[v2,0/3] Improve pwm-ir-tx precision

The pwm-ir-tx driver has to turn the pwm signal on and off, and suffers
from delays as this is done in process context. Make this work in atomic
context.

changes:

v2:
 - Simplified the pwm ir tx based on Ivaylo Dimitrov suggestions
 - Renamed pwm_can_sleep() to pwm_is_atomic
 - Fix sleeping issue in pwm-bcm2835 (clk_get_rate() can sleep)
 - use non_block_start() in pwm to check atomic-ness

Sean Young (3):
  pwm: make it possible to apply pwm changes in atomic context
  pwm: bcm2835: allow pwm driver to be used in atomic context
  media: pwm-ir-tx: trigger edges from hrtimer interrupt context

 drivers/media/rc/pwm-ir-tx.c  | 79 +++++++++++++++++++++++++++++++++--
 drivers/pwm/core.c            | 62 ++++++++++++++++++++++-----
 drivers/pwm/pwm-bcm2835.c     | 21 ++++++----
 drivers/pwm/pwm-renesas-tpu.c |  1 -
 include/linux/pwm.h           | 29 ++++++++++++-
 5 files changed, 166 insertions(+), 26 deletions(-)

On Fri, Oct 13, 2023 at 01:51:40PM +0200, Thierry Reding wrote:
> On Fri, Oct 13, 2023 at 11:46:14AM +0100, Sean Young wrote:
> [...]
> > diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> > index d2f9f690a9c1..93f166ab03c1 100644
> > --- a/include/linux/pwm.h
> > +++ b/include/linux/pwm.h
> > @@ -267,6 +267,7 @@ struct pwm_capture {
> >   * @get_state: get the current PWM state. This function is only
> >   *	       called once per PWM device when the PWM chip is
> >   *	       registered.
> > + * @atomic: can the driver execute pwm_apply_state in atomic context
> >   * @owner: helps prevent removal of modules exporting active PWMs
> >   */
> >  struct pwm_ops {
> > @@ -278,6 +279,7 @@ struct pwm_ops {
> >  		     const struct pwm_state *state);
> >  	int (*get_state)(struct pwm_chip *chip, struct pwm_device *pwm,
> >  			 struct pwm_state *state);
> > +	bool atomic;
> >  	struct module *owner;
> >  };
> 
> As I mentioned earlier, this really belongs in struct pwm_chip rather
> than struct pwm_ops. I know that Uwe said this is unlikely to happen,
> and that may be true, but at the same time it's not like I'm asking
> much. Whether you put this in struct pwm_ops or struct pwm_chip is
> about the same amount of code, and putting it into pwm_chip is much
> more flexible, so it's really a no-brainer.

Happy to change this of course. I changed it and then changed it back after
Uwe's comment, I'll fix this in the next version.

One tiny advantage is that pwm_ops is static const while pwm_chip is
allocated per-pwm, so will need instructions for setting the value. Having
said that, the difference is tiny, it's a single bool.


Sean

Hello,

On Fri, Oct 13, 2023 at 05:34:34PM +0200, Thierry Reding wrote:
> On Fri, Oct 13, 2023 at 03:58:30PM +0100, Sean Young wrote:
> > On Fri, Oct 13, 2023 at 01:51:40PM +0200, Thierry Reding wrote:
> > > On Fri, Oct 13, 2023 at 11:46:14AM +0100, Sean Young wrote:
> > > [...]
> > > > diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> > > > index d2f9f690a9c1..93f166ab03c1 100644
> > > > --- a/include/linux/pwm.h
> > > > +++ b/include/linux/pwm.h
> > > > @@ -267,6 +267,7 @@ struct pwm_capture {
> > > >   * @get_state: get the current PWM state. This function is only
> > > >   *	       called once per PWM device when the PWM chip is
> > > >   *	       registered.
> > > > + * @atomic: can the driver execute pwm_apply_state in atomic context
> > > >   * @owner: helps prevent removal of modules exporting active PWMs
> > > >   */
> > > >  struct pwm_ops {
> > > > @@ -278,6 +279,7 @@ struct pwm_ops {
> > > >  		     const struct pwm_state *state);
> > > >  	int (*get_state)(struct pwm_chip *chip, struct pwm_device *pwm,
> > > >  			 struct pwm_state *state);
> > > > +	bool atomic;
> > > >  	struct module *owner;
> > > >  };
> > > 
> > > As I mentioned earlier, this really belongs in struct pwm_chip rather
> > > than struct pwm_ops. I know that Uwe said this is unlikely to happen,
> > > and that may be true, but at the same time it's not like I'm asking
> > > much. Whether you put this in struct pwm_ops or struct pwm_chip is
> > > about the same amount of code, and putting it into pwm_chip is much
> > > more flexible, so it's really a no-brainer.
> > 
> > Happy to change this of course. I changed it and then changed it back after
> > Uwe's comment, I'll fix this in the next version.
> > 
> > One tiny advantage is that pwm_ops is static const while pwm_chip is
> > allocated per-pwm, so will need instructions for setting the value. Having
> > said that, the difference is tiny, it's a single bool.
> 
> Yeah, it's typically a single assignment, so from a code point of view
> it should be pretty much the same. I suppose from an instruction level
> point of view, yes, this might add a teeny-tiny bit of overhead.
> 
> On the other hand it lets us do interesting things like initialize
> chip->atomic = !regmap_might_sleep() for those drivers that use regmap
> and then not worry about it any longer.
> 
> Given that, I'm also wondering if we should try to keep the terminology
> a bit more consistent. "Atomic" is somewhat overloaded because ->apply()
> and ->get_state() are part of the "atomic" PWM API (in the sense that
> applying changes are done as a single, atomic operation, rather than in
> the sense of "non-sleeping" operation).
> 
> So pwm_apply_state_atomic() is then doubly atomic, which is a bit weird.
> On the other hand it's a bit tedious to convert all existing users to
> pwm_apply_state_might_sleep().
> 
> Perhaps as a compromise we can add pwm_apply_state_might_sleep() and
> make pwm_apply_state() a (deprecated) alias for that, so that existing
> drivers can be converted one by one.

To throw in my green for our bike shed: I'd pick

	pwm_apply_state_cansleep()

to match what gpio does (with gpiod_set_value_cansleep()). (Though I
have to admit that semantically Thierry's might_sleep is nicer as it
matches might_sleep().)

If we don't want to have an explicit indicator for the atomic/fast
variant (again similar to the gpio framework), maybe we can drop
"_state" which I think is somehow redundant and go for:

	pwm_apply (fast)
	pwm_apply_cansleep (sleeping)
	pwm_apply_state (compat alias for pwm_apply_cansleep())

(maybe replace cansleep with might_sleep). Similar for pwm_get_state()
we could use the opportunity and make

	pwm_get()

actually call ->get_state() and introduce

	pwm_get_lastapplied()

with the semantic of todays pwm_get_state(). Do we need a
pwm_get_cansleep/might_sleep()?

Best regards
Uwe