On Wed, Jun 07, 2017 at 05:09:08PM +0200, Alexandre Belloni wrote:
> On 07/06/2017 at 16:17:35 +0200, Daniel Lezcano wrote:
> > > > > This driver uses regmap and syscon to be able to probe early in the
> > > > > boot
> > > > > and avoid having to switch on the TCB clocksource later. Using regmap
> > > > > also
> > > > > means that unused TCB channels may be used by other drivers (PWM for
> > > > > example).
> > > >
> > > > Can you give more details, I fail to understand how regmap and syscon
> > > > help to
> > > > probe sooner than timer_init()?
> > >
> > >
> > > Because before that, the tcb driver relied on atmel_tclib to share the
> > > TCBs and it happened way too late, at arch_initcall() time.
> >
> > So is it still necesary to use regmap? I would like to take the opportunity
> > to
> > move the init routine to the common init routine if possible:
> >
> > https://patchwork.kernel.org/patch/9768845/
> >
>
> It is still necessary because we want to be able to share the timer
> between multiple drivers. For example, you can have the clocksource on
> channel 0, clockevent on channel 1 and a pwm on channel 2
The hardware timer can be shared, the channels used in different subsystem.
Each channel are used exclusively.
What is the benefit of regmap? It has a cost, and takes a lock at each read.
For instance:
+static u64 tc_get_cycles(struct clocksource *cs)
+{
+ u32 lower, upper, tmp;
+
+ do {
+ regmap_read(tc.regmap, ATMEL_TC_CV(1), &upper);
+ regmap_read(tc.regmap, ATMEL_TC_CV(0), &lower);
+ regmap_read(tc.regmap, ATMEL_TC_CV(1), &tmp);
+ } while (upper != tmp);
+
+ return (upper << 16) | lower;
+}
Is:
+static u64 tc_get_cycles(struct clocksource *cs)
+{
+ u32 lower, upper, tmp;
+
+ do {
+ regmap_read(tc.regmap, ATMEL_TC_CV(1), &upper);
lock();
lot-of-things();
unlock();
+ regmap_read(tc.regmap, ATMEL_TC_CV(0), &lower);
lock();
lot-of-things();
unlock();
+ regmap_read(tc.regmap, ATMEL_TC_CV(1), &tmp);
lock();
lot-of-things();
unlock();
+ } while (upper != tmp);
+
+ return (upper << 16) | lower;
+}
I suggest to look what is in 'lot-of-things()' and especially what is doing
regcache_read().
May be you can reconsider the regmap? This driver is the only one use the
regmap AFAICT and I don't think it is adequate.
> > > > Can you explain why we have two clocks here?
> > > >
> > >
> > > Each channel have its clock, I can add a comment if you want.
> >
> > I don't understand. Why do we have two clocks?
> >
> > One channel is driven by one clock and the second one takes the overflow
> > signal
> > from the first one, so no second clock is involved there, no?
> >
>
> Those are the peripheral clocks, they are not used by the counters but
> used to be able to read/write the registers.
Mmh, strange. Why is the clk[0]'s rate used in this case?
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