linux-kernel - Re: [PATCH 42/48] clocksource/drivers: Add a new driver for the Atmel ARM TC blocks

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Message-ID: <20170516115916.ip3ea34bofg3jpuo@piout.net>
Date:   Tue, 16 May 2017 13:59:16 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Nicolas Ferre <nicolas.ferre@...el.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 42/48] clocksource/drivers: Add a new driver for the
 Atmel ARM TC blocks

Hi Daniel,

Almost one year later, I'm back on that topic.

On 24/06/2016 at 12:07:01 +0200, Daniel Lezcano wrote:
> > diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> > index 47352d25c15e..ff7f4022c749 100644
> > --- a/drivers/clocksource/Kconfig
> > +++ b/drivers/clocksource/Kconfig
> > @@ -258,6 +258,19 @@ config ATMEL_ST
> >   	select CLKSRC_OF
> >   	select MFD_SYSCON
> > 
> > +config ATMEL_ARM_TCB_CLKSRC
> > +	bool "TC Block Clocksource"
> 
> The Kconfig options are set now with the COMPILE_TEST option in order to
> increase the compilation test coverage.
> 
> Please, add bool "TC Block Clocksource" if COMPILE_TEST, ...
> 
> > +	select REGMAP_MMIO
> > +	depends on GENERIC_CLOCKEVENTS
> > +	depends on SOC_AT91RM9200 || SOC_AT91SAM9 || SOC_SAMA5
> > +	default SOC_AT91RM9200 || SOC_AT91SAM9 || SOC_SAMA5
> 
> ... remove these dependencies and let the SoC's Kconfig to select the timer
> like the other timers are.
> 

The main issue with what you suggest is that it removes the possibility
to not compile the driver. This may be interesting for people using the
PIT as the clocksource/clockevent and the TCBs for something else.

> > +static cycle_t tc_get_cycles(struct clocksource *cs)
> > +{
> > +	unsigned long	flags;
> > +	u32		lower, upper, tmp;
> > +
> > +	raw_local_irq_save(flags);
> > +	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);
> > +
> > +	raw_local_irq_restore(flags);
> 
> Why is this lock needed ?
> 

I've taken that from the old driver, I'm not sure this is needed. Maybe
to lower the chance to have upper != tmp.

> > +	return (upper << 16) | lower;
> > +}
> > +


> > +static int tcb_clkevt_oneshot(struct clock_event_device *dev)
> > +{
> > +	int ret;
> > +
> > +	if (tc.irq_requested)
> > +		return 0;
> > +
> > +	ret = request_irq(tc.irq, tc_clkevt_irq, IRQF_TIMER | IRQF_SHARED,
> > +			  "tcb_clkevt", &tc);
> > +	if (!ret)
> > +		tc.irq_requested = true;
> 
> The legacy driver checks clockevent_state_detached() and disables the clock.
> 

This feature is different from the legacy driver. Here, the driver is
using a single TCB channel for both clocksource and clockevent while the
legacy driver always uses at least two channels.

> Why is 'irq_requested' and request_irq/free_irq cleaner ?
> 
> Isn't there a configuration with the TCB register to disable the clockevent
> only ?
> 

I'll try something cleaner anyway.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com