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 [off-list ref] wrote:

On Wed, Mar 30, 2016 at 10:03:31PM +0200, Boris Brezillon wrote:

quoted

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 <redacted>
---
 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