[PATCH 1/3] dt-bindings: chosen: Add clocksource and clockevent selection

[Boris Brezillon]

On Fri, 15 Dec 2017 11:40:04 +0000
Mark Rutland <mark.rutland at arm.com> wrote:

> Hi,
> 
> On Thu, Dec 14, 2017 at 09:01:20PM +0100, Boris Brezillon wrote:
> > On Wed, 13 Dec 2017 16:57:50 -0600
> > Rob Herring <robh+dt at kernel.org> wrote:  
> > > On Wed, Dec 13, 2017 at 12:53 PM, Alexandre Belloni
> > > <alexandre.belloni at free-electrons.com> wrote:  
> 
> > > > The clocksource and clockevent timer are probed early in the boot process.
> > > > At that time it is difficult for linux to know whether a particular timer
> > > > can be used as the clocksource or the clockevent or by another driver,
> > > > especially when they are all identical or have similar features.    
> > > 
> > > If all identical, then it shouldn't matter. "similar" means some
> > > difference. Describe those differences.  
> > 
> > We had this discussion already. Those timers might be connected to
> > external pins and may serve the role of PWM generators or capture
> > devices. We can also chain timers and provide a clocksource with a
> > better resolution or one that wraps less often.   
> 
> Could you elaborate on the chaining case? I haven't encountered that,
> and at the moment I'm not sure I follow how that works.

In a TCB (Timer Counter Block) you have 3 TC (Timer Counters). Each
timer can take a regular clock (+a divider) as a source, but it can
also take the output of the previous channel (so channel 1 can take
the output of channel 0, channel 2 the output of channel 1, and channel
0 the output of channel 2). A TC output can be configured to toggle
every time the counter overflows. So when you chain 2 channels, you
double the number of bits of your counter. This is particularly
interesting if you want to create a precise timer that has an
acceptable wraparound period, otherwise, you'll have to choose between
a timer with a poor precision and an acceptable wraparound period, and
a timer with a good precision and a small wraparound.

> 
> > > >  - registering the first seen timer as a clockevent and the second one as
> > > >  a clocksource as in rk_timer_init or dw_apb_timer_init
> > > >
> > > > Add a linux,clocksource and a linux,clockevent node in chosen with a timer
> > > > property pointing to the timer to use. Other properties, like the targeted
> > > > precision may be added later.    
> > > 
> > > Open ended expansion of this does not help convince me it is needed.  
> > 
> > It's not an open ended expansion, we're just trying to find a way to
> > describe which timer blocks should be used as free running timers
> > (clksource) and which one should be used as programmable timers
> > (clkevent). Automatically selecting timer blocks to assign to the
> > clkevent or clocksource is not so easy (as has been explained earlier)
> > because at the time the system registers its clksource/clkevent devices
> > we might not have all the necessary information to know which timer
> > blocks will be reserved for other usage later on. The use case I have
> > in mind is DT overlays, where one of the overlay is using a timer as a
> > PWM generator. If the clkevent or clksource has already claimed the
> > timer connected to the pins the overlay is using, then we're screwed,
> > and there's no way the system can know that ahead of time except by
> > pre-assigning a timer to the clksource or clkevent feature.  
> 
> I guess that might work for the boot-time overlay case, where the user
> knows ahead-of-time that there will be a conflict for resources, but
> that doesn't help with the dynamic overlay case, since the user can't
> know what conflicts there will be.
> 
> Can we attempt to unregister the clock device in that case, when the PWM
> is requested? If the timekeeping core can select another device, then
> we're free to use this one as a PWM. If not, then we're stuck anyway.

Actually, the problem had already been solved with the "atmel,tcb-timer"
compatible. When this compatible is used we know the TC(s) can be used
as generic timers. If you want to reserve TC(s) for other usage (like a
PWM), you just leave the TC undefined in the device tree and an overlay
can add a new node reserving this TC afterwards. This approach has
already been accepted by Rob [1].

So right now, the problem we have is how to assign a specific timer to
a clockevent or clocksource 'device'.

[1]https://patchwork.kernel.org/patch/9755341/