linux-kernel - Re: [RESEND v5 2/2] pwm: core: Convert period and duty cycle to u64

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Message-ID: <20200213223806.GA12212@codeaurora.org>
Date:   Thu, 13 Feb 2020 14:38:09 -0800
From:   Guru Das Srinagesh <gurus@...eaurora.org>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND v5 2/2] pwm: core: Convert period and duty cycle to u64

On Thu, Feb 13, 2020 at 10:58:57PM +0100, Uwe Kleine-König wrote:
> On Thu, Feb 13, 2020 at 01:06:49PM -0800, Guru Das Srinagesh wrote:
> > On Thu, Feb 13, 2020 at 09:28:04PM +0100, Uwe Kleine-König wrote:
> > > On Thu, Feb 13, 2020 at 11:39:26AM -0800, Guru Das Srinagesh wrote:
> > > > On Thu, Feb 13, 2020 at 11:18:02AM +0100, Uwe Kleine-König wrote:
> > > > > Hello,
> > > > > 
> > > > > On Wed, Feb 12, 2020 at 10:54:08AM -0800, Guru Das Srinagesh wrote:
> > > > > > @@ -305,8 +305,8 @@ struct pwm_chip {
> > > > > >   * @duty_cycle: duty cycle of the PWM signal (in nanoseconds)
> > > > > >   */
> > > > > >  struct pwm_capture {
> > > > > > -	unsigned int period;
> > > > > > -	unsigned int duty_cycle;
> > > > > > +	u64 period;
> > > > > > +	u64 duty_cycle;
> > > > > >  };
> > > > > 
> > > > > Is this last hunk a separate change?
> > > > > 
> > > > > Otherwise looks fine.
> > > > 
> > > > No, this is very much a part of the change and not a separate one.
> > > 
> > > Not sure we understand each other. I wondered if conversion of the
> > > pwm_capture stuff should be done separately. (OTOH I wonder if this is
> > > used at all and already considered deleting it.)
> > 
> > I see. Could you please elaborate on your concerns? I think this hunk's
> > being in this patch makes sense as duty and period should be converted
> > to u64 throughout the file in one go.
> 
> I guess that capturing isn't much used if at all. So keeping it as
> limited as it is today doesn't seem like a bad idea to me. (OK, you
> could also accidentally break it such that we can say in a few years
> time: This is broken since v5.6, obviously nobody cares and we remove it
> for good :-))
> 
> > Also, it looks like pwm_capture is being used by pwm-sti.c and
> > pwm-stm32.c currently.
> 
> Yeah, these two drivers provide the needed callback. That doesn't
> necessarily mean there are active users. Also I'm convinced that there
> are implementation problems. For example the commit that added capture
> support for stm32 has in its commit log:
> 
> 	Capture requires exclusive access (e.g. no pwm output running at
> 	the same time, to protect common prescaler).
> 
> but the capture callback doesn't even check if the PWMs are in use (but
> I only looked quickly, so I might have missed something).
> 
> Apart from that I think that the capturing stuff doesn't really fit into
> the PWM framework which is (apart from the capture callback and API
> function) about PWM *outputs* and most hardware's I know about either
> don't support capturing or it is located in a different IP than the PWM
> output. (And it is not even possible today to register a driver that can
> only capture but not drive a PWM output.)

Thanks for the explanation. So what would you recommend - dropping that
hunk entirely or separating that out in a separate patch?

Thank you.

Guru Das.