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Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept

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  Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Thierry Reding <thierry.reding@gmail.com> - 2016-04-12 13:50 +0200
    Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-04-12 14:20 +0200
      Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Thierry Reding <thierry.reding@gmail.com> - 2016-04-12 14:30 +0200
        Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-04-12 14:50 +0200
          Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Thierry Reding <thierry.reding@gmail.com> - 2016-04-12 15:20 +0200
            Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-04-12 15:30 +0200
              Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Thierry Reding <thierry.reding@gmail.com> - 2016-04-12 16:10 +0200
                Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept Boris Brezillon <boris.brezillon@free-electrons.com> - 2016-04-12 16:20 +0200

#1376726 — Re: [PATCH v5 15/46] pwm: introduce the pwm_state concept

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On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> The PWM state, represented by its period, duty_cycle and polarity,
> is currently directly stored in the PWM device.
> Declare a pwm_state structure embedding those field so that we can later
> use this struct to atomically update all the PWM parameters at once.
> 
> All pwm_get_xxx() helpers are now implemented as wrappers around
> pwm_get_state().
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> ---
>  drivers/pwm/core.c  |  8 ++++----
>  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
>  2 files changed, 46 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> index 6433059..f3f91e7 100644
> --- a/drivers/pwm/core.c
> +++ b/drivers/pwm/core.c
> @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
>  		pwm->chip = chip;
>  		pwm->pwm = chip->base + i;
>  		pwm->hwpwm = i;
> -		pwm->polarity = polarity;
> +		pwm->state.polarity = polarity;

Would this not more correctly be assigned to pwm->args.polarity? After
all this is setting up the "initial" state, much like DT or the lookup
tables would for duty cycle and period.

Thierry

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#1376740

On Tue, 12 Apr 2016 13:49:04 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > The PWM state, represented by its period, duty_cycle and polarity,
> > is currently directly stored in the PWM device.
> > Declare a pwm_state structure embedding those field so that we can later
> > use this struct to atomically update all the PWM parameters at once.
> > 
> > All pwm_get_xxx() helpers are now implemented as wrappers around
> > pwm_get_state().
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > ---
> >  drivers/pwm/core.c  |  8 ++++----
> >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> >  2 files changed, 46 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > index 6433059..f3f91e7 100644
> > --- a/drivers/pwm/core.c
> > +++ b/drivers/pwm/core.c
> > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> >  		pwm->chip = chip;
> >  		pwm->pwm = chip->base + i;
> >  		pwm->hwpwm = i;
> > -		pwm->polarity = polarity;
> > +		pwm->state.polarity = polarity;
> 
> Would this not more correctly be assigned to pwm->args.polarity? After
> all this is setting up the "initial" state, much like DT or the lookup
> tables would for duty cycle and period.

Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
all the reference info should be extracted from DT, PWM lookup table or
driver specific ->request() implementation, but I can definitely
initialize the args.polarity here too.

Should I keep the pwm->state.polarity assignment (to set the initial
polarity when the driver does not support hardware readout)?

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

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#1376746

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On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> On Tue, 12 Apr 2016 13:49:04 +0200
> Thierry Reding <thierry.reding@gmail.com> wrote:
> 
> > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > The PWM state, represented by its period, duty_cycle and polarity,
> > > is currently directly stored in the PWM device.
> > > Declare a pwm_state structure embedding those field so that we can later
> > > use this struct to atomically update all the PWM parameters at once.
> > > 
> > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > pwm_get_state().
> > > 
> > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > ---
> > >  drivers/pwm/core.c  |  8 ++++----
> > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > index 6433059..f3f91e7 100644
> > > --- a/drivers/pwm/core.c
> > > +++ b/drivers/pwm/core.c
> > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > >  		pwm->chip = chip;
> > >  		pwm->pwm = chip->base + i;
> > >  		pwm->hwpwm = i;
> > > -		pwm->polarity = polarity;
> > > +		pwm->state.polarity = polarity;
> > 
> > Would this not more correctly be assigned to pwm->args.polarity? After
> > all this is setting up the "initial" state, much like DT or the lookup
> > tables would for duty cycle and period.
> 
> Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> all the reference info should be extracted from DT, PWM lookup table or
> driver specific ->request() implementation, but I can definitely
> initialize the args.polarity here too.
> 
> Should I keep the pwm->state.polarity assignment (to set the initial
> polarity when the driver does not support hardware readout)?

