linux-kernel - Re: [RESEND 01/11] pwm: Add PWM Capture support

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Date:	Wed, 13 Apr 2016 16:50:24 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, maxime.coquelin@...com,
	linux-pwm@...r.kernel.org, ajitpal.singh@...com
Subject: Re: [RESEND 01/11] pwm: Add PWM Capture support

On Wed, Apr 13, 2016 at 10:36:05AM +0100, Lee Jones wrote:
> On Tue, 12 Apr 2016, Thierry Reding wrote:
> 
> > On Wed, Mar 02, 2016 at 03:31:59PM +0000, Lee Jones wrote:
> > > Supply a PWM Capture call-back Op in order to pass back
> > > information obtained by running analysis on PWM a signal.
> > > This would normally (at least during testing) be called from
> > > the Sysfs routines with a view to printing out PWM Capture
> > > data which has been encoded into a string.
> > > 
> > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > ---
> > >  drivers/pwm/core.c  | 26 ++++++++++++++++++++++++++
> > >  include/linux/pwm.h | 13 +++++++++++++
> > >  2 files changed, 39 insertions(+)
> > 
> > Overall I like the concept of introducing this capture functionality.
> > 
> > However I have a couple of questions, see below.
> > 
> > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > index d24ca5f..8f4a8a9 100644
> > > --- a/drivers/pwm/core.c
> > > +++ b/drivers/pwm/core.c
> > > @@ -494,6 +494,32 @@ unlock:
> > >  EXPORT_SYMBOL_GPL(pwm_set_polarity);
> > >  
> > >  /**
> > > + * pwm_capture() - capture and report a PWM signal
> > > + * @pwm: PWM device
> > > + * @channel: PWM capture channel to use
> > > + * @buf: buffer to place output message into
> > > + *
> > > + * Returns: 0 on success or a negative error code on failure.
> > > + */
> > > +int pwm_capture(struct pwm_device *pwm, int channel, char *buf)
> > 
> > This public interface seems to be targetted specifically at sysfs. As
> > such I'm not sure if there is reason to make it public, since the code
> > is unlikely to ever be called by other users in the kernel.
> > 
> > Do you think it would be possible to make the interface more generic by
> > passing back some form of structure containing the capture result? That
> > way users within the kernel could use the result without having to go
> > and parse a string filled in by the driver. It would also be easy to
> > implement sysfs support on top of that. Another advantage is that there
> > would be a standard result structure rather than a free-form string
> > filled by drivers that can't be controlled.
> > 
> > What kind of result does the STi hardware return? Looking at the driver
> > later in the series it seems to support triggering interrupts on rising
> > and falling edges and capture some running counter at these events. If
> > the frequency of the counter increment is known, these numbers should
> > allow us to determine both the period and duty cycle of the PWM signal
> > in nanoseconds. Would it be possible to rewrite this function and the
> > driver patch to something like this:
> > 
> > 	int pwm_capture(struct pwm_device *pwm, struct pwm_capture *result);
> > 
> > Where
> > 
> > 	struct pwm_capture {
> > 		unsigned int period;
> > 		unsigned int duty_cycle;
> > 	};
> > 
> > ?
> 
> Yes, I think that sounds feasible.
> 
> > Another thing I noticed is that the code here seems to be confusing
> > channels and devices. In the PWM subsystem a struct pwm_device
> > represents a single channel. Allowing the channel to be specified is
> > redundant at best, and confusing at worst.
> 
> On the STi platform I'm working on, we have 2 devices PWM{0,1} and
> each device has 4 separate channels [0..3].  Not all of them support
> PWM capture, but the channels are 'a thing'.  I'd need to look into it
> further, but I guess you'd like the driver to pretend we have 8
> devices?  If that's the case, what's the point in the core 'npwm'
> parameter?  Surely that's "channels per device"?

Well, it's technically "channels per _chip_". Perhaps the confusion is
with the historical naming: a PWM channel is represented by a struct
pwm_device, whereas what I think you're referring to as device (as in
"channels per device") is represented as a struct pwm_chip.

Thierry

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