.

> There is a regcache_mark_dirty call that will ensure all registers in the
> cache are read..

The regcache_mark_dirty function is used to notify the cache that the 
device has been reset to the known default values. I attempted to do 
something like:

	regcache_mark_dirty()
	regcache_sync()

This doesn't work because this explicitly avoid overwriting known defaults.

> If you then need any particular values they should be explicitly written
> on the assumption you have no idea what the state is.  Brute force writing
> all the defaults is nasty and doesn't give any information as to what
> is happening.

If the device had a reset command I should have used that, right? What 
is happening is that I am implementing a reset command in software.

I can skip writing values like REG_TRIM_* which are used for calibration 
and are not exposed by the driver. This would allow these values to be 
configured externally using something like i2cset at boot time. Is that 
what you mean?

I could also skip initializing scaling parameters but then stuff like 
in_illuminance_scale would persist after rmmod/insmod. This seems 
undesirable.

--
Regards,
Leonard