Wouldn't this work automatically as part of the pwm_apply_args() helper
if we extended it with this setting?

Thierry

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#1376769

On Tue, 12 Apr 2016 14:21:41 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> > On Tue, 12 Apr 2016 13:49:04 +0200
> > Thierry Reding <thierry.reding@gmail.com> wrote:
> > 
> > > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > > The PWM state, represented by its period, duty_cycle and polarity,
> > > > is currently directly stored in the PWM device.
> > > > Declare a pwm_state structure embedding those field so that we can later
> > > > use this struct to atomically update all the PWM parameters at once.
> > > > 
> > > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > > pwm_get_state().
> > > > 
> > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > ---
> > > >  drivers/pwm/core.c  |  8 ++++----
> > > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > > 
> > > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > > index 6433059..f3f91e7 100644
> > > > --- a/drivers/pwm/core.c
> > > > +++ b/drivers/pwm/core.c
> > > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > > >  		pwm->chip = chip;
> > > >  		pwm->pwm = chip->base + i;
> > > >  		pwm->hwpwm = i;
> > > > -		pwm->polarity = polarity;
> > > > +		pwm->state.polarity = polarity;
> > > 
> > > Would this not more correctly be assigned to pwm->args.polarity? After
> > > all this is setting up the "initial" state, much like DT or the lookup
> > > tables would for duty cycle and period.
> > 
> > Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> > all the reference info should be extracted from DT, PWM lookup table or
> > driver specific ->request() implementation, but I can definitely
> > initialize the args.polarity here too.
> > 
> > Should I keep the pwm->state.polarity assignment (to set the initial
> > polarity when the driver does not support hardware readout)?
> 
> Wouldn't this work automatically as part of the pwm_apply_args() helper
> if we extended it with this setting?

Well, as you explained in you answer to patch 5, pwm_apply_args()
should be called on a per-request basis (each time a PWM device is
requested), while the initial polarity setting should only be applied
when registering the PWM chip (and its devices). After that, the
framework takes care of keeping the PWM state in sync with the hardware
state.

Let's take a real (though a bit unusual) example. Say you have a single
PWM device referenced by two different users. Only one user can be
enabled at a time, but each of them has its own reference config
(different polarity, different period).

User1 calls pwm_get() and applies its own polarity and period. Then
user1 is unregistered and release the PWM device, leaving the polarity
and period untouched.

User2 is registered and request the same PWM device, but user2 is
smarter and tries to extract the current PWM state before adapting the
config according to pwm_args. If you just reset pwm->state.polarity
each time pwm_apply_args() is called (and you suggested to call it as
part of the request procedure), then this means the PWM state is no
longer in sync with the hardware state.


