Re: [PATCH 46/58] clocksource/drivers: Add a new driver for the Atmel ARM TC blocks

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On Tue, Jun 06, 2017 at 08:05:59PM +0200, Alexandre Belloni wrote:
> On 06/06/2017 at 17:21:05 +0200, Daniel Lezcano wrote:
> > On Tue, May 30, 2017 at 11:51:27PM +0200, Alexandre Belloni wrote:
> > > Add a driver for the Atmel Timer Counter Blocks. This driver provides a
> > > clocksource and a clockevent device. The clockevent device is linked to the
> > > clocksource counter and so it will run at the same frequency.
> > 
> > Where is the clockevent in this driver?
> > 
> 
> That's a leftover from v1, it seems I forgot to rework that commit
> message.
> 
> > It seems the cutting out of this driver is a bit fuzzy and hard to follow.
> > 
> > Please re-org the changes in a logical manner when resubmitting.
> > 
> 
> I can submit the whole driver as a single patch if that is easier for
> you to review.

As you wish. A single driver or a split into consistent parts.

> > > 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/

> > > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
> > > ---
> > >  drivers/clocksource/Kconfig                 |  13 ++
> > >  drivers/clocksource/Makefile                |   3 +-
> > >  drivers/clocksource/timer-atmel-tcbclksrc.c | 252 ++++++++++++++++++++++++++++
> > 
> > As it is a clksrc + clkevt, please change the name to something more adequate:
> > 
> > eg. timer-tcb.c
> > 
> > >  3 files changed, 267 insertions(+), 1 deletion(-)
> > >  create mode 100644 drivers/clocksource/timer-atmel-tcbclksrc.c
> > > 
> > > diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> > > index 545d541ae20e..cbd710db3c09 100644
> > > --- a/drivers/clocksource/Kconfig
> > > +++ b/drivers/clocksource/Kconfig
> > > @@ -418,6 +418,19 @@ config ATMEL_ST
> > >  	help
> > >  	  Support for the Atmel ST timer.
> > >  
> > > +config ATMEL_ARM_TCB_CLKSRC
> > > +	bool "TC Block Clocksource"
> > > +	select REGMAP_MMIO
> > > +	depends on GENERIC_CLOCKEVENTS
> > > +	depends on SOC_AT91RM9200 || SOC_AT91SAM9 || SOC_SAMA5 || COMPILE_TEST
> > > +	default SOC_AT91RM9200 || SOC_AT91SAM9 || SOC_SAMA5
> > > +	help
> > > +	  Select this to get a high precision clocksource based on a
> > > +	  TC block with a 5+ MHz base clock rate.
> > > +	  On platforms with 16-bit counters, two timer channels are combined
> > > +	  to make a single 32-bit timer.
> > > +	  It can also be used as a clock event device supporting oneshot mode.
> > > +
> > >  config CLKSRC_METAG_GENERIC
> > >  	def_bool y if METAG
> > >  	help
> > > diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
> > > index 2b5b56a6f00f..53a0b40e0db2 100644
> > > --- a/drivers/clocksource/Makefile
> > > +++ b/drivers/clocksource/Makefile
> > > @@ -2,7 +2,8 @@ obj-$(CONFIG_CLKSRC_PROBE)	+= clksrc-probe.o
> > >  obj-$(CONFIG_CLKEVT_PROBE)	+= clkevt-probe.o
> > >  obj-$(CONFIG_ATMEL_PIT)		+= timer-atmel-pit.o
> > >  obj-$(CONFIG_ATMEL_ST)		+= timer-atmel-st.o
> > > -obj-$(CONFIG_ATMEL_TCB_CLKSRC)	+= tcb_clksrc.o
> > > +obj-$(CONFIG_ATMEL_TCB_CLKSRC) += tcb_clksrc.o
> > > +obj-$(CONFIG_ATMEL_ARM_TCB_CLKSRC)	+= timer-atmel-tcbclksrc.o
> > >  obj-$(CONFIG_X86_PM_TIMER)	+= acpi_pm.o
> > >  obj-$(CONFIG_SCx200HR_TIMER)	+= scx200_hrt.o
> > >  obj-$(CONFIG_CS5535_CLOCK_EVENT_SRC)	+= cs5535-clockevt.o
> > > diff --git a/drivers/clocksource/timer-atmel-tcbclksrc.c b/drivers/clocksource/timer-atmel-tcbclksrc.c
> > > new file mode 100644
> > > index 000000000000..f18d177bfdea
> > > --- /dev/null
> > > +++ b/drivers/clocksource/timer-atmel-tcbclksrc.c
> > > @@ -0,0 +1,252 @@
> > > +#include <linux/clk.h>
> > > +#include <linux/clockchips.h>
> > > +#include <linux/clocksource.h>
> > > +#include <linux/interrupt.h>
> > > +#include <linux/kernel.h>
> > > +#include <linux/mfd/syscon.h>
> > > +#include <linux/of_irq.h>
> > > +#include <linux/regmap.h>
> > > +#include <linux/sched_clock.h>
> > > +#include <soc/at91/atmel_tcb.h>
> > > +
> > > +static struct atmel_tcb_clksrc {
> > > +	char name[20];
> > 
> > 	^^^^^^^^^^^^^^
> > This field is pointless.
> > 
> 
> You mean you don't like how it is used? Or you don't think having the
> timer full name is useful?

The field is not needed, the only place where it is used is where we affect it.
 
> > > +	struct clocksource clksrc;
> > 
> > Why is this field defined and passed around to functions which do not use it?
> > 
> > Please consider using clocksource_mmio_init() and remove this field.
> > 
> 
> Well, this doesn't work with a regmap so it doesn't fit.
>
> > > +	struct regmap *regmap;
> > > +	struct clk *clk[2];
> > 
> > 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?

> > > +	int channels[2];
> > 
> > Instead of dealing with 2 channels and a costly bits shifting in the hot path,
> > why not use a single channel with a different wrap up? IOW mask is 16 or 32.
> > 
> > The resulting code will be simpler, nicer and perhaps more efficient if you
> > save the tc_get_cycles() loop?
> > 
> 
> I think the rationale behind it is that 16 bits at 5MHz makes it
> wrap every 13ms which is too fast to be useful.

Ok, I see.

> > > +	int bits;
> > 
> > 	^^^^^^^^
> > 
> > This field is pointless.
> > 
> > > +	int irq;
> > 
> > irq belongs to clockevents changes.
> > 
> > > +	bool registered;
> > 
> > What is the purpose of this registered boolean?
> > 
> 
> The main reason is that RobH doesn't want to have the use (clocksource
> or clockevent) of the timer in the DT so when probing a timer, I need to
> know whether I already have a clocksource to decide when it is time to
> register a clockevent.

Yes, we had this discussion some weeks ago.

This registered hack forces the DT to define first the clocksource, then the
clockevent.

So, I suggest you fold the timer definition into a single one like the other
drivers.


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