Re: [RESEND 09/11] pwm: sti: Add PWM Capture call-back

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Apr 15, 2016 at 09:29:00AM +0100, Lee Jones wrote:
> On Wed, 13 Apr 2016, Thierry Reding wrote:
> 
> > On Wed, Apr 13, 2016 at 11:25:54AM +0100, Lee Jones wrote:
> > > On Tue, 12 Apr 2016, Thierry Reding wrote:
> > > 
> > > > On Wed, Mar 02, 2016 at 03:32:07PM +0000, Lee Jones wrote:
> > > > > Once a PWM Capture has been initiated, the capture call
> > > > > enables a rising edge detection IRQ, then waits.  Once each
> > > > > of the 3 phase changes have been recorded the thread then
> > > > > wakes.  The remaining part of the call carries out the
> > > > > relevant calculations and passes back a formatted string to
> > > > > the caller.
> > > > > 
> > > > > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > > > > ---
> > > > >  drivers/pwm/pwm-sti.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > >  1 file changed, 72 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/pwm/pwm-sti.c b/drivers/pwm/pwm-sti.c
> > > > > index 82a69e4..8de9b4a 100644
> > > > > --- a/drivers/pwm/pwm-sti.c
> > > > > +++ b/drivers/pwm/pwm-sti.c
> 
> [...]
> 
> > > > > +	/* Prepare capture measurement */
> > > > > +	d->index = 0;
> > > > > +	regmap_write(pc->regmap, PWM_CPT_EDGE(channel), CPT_EDGE_RISING);
> > > > > +	regmap_field_write(pc->pwm_cpt_int_en, BIT(channel));
> > > > > +	ret = wait_event_interruptible_timeout(d->wait, d->index > 1, HZ);
> > > > 
> > > > The timeout here should make sure callers don't hang forever. But maybe
> > > > you can still make sure that when the PWM gets disabled the wait queue
> > > > is woken and perhaps return an appropriate error code to let users know
> > > > that the operation was interrupted.
> > > 
> > > Sure.  I'll look into that.
> > > 
> > > > Also, how about letting callers choose the value of the timeout? In some
> > > > cases they may be interested in long-running signals. In other cases the
> > > > whole second timeout may be much too long.
> > > 
> > > I'm not opposed to it.  How do you suggest we do that?
> > 
> > The easiest would probably be to add an unsigned long timeout parameter
> > to the pwm_capture() function and ->capture() callbacks.
> > 
> > But thinking about this further I'm wondering if it might not be easier
> > and more flexible to move the timeout completely outside of this code
> > and into callers. I suspect that the most simple way to do that would be
> > to add a completion to struct pwm_capture that callers can use to wait
> > for completion of a capture. This would make the whole process
> > asynchronous and allow interesting things like making the sysfs capture
> > file pollable, for example.
> 
> Okay, so how do you propose we handle this with sysfs?  Perhaps
> another RW file to set it?

I'm unfamiliar with how this is done in other drivers, so I'd have to
look at them first. I suspect that it would be fine for now to simply
redesign the PWM API parts and keep some default timeout in sysfs. It
could be extended with some mechanism to override the default timeout
in the future.

Thierry

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