Re: [PATCH v4 6/6] hwmon: pwm-fan: Remove internal duplicated pwm_state

[Alexander Stein <alexander.stein@ew.tq-group.com>]

Hello Guenter, Uwe,

Am Montag, 23. Mai 2022, 16:18:57 CEST schrieb Guenter Roeck:
> On 5/23/22 06:55, Alexander Stein wrote:
> > Hi Uwe,
> > 
> > Am Montag, 23. Mai 2022, 14:46:14 CEST schrieb Uwe Kleine-König:
> >> * PGP Signed by an unknown key
> >> 
> >> Hello,
> >> 
> >> On Mon, May 23, 2022 at 01:05:13PM +0200, Alexander Stein wrote:
> >>> Each pwm device has already a pwm_state. Use this one instead of
> >>> managing an own copy of it.
> >>> 
> >>> Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
> >>> ---
> >>> 
> >>>   drivers/hwmon/pwm-fan.c | 49 +++++++++++++++++++++++++----------------
> >>>   1 file changed, 30 insertions(+), 19 deletions(-)
> >>> 
> >>> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> >>> index e5d4b3b1cc49..e0ce81cdf5e0 100644
> >>> --- a/drivers/hwmon/pwm-fan.c
> >>> +++ b/drivers/hwmon/pwm-fan.c
> >>> @@ -40,7 +40,6 @@ struct pwm_fan_ctx {
> >>> 
> >>>   	struct mutex lock;
> >>>   	struct pwm_device *pwm;
> >>> 
> >>> -	struct pwm_state pwm_state;
> >>> 
> >>>   	struct regulator *reg_en;
> >>>   	enum pwm_fan_enable_mode enable_mode;
> >>>   	bool regulator_enabled;
> >>> 
> >>> @@ -142,7 +141,7 @@ static int pwm_fan_switch_power(struct pwm_fan_ctx
> >>> *ctx, bool on)>
> >>> 
> >>>   static int pwm_fan_power_on(struct pwm_fan_ctx *ctx)
> >>>   {
> >>> 
> >>> -	struct pwm_state *state = &ctx->pwm_state;
> >>> +	struct pwm_state state;
> >>> 
> >>>   	int ret;
> >>>   	
> >>>   	if (ctx->enabled)
> >>> 
> >>> @@ -154,8 +153,9 @@ static int pwm_fan_power_on(struct pwm_fan_ctx *ctx)
> >>> 
> >>>   		return ret;
> >>>   	
> >>>   	}
> >>> 
> >>> -	state->enabled = true;
> >>> -	ret = pwm_apply_state(ctx->pwm, state);
> >>> +	pwm_get_state(ctx->pwm, &state);
> >>> +	state.enabled = true;
> >>> +	ret = pwm_apply_state(ctx->pwm, &state);
> >>> 
> >>>   	if (ret) {
> >>>   	
> >>>   		dev_err(ctx->dev, "failed to enable PWM\n");
> >>>   		goto disable_regulator;
> >> 
> >> IMHO this isn't a net win. You trade the overhead of pwm_get_state
> >> against some memory savings. I personally am not a big fan of the
> >> get_state + modify + apply codeflow. The PWM framework does internal
> >> caching of the last applied state, but the details are a bit ugly. (i.e.
> >> pwm_get_state returns the last applied state, unless there was no state
> >> applied before. In that case it returns what .get_state returned during
> >> request time, unless there is no .get_state callback ... not sure if the
> >> device tree stuff somehow goes into that, didn't find it on a quick
> >> glance)
> >> 
> >> Also there is a (small) danger, that pwm_state contains something that
> >> isn't intended by the driver, e.g. a wrong polarity. So I like the
> >> consumer to fully specify what they intend and not use pwm_get_state().
> > 
> > Ah, I see. I have no hard feelings for this patch. I just wondered why the
> > PWM state is duplicated. and wanted to get rid of it. If there is a
> > specific reason for this, I'm ok with that.
> 
> I don't see the value of continuous runtime overhead to save a few bytes of
> data, so I don't see a reason to _not_ cache the state locally. This is
> similar to caching a clock frequency locally instead of calling the clock
> subsystem again and again to read it. Sure, nowadays CPUs are more powerful
> than they used to be, but I don't see that as reason or argument for
> wasting their power.

Ok, seems reasonable. I'm fully fine with patch 6 being dropped. What about 
the other patches?

Best regards,
Alexander