linux-kernel - Re: [PATCH v4 4/5] iio: light: ROHM BU27008 color sensor

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Message-ID: <ZFjpqwOo3DxnWahM@smile.fi.intel.com>
Date:   Mon, 8 May 2023 15:23:07 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
Cc:     Matti Vaittinen <mazziesaccount@...il.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shreeya Patel <shreeya.patel@...labora.com>,
        Zhigang Shi <Zhigang.Shi@...eon.com>,
        Paul Gazzillo <paul@...zz.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/5] iio: light: ROHM BU27008 color sensor

On Fri, May 05, 2023 at 04:56:47AM +0000, Vaittinen, Matti wrote:
> On 5/4/23 17:33, Andy Shevchenko wrote:
> > On Wed, May 03, 2023 at 12:50:14PM +0300, Matti Vaittinen wrote:

...

> >> +config ROHM_BU27008
> >> +	tristate "ROHM BU27008 color (RGB+C/IR) sensor"
> >> +	depends on I2C
> >> +	select REGMAP_I2C
> >> +	select IIO_GTS_HELPER
> >> +	help
> >> +	  Enable support for the ROHM BU27008 color sensor.
> >> +	  The ROHM BU27008 is a sensor with 5 photodiodes (red, green,
> >> +	  blue, clear and IR) with four configurable channels. Red and
> >> +	  green being always available and two out of the rest three
> >> +	  (blue, clear, IR) can be selected to be simultaneously measured.
> >> +	  Typical application is adjusting LCD backlight of TVs,
> >> +	  mobile phones and tablet PCs.
> > 
> > Module name?
> 
> We have discussed this several times already.
> 
> https://lore.kernel.org/all/10c4663b-dd65-a545-786d-10aed6e6e5e9@fi.rohmeurope.com/
> 
> Module name is completely irrelevant when selecting a kernel configuration.

This option is also selectable by user.

...

> > Do you need regmap lock? If so, why (since you have mutex)?
> 
> I believe you know that regmap uses a default lock when no external lock 
> is given. So, I assume you mean that maybe we could set 
> 'disable_locking' for the regmap here.

Correct.

> It's nice to be occasionally pushed to think "out of the box". And yes, 
> disabling regmap lock is really out of my "normal box" :)
> 
> I didn't go through all of the code yet, but I think pretty much all of 
> the sequences which end up to register writes are indeed protected by 
> the mutex. (Well, probe is not but it is expected to only update one bit 
> while rest of the register should stay fixed).
> 
> It may be we could live without regmap_lock when driver is in it's 
> current state, but I am not convinced the performance improvement is 
> worth the risk. Having regmap unprotected is not common, and it is also 
> not easy to spot when making changes to the driver. In my opinion it is 
> a bit like asking for a nose-bleed unless there is really heavy reasons 
> to drop the lock... In this case, having the regmap_lock (which is 
> pretty much never locked because we have the mutex as you said) is 
> probably not a penalty that matters.

Basically you try to justify a hidden mine field in case somebody will think
"oh, we are protected by regmap lock, so why to bother call mutex_lock()" and
at the end it become a subtle bugs in the code. With disable_locking = true
I can see that code author _carefully thought through_ the locking schema and
understands the hardware and the code.

P.S. I'm wondering why your lines of text have a single trailing whitespace
but the last line.

-- 
With Best Regards,
Andy Shevchenko