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

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#1376791

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On Tue, Apr 12, 2016 at 02:45:08PM +0200, Boris Brezillon wrote:
> On Tue, 12 Apr 2016 14:21:41 +0200
> Thierry Reding <thierry.reding@gmail.com> wrote:
> 
> > On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> > > On Tue, 12 Apr 2016 13:49:04 +0200
> > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > 
> > > > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > > > The PWM state, represented by its period, duty_cycle and polarity,
> > > > > is currently directly stored in the PWM device.
> > > > > Declare a pwm_state structure embedding those field so that we can later
> > > > > use this struct to atomically update all the PWM parameters at once.
> > > > > 
> > > > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > > > pwm_get_state().
> > > > > 
> > > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > > ---
> > > > >  drivers/pwm/core.c  |  8 ++++----
> > > > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > > > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > > > index 6433059..f3f91e7 100644
> > > > > --- a/drivers/pwm/core.c
> > > > > +++ b/drivers/pwm/core.c
> > > > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > > > >  		pwm->chip = chip;
> > > > >  		pwm->pwm = chip->base + i;
> > > > >  		pwm->hwpwm = i;
> > > > > -		pwm->polarity = polarity;
> > > > > +		pwm->state.polarity = polarity;
> > > > 
> > > > Would this not more correctly be assigned to pwm->args.polarity? After
> > > > all this is setting up the "initial" state, much like DT or the lookup
> > > > tables would for duty cycle and period.
> > > 
> > > Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> > > all the reference info should be extracted from DT, PWM lookup table or
> > > driver specific ->request() implementation, but I can definitely
> > > initialize the args.polarity here too.
> > > 
> > > Should I keep the pwm->state.polarity assignment (to set the initial
> > > polarity when the driver does not support hardware readout)?
> > 
> > Wouldn't this work automatically as part of the pwm_apply_args() helper
> > if we extended it with this setting?
> 
> Well, as you explained in you answer to patch 5, pwm_apply_args()
> should be called on a per-request basis (each time a PWM device is
> requested), while the initial polarity setting should only be applied
> when registering the PWM chip (and its devices). After that, the
> framework takes care of keeping the PWM state in sync with the hardware
> state.
> 
> Let's take a real (though a bit unusual) example. Say you have a single
> PWM device referenced by two different users. Only one user can be
> enabled at a time, but each of them has its own reference config
> (different polarity, different period).
> 
> User1 calls pwm_get() and applies its own polarity and period. Then
> user1 is unregistered and release the PWM device, leaving the polarity
> and period untouched.
> 
> User2 is registered and request the same PWM device, but user2 is
> smarter and tries to extract the current PWM state before adapting the
> config according to pwm_args. If you just reset pwm->state.polarity
> each time pwm_apply_args() is called (and you suggested to call it as
> part of the request procedure), then this means the PWM state is no
> longer in sync with the hardware state.

In that case neither will be the period or duty cycle. Essentially this
gets us back to square one where we need to decide how to handle current
state vs. initial arguments.

But I don't think this is really going to be an issue because this is
all moot until we've moved over to the atomic API, at which point this
is all going to go away anyway.

Thierry

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#1376794

On Tue, 12 Apr 2016 15:11:18 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Tue, Apr 12, 2016 at 02:45:08PM +0200, Boris Brezillon wrote:
> > On Tue, 12 Apr 2016 14:21:41 +0200
> > Thierry Reding <thierry.reding@gmail.com> wrote:
> > 
> > > On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> > > > On Tue, 12 Apr 2016 13:49:04 +0200
> > > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > > 
> > > > > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > > > > The PWM state, represented by its period, duty_cycle and polarity,
> > > > > > is currently directly stored in the PWM device.
> > > > > > Declare a pwm_state structure embedding those field so that we can later
> > > > > > use this struct to atomically update all the PWM parameters at once.
> > > > > > 
> > > > > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > > > > pwm_get_state().
> > > > > > 
> > > > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > > > ---
> > > > > >  drivers/pwm/core.c  |  8 ++++----
> > > > > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > > > > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > > > > index 6433059..f3f91e7 100644
> > > > > > --- a/drivers/pwm/core.c
> > > > > > +++ b/drivers/pwm/core.c
> > > > > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > > > > >  		pwm->chip = chip;
> > > > > >  		pwm->pwm = chip->base + i;
> > > > > >  		pwm->hwpwm = i;
> > > > > > -		pwm->polarity = polarity;
> > > > > > +		pwm->state.polarity = polarity;
> > > > > 
> > > > > Would this not more correctly be assigned to pwm->args.polarity? After
> > > > > all this is setting up the "initial" state, much like DT or the lookup
> > > > > tables would for duty cycle and period.
> > > > 
> > > > Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> > > > all the reference info should be extracted from DT, PWM lookup table or
> > > > driver specific ->request() implementation, but I can definitely
> > > > initialize the args.polarity here too.
> > > > 
> > > > Should I keep the pwm->state.polarity assignment (to set the initial
> > > > polarity when the driver does not support hardware readout)?
> > > 
> > > Wouldn't this work automatically as part of the pwm_apply_args() helper
> > > if we extended it with this setting?
> > 
> > Well, as you explained in you answer to patch 5, pwm_apply_args()
> > should be called on a per-request basis (each time a PWM device is
> > requested), while the initial polarity setting should only be applied
> > when registering the PWM chip (and its devices). After that, the
> > framework takes care of keeping the PWM state in sync with the hardware
> > state.
> > 
> > Let's take a real (though a bit unusual) example. Say you have a single
> > PWM device referenced by two different users. Only one user can be
> > enabled at a time, but each of them has its own reference config
> > (different polarity, different period).
> > 
> > User1 calls pwm_get() and applies its own polarity and period. Then
> > user1 is unregistered and release the PWM device, leaving the polarity
> > and period untouched.
> > 
> > User2 is registered and request the same PWM device, but user2 is
> > smarter and tries to extract the current PWM state before adapting the
> > config according to pwm_args. If you just reset pwm->state.polarity
> > each time pwm_apply_args() is called (and you suggested to call it as
> > part of the request procedure), then this means the PWM state is no
> > longer in sync with the hardware state.
> 
> In that case neither will be the period or duty cycle. Essentially this
> gets us back to square one where we need to decide how to handle current
> state vs. initial arguments.

