linux-kernel - Re: [PATCH v5 08/46] hwmon: pwm-fan: use pwm_get

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Date:	Thu, 31 Mar 2016 09:07:09 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>, linux-clk@...r.kernel.org,
	Mark Brown <broonie@...nel.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Kamil Debski <k.debski@...sung.com>, lm-sensors@...sensors.org,
	Jean Delvare <jdelvare@...e.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Jacek Anaszewski <j.anaszewski@...sung.com>,
	linux-leds@...r.kernel.org,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...glegroups.com,
	Joachim Eastwood <manabian@...il.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Heiko Stuebner <heiko@...ech.de>,
	linux-rockchip@...ts.infradead.org,
	Jingoo Han <jingoohan1@...il.com>,
	Lee Jones <lee.jones@...aro.org>, linux-fbdev@...r.kernel.org,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-samsung-soc@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...el.com>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	David Airlie <airlied@...ux.ie>,
	Daniel Vetter <daniel@...ll.ch>,
	dri-devel@...ts.freedesktop.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Hartley Sweeten <hsweeten@...ionengravers.com>,
	Ryan Mallon <rmallon@...il.com>,
	Alexander Shiyan <shc_work@...l.ru>, Milo Kim <milo.kim@...com>
Subject: Re: [PATCH v5 08/46] hwmon: pwm-fan: use pwm_get_args() where
 appropriate

Hi Guenter,

On Wed, 30 Mar 2016 15:52:44 -0700
Guenter Roeck <linux@...ck-us.net> wrote:

> On Wed, Mar 30, 2016 at 10:03:31PM +0200, Boris Brezillon wrote:
> > The PWM framework has clarified the concept of reference PWM config
> > (the platform dependent config retrieved from the DT or the PWM
> > lookup table) and real PWM state.
> > 
> > Use pwm_get_args() when the PWM user wants to retrieve this reference
> > config and not the current state.
> > 
> > This is part of the rework allowing the PWM framework to support
> > hardware readout and expose real PWM state even when the PWM has
> > just been requested (before the user calls pwm_config/enable/disable()).
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
> > ---
> >  drivers/hwmon/pwm-fan.c | 19 +++++++++++++------
> >  1 file changed, 13 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> > index 3e23003..82c5656 100644
> > --- a/drivers/hwmon/pwm-fan.c
> > +++ b/drivers/hwmon/pwm-fan.c
> > @@ -40,15 +40,18 @@ struct pwm_fan_ctx {
> >  
> >  static int  __set_pwm(struct pwm_fan_ctx *ctx, unsigned long pwm)
> >  {
> > +	struct pwm_args pargs = { };
> 
> Hi Boris,
> 
> I guess I am missing some context; sorry for that. Unfortunately,
> I did not easily find an explanation, so please bear with me.
> 
> Two questions: Why do we need a local copy of struct pwm_args instead
> of a pointer to it ? If it can change while being used, isn't it
> inconsistent anyway ?

It cannot change after pwm_get() is called. For the reason behind
prototype: I just followed the Thierry's proposal, but I'm perfectly
fine returning a const struct pwm_args pointer intead of passing
pwm_args as a parameter.

Thierry, what's your opinion?


> 
> Also, assuming the local copy is necessary, why initialize pargs ? 
> After all, pwm_get_args() just overwrites it.

It's a leftover from a previous version where pwm_get_args was
implemented this way:

static inline void pwm_get_args(pwm, args)
{
	if (pwm)
		*args = pwm->args
}

and this implementation was generating a lot of 'uninitialized
variable' warnings.

I just decided to drop the 'if (pwm)' test, because, IMO, this
should be checked way before calling pwm_get_args() is called.

Anyway, apparently I forgot to modify a few patches after this
modification.

Thanks for the review.

Boris


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com