linux-kernel - Re: [RFC PATCH RESEND 2/3] leds: upboard: Add LED support

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Date:   Thu, 26 Apr 2018 13:49:22 +0100
From:   Javier Arteaga <javier@...tex.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Dan O'Donovan <dan@...tex.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Lee Jones <lee.jones@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-leds@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RESEND 2/3] leds: upboard: Add LED support

Thanks Andy for these pointers :)

On Thu, Apr 26, 2018 at 10:55:49AM +0300, Andy Shevchenko wrote:
> On Thu, 2018-04-26 at 03:34 +0100, Javier Arteaga wrote:
> > My understanding was that in this context, __init allows this probe()
> > to
> > be dropped from memory after module load.
> > 
> > What am I doing wrong there?
> 
> Just give another thought about it. The keyword(s) here is(are): time to
> live of the objects in question. It's good to get knowing what unbind-
> bind means (for built-in drivers).

So this is the bit that I _believed_ applied to the platform drivers for
these MFD-registered devices (from driver-model/platform.txt):

  Or, in common situations where the device is known not to be hot-pluggable,
  the probe() routine can live in an init section to reduce the driver's
  runtime memory footprint:

  	int platform_driver_probe(struct platform_driver *drv,
  			  int (*probe)(struct platform_device *))

I'm thinking my misunderstanding probably stems from assuming that these
leds/pinctrl drivers will always find all devices registered at init
time. Can't say I've validated that assumption - I just didn't see
anything obviously blowing up in my tests so far :)

I'll keep reading and test out a few more things so I fully understand.
Until then, I've taken out __init annotations from next version.

> > > Don't use direct dereference to platform_data.
> > 
> > Sorry, I don't understand this one. What's the alternative?
> 
> See other drivers how they do that stuff. Hint: check inline functions
> in include/linux/device.h.

I wasn't looking at the right other drivers :)
I'll use the dev_get_platdata() wrapper going forwards.