That's not true. Now we clearly differentiate the reference config
(content of pwm_args which is only a subset of what you'll find in
pwm_state) and the PWM state (represented by pwm_state).

We should be safe as long as we keep those 2 elements as 2 orthogonal
concepts:
- pwm_args is supposed to give some hint to the PWM user to help him
  configure it's PWM appropriately
- pwm_state is here to reflect the real PWM state, and apply new
  configs

> 
> But I don't think this is really going to be an issue because this is
> all moot until we've moved over to the atomic API, at which point this
> is all going to go away anyway.

As stated in my answer to patch 5, I think I misunderstood your
suggestion. pwm_apply_args() is supposed to adjust the PWM config to
match the period and polarity specified in pwm_args, right?

If that's the case, my question is, should we really call this function
each time a new user requests a PWM instead of letting those users call
the function on-demand (not all users want to adapt the current PWM
config to the pwm_args, some may just want to apply a completely new
config).

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

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#1376829

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On Tue, Apr 12, 2016 at 03:26:44PM +0200, Boris Brezillon wrote:
> On Tue, 12 Apr 2016 15:11:18 +0200
> Thierry Reding <thierry.reding@gmail.com> wrote:
> 
> > On Tue, Apr 12, 2016 at 02:45:08PM +0200, Boris Brezillon wrote:
> > > On Tue, 12 Apr 2016 14:21:41 +0200
> > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > 
> > > > On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> > > > > On Tue, 12 Apr 2016 13:49:04 +0200
> > > > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > > > 
> > > > > > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > > > > > The PWM state, represented by its period, duty_cycle and polarity,
> > > > > > > is currently directly stored in the PWM device.
> > > > > > > Declare a pwm_state structure embedding those field so that we can later
> > > > > > > use this struct to atomically update all the PWM parameters at once.
> > > > > > > 
> > > > > > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > > > > > pwm_get_state().
> > > > > > > 
> > > > > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > > > > ---
> > > > > > >  drivers/pwm/core.c  |  8 ++++----
> > > > > > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > > > > > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > > > > > index 6433059..f3f91e7 100644
> > > > > > > --- a/drivers/pwm/core.c
> > > > > > > +++ b/drivers/pwm/core.c
> > > > > > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > > > > > >  		pwm->chip = chip;
> > > > > > >  		pwm->pwm = chip->base + i;
> > > > > > >  		pwm->hwpwm = i;
> > > > > > > -		pwm->polarity = polarity;
> > > > > > > +		pwm->state.polarity = polarity;
> > > > > > 
> > > > > > Would this not more correctly be assigned to pwm->args.polarity? After
> > > > > > all this is setting up the "initial" state, much like DT or the lookup
> > > > > > tables would for duty cycle and period.
> > > > > 
> > > > > Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> > > > > all the reference info should be extracted from DT, PWM lookup table or
> > > > > driver specific ->request() implementation, but I can definitely
> > > > > initialize the args.polarity here too.
> > > > > 
> > > > > Should I keep the pwm->state.polarity assignment (to set the initial
> > > > > polarity when the driver does not support hardware readout)?
> > > > 
> > > > Wouldn't this work automatically as part of the pwm_apply_args() helper
> > > > if we extended it with this setting?
> > > 
> > > Well, as you explained in you answer to patch 5, pwm_apply_args()
> > > should be called on a per-request basis (each time a PWM device is
> > > requested), while the initial polarity setting should only be applied
> > > when registering the PWM chip (and its devices). After that, the
> > > framework takes care of keeping the PWM state in sync with the hardware
> > > state.
> > > 
> > > Let's take a real (though a bit unusual) example. Say you have a single
> > > PWM device referenced by two different users. Only one user can be
> > > enabled at a time, but each of them has its own reference config
> > > (different polarity, different period).
> > > 
> > > User1 calls pwm_get() and applies its own polarity and period. Then
> > > user1 is unregistered and release the PWM device, leaving the polarity
> > > and period untouched.
> > > 
> > > User2 is registered and request the same PWM device, but user2 is
> > > smarter and tries to extract the current PWM state before adapting the
> > > config according to pwm_args. If you just reset pwm->state.polarity
> > > each time pwm_apply_args() is called (and you suggested to call it as
> > > part of the request procedure), then this means the PWM state is no
> > > longer in sync with the hardware state.
> > 
> > In that case neither will be the period or duty cycle. Essentially this
> > gets us back to square one where we need to decide how to handle current
> > state vs. initial arguments.
> 
> That's not true. Now we clearly differentiate the reference config
> (content of pwm_args which is only a subset of what you'll find in
> pwm_state) and the PWM state (represented by pwm_state).
> 
> We should be safe as long as we keep those 2 elements as 2 orthogonal
> concepts:
> - pwm_args is supposed to give some hint to the PWM user to help him
>   configure it's PWM appropriately
> - pwm_state is here to reflect the real PWM state, and apply new
>   configs
> 
> > 
> > But I don't think this is really going to be an issue because this is
> > all moot until we've moved over to the atomic API, at which point this
> > is all going to go away anyway.
> 
> As stated in my answer to patch 5, I think I misunderstood your
> suggestion. pwm_apply_args() is supposed to adjust the PWM config to
> match the period and polarity specified in pwm_args, right?
> 
> If that's the case, my question is, should we really call this function
> each time a new user requests a PWM instead of letting those users call
> the function on-demand (not all users want to adapt the current PWM
> config to the pwm_args, some may just want to apply a completely new
> config).

