How to set PWM tunable name to ehrpwm.1 ?

[Warner Losh]

On Fri, Jun 7, 2019 at 9:11 AM Emmanuel Vadot <manu at bidouilliste.com> wrote:

> On Fri, 07 Jun 2019 08:58:49 -0600
> Ian Lepore <ian at freebsd.org> wrote:
>
> > On Fri, 2019-06-07 at 02:08 -0700, Nicola Mingotti wrote:
> > >
> > > On 6/6/19 3:40 PM, Ian Lepore wrote:
> > > > On Thu, 2019-06-06 at 16:06 -0600, Sergey Manucharian wrote:
> > > > > Excerpts from Nicola Mingotti's message from Thu 06-Jun-19 12:33:
> > > > > > In my BeagleBone Black, FreeBSD-12 RELEASE, i created two
> > > > > > overlays,
> > > > > > pwm.dtso and pwm1.dtso. They enable the PWM pins p9.21, p9.22
> > > > > > and
> > > > > > respectively p9.14, p9.16. DTSO files are below.
> > > > > >
> > > > > > If I load both the DTBO at boot I see
> > > > > > correctly|ehrpwm.0|and|ehrpwm.1|,
> > > > > > associated to the correct pins. But, if i remove the
> > > > > > overlay|pwm.dtbo|then i seen only|ehrpwm.0|in|sysctl -a|, which
> > > > > > is
> > > > > > not
> > > > > > what i want, i would like to see the name|ehrpwm.1|.
> > > > > >
> > > > > > This is important because i must be 100% sure a certain pin
> > > > > > corresponds
> > > > > > the a certain tunable.This must be true even if i remove non
> > > > > > relevant
> > > > > > overlays in the future. I guess there must be some parameter in
> > > > > > the
> > > > > > DTSO
> > > > > > which i don't know, i hope you can give me some directions
> > > > > > about
> > > > > > that.
> > > > >
> > > > > It is not related to your DTBO's. That's how everything works (at
> > > > > least
> > > > > by default). You will see the same naming issue with serial
> > > > > ports,
> > > > > for
> > > > > example. And not just in BBB.
> > > > >
> > > > > E.g. when I have enabled uart0 and uart2 they are named ttyu0 and
> > > > > ttyu1,
> > > > > if I have only uart2, it becomes ttyu0.
> > > > >
> > > > > It's easier if there is a device node in /dev, so you can create
> > > > > a
> > > > > symlink
> > > > > with a fixed name (I have a script called by devd for my multiple
> > > > > serial
> > > > > ports). However, that's not the case with PWM...
> > > > >
> > > > > Maybe there is an option to use persistent names for devices that
> > > > > somebody
> > > > > can point to.
> > > > >
> > > >
> > > > Nope, there's no magic thing you're missing that fixes
> > > > this.  Devices
> > > > get named-and-numbered based on the order of instantiation.
> > > >
> > > > Since what really matters here is the sysctl names, we could change
> > > > the
> > > > driver to install the sysctl nodes using the fdt device node names
> > > > instead of the freebsd newbus device names.  Hmm, actually, since
> > > > people may be relying on the current names, I guess what we'd have
> > > > to
> > > > do is install another set of sysctl names based on fdt name
> > > > (basically
> > > > a set of alias names).
> > > >
> > > > -- Ian
> > > >
> > >
> > > I see, I agree changing the default naming scheme may damage who is
> > > relaying on it. It is not a good idea. Maybe it could be implemented
> > > in
> > > release 13.
> > >
> > > To Sergey. I used devd in the past, it works well. But i would
> > > prefer
> > > not to use it in this case, even if I had a /dev/xyz file available.
> > > The
> > > reason is that the /dev/xyz file would appear before the the devd
> > > daemon
> > > starts up (i guess), so the case would not stricly be covered by
> > > what
> > > the devd man page says devd should do.
> > > $> man devd
> > > => " ... Whenever a device is added to or removed from the device
> > > tree ... "
> > >
> > > To Ian. The idea of the alias seems good. I don't know at all what
> > > you
> > > can manage to do at the kernel level with the tunables. I imagine
> > > something like |dev.alias.am335x_ehrpwm.1| which actually refers to
> > > > EHRPWM1| not the second |ehrpwm| that got plugged into the system
> > > > via
> > >
> > > overlay.
> > >
> > > Thank you for your answers
> > >
> > >
> >
> > The dev.* hierarchy is managed by newbus; what I was picturing was
> > something like hw.ehrpwm1.freq and so on, settable as either tunable or
> > sysctl.  But it turns out ehrpwm1 is just a label in the dts, not
> > accessible at runtime.  The actual node name is just 'pwm' and really,
> > nothing prevents upstream from changing that name on a whim next time
> > we import new dts files.  (And linux sure seems to have a lot of
> > arbitrary whims when it comes to changing dts.)
>
>  Relying on the name is clearly not a good idea, especially for TI
> since they change stuff A LOT.
>
> > Since an overlay is required to use this stuff anyway, I'm now thinking
> > a custom property in the overlay that names the sysctl nodes might be a
> > good option.  So you'd add a property like:
> >
> >    &ehrpwm0 {
> >        status = "okay";
> >        pinctrl-names = "default";
> >        pinctrl-0 = <&ehrpwm0_AB_pins>;
> >        freebsd,sysctl = "backlight";
> >    };
> >
> > And that would make it install names like hw.backlight.freq and
> > hw.backlight.period and so on.  If you don't add that property, it just
> > installs the names it uses now (dev.ehrpwm.*) for compatibility.
> >
> > -- Ian
>
>  What would be better is to add support to the pwm(9) api for this
> driver and make the api and pwm(8) using the "pwm-names" property which
> is a standard one from the bindings.
>

For this specific case, I think that's great. I'd go farther than say we
should have the FDT/OFW node name, if any, associated with something like
dev.<dev>.<unit>.node_name.

Warner