On Tue, 12 Apr 2016 15:11:18 +0200
Thierry Reding [off-list ref] wrote:

On Tue, Apr 12, 2016 at 02:45:08PM +0200, Boris Brezillon wrote:

quoted

On Tue, 12 Apr 2016 14:21:41 +0200
Thierry Reding [off-list ref] wrote:

quoted

On Tue, Apr 12, 2016 at 02:17:18PM +0200, Boris Brezillon wrote:

quoted

On Tue, 12 Apr 2016 13:49:04 +0200
Thierry Reding [off-list ref] wrote:

quoted

On Wed, Mar 30, 2016 at 10:03:38PM +0200, Boris Brezillon wrote:

quoted

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-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
---
 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