I think we're still talking past each other. I didn't mean for this to
be a proper part of the API. Like you said the struct pwm_args doesn't
contain enough data to construct a complete state and apply it.

What I was suggesting is to factor out the individual calls to the
various pwm_set_*() functions into a single call. So we wouldn't be
changing semantics, just refactoring to make it easier to get rid of
again in one of the subsequent patches.

That is, pwm_apply_args() would go away again within this very series,
at the same point that you're currently removing the pwm_set_*() calls.

Thierry

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#1376832

On Tue, 12 Apr 2016 16:05:46 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Tue, Apr 12, 2016 at 03:26:44PM +0200, Boris Brezillon wrote:
> > On Tue, 12 Apr 2016 15:11:18 +0200
> > Thierry Reding <thierry.reding@gmail.com> wrote:
> > 
> > > On Tue, Apr 12, 2016 at 02:45:08PM +0200, Boris Brezillon wrote:
> > > > On Tue, 12 Apr 2016 14:21:41 +0200
> > > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > > 
> > > > > On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:
> > > > > > On Tue, 12 Apr 2016 13:49:04 +0200
> > > > > > Thierry Reding <thierry.reding@gmail.com> wrote:
> > > > > > 
> > > > > > > On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:
> > > > > > > > The PWM state, represented by its period, duty_cycle and polarity,
> > > > > > > > is currently directly stored in the PWM device.
> > > > > > > > Declare a pwm_state structure embedding those field so that we can later
> > > > > > > > use this struct to atomically update all the PWM parameters at once.
> > > > > > > > 
> > > > > > > > All pwm_get_xxx() helpers are now implemented as wrappers around
> > > > > > > > pwm_get_state().
> > > > > > > > 
> > > > > > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > > > > > ---
> > > > > > > >  drivers/pwm/core.c  |  8 ++++----
> > > > > > > >  include/linux/pwm.h | 54 +++++++++++++++++++++++++++++++++++++++++------------
> > > > > > > >  2 files changed, 46 insertions(+), 16 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > > > > > > index 6433059..f3f91e7 100644
> > > > > > > > --- a/drivers/pwm/core.c
> > > > > > > > +++ b/drivers/pwm/core.c
> > > > > > > > @@ -268,7 +268,7 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
> > > > > > > >  		pwm->chip = chip;
> > > > > > > >  		pwm->pwm = chip->base + i;
> > > > > > > >  		pwm->hwpwm = i;
> > > > > > > > -		pwm->polarity = polarity;
> > > > > > > > +		pwm->state.polarity = polarity;
> > > > > > > 
> > > > > > > Would this not more correctly be assigned to pwm->args.polarity? After
> > > > > > > all this is setting up the "initial" state, much like DT or the lookup
> > > > > > > tables would for duty cycle and period.
> > > > > > 
> > > > > > Yes, I wasn't sure about the pwm_add_with_polarity() meaning. To me,
> > > > > > all the reference info should be extracted from DT, PWM lookup table or
> > > > > > driver specific ->request() implementation, but I can definitely
> > > > > > initialize the args.polarity here too.
> > > > > > 
> > > > > > Should I keep the pwm->state.polarity assignment (to set the initial
> > > > > > polarity when the driver does not support hardware readout)?
> > > > > 
> > > > > Wouldn't this work automatically as part of the pwm_apply_args() helper
> > > > > if we extended it with this setting?
> > > > 
> > > > Well, as you explained in you answer to patch 5, pwm_apply_args()
> > > > should be called on a per-request basis (each time a PWM device is
> > > > requested), while the initial polarity setting should only be applied
> > > > when registering the PWM chip (and its devices). After that, the
> > > > framework takes care of keeping the PWM state in sync with the hardware
> > > > state.
> > > > 
> > > > Let's take a real (though a bit unusual) example. Say you have a single
> > > > PWM device referenced by two different users. Only one user can be
> > > > enabled at a time, but each of them has its own reference config
> > > > (different polarity, different period).
> > > > 
> > > > User1 calls pwm_get() and applies its own polarity and period. Then
> > > > user1 is unregistered and release the PWM device, leaving the polarity
> > > > and period untouched.
> > > > 
> > > > User2 is registered and request the same PWM device, but user2 is
> > > > smarter and tries to extract the current PWM state before adapting the
> > > > config according to pwm_args. If you just reset pwm->state.polarity
> > > > each time pwm_apply_args() is called (and you suggested to call it as
> > > > part of the request procedure), then this means the PWM state is no
> > > > longer in sync with the hardware state.
> > > 
> > > In that case neither will be the period or duty cycle. Essentially this
> > > gets us back to square one where we need to decide how to handle current
> > > state vs. initial arguments.
> > 
> > That's not true. Now we clearly differentiate the reference config
> > (content of pwm_args which is only a subset of what you'll find in
> > pwm_state) and the PWM state (represented by pwm_state).
> > 
> > We should be safe as long as we keep those 2 elements as 2 orthogonal
> > concepts:
> > - pwm_args is supposed to give some hint to the PWM user to help him
> >   configure it's PWM appropriately
> > - pwm_state is here to reflect the real PWM state, and apply new
> >   configs
> > 
> > > 
> > > But I don't think this is really going to be an issue because this is
> > > all moot until we've moved over to the atomic API, at which point this
> > > is all going to go away anyway.
> > 
> > As stated in my answer to patch 5, I think I misunderstood your
> > suggestion. pwm_apply_args() is supposed to adjust the PWM config to
> > match the period and polarity specified in pwm_args, right?
> > 
> > If that's the case, my question is, should we really call this function
> > each time a new user requests a PWM instead of letting those users call
> > the function on-demand (not all users want to adapt the current PWM
> > config to the pwm_args, some may just want to apply a completely new
> > config).
> 
> I think we're still talking past each other. I didn't mean for this to
> be a proper part of the API. Like you said the struct pwm_args doesn't
> contain enough data to construct a complete state and apply it.
> 
> What I was suggesting is to factor out the individual calls to the
> various pwm_set_*() functions into a single call. So we wouldn't be
> changing semantics, just refactoring to make it easier to get rid of
> again in one of the subsequent patches.
> 
> That is, pwm_apply_args() would go away again within this very series,
> at the same point that you're currently removing the pwm_set_*() calls.

Okay, eventually got it :).

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

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