linux-kernel - Re: [PATCH v5 2/6] clocksource/drivers: Add a new driver for the Atmel ARM TC blocks

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Date:   Wed, 20 Jun 2018 12:32:59 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexander Dahl <ada@...rsis.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] clocksource/drivers: Add a new driver for the
 Atmel ARM TC blocks

On 20/06/2018 12:07:00+0200, Thomas Gleixner wrote:
> > > > +static int tcb_clkevt_next_event(unsigned long delta,
> > > > +				 struct clock_event_device *d)
> > > > +{
> > > > +	u32 old, next, cur;
> > > > +
> > > > +	old = readl(tc.base + ATMEL_TC_CV(tc.channels[0]));
> > > > +	next = old + delta;
> > > > +	writel(next, tc.base + ATMEL_TC_RC(tc.channels[0]));
> > > > +	cur = readl(tc.base + ATMEL_TC_CV(tc.channels[0]));
> > > > +
> > > > +	/* check whether the delta elapsed while setting the register */
> > > > +	if ((next < old && cur < old && cur > next) ||
> > > > +	    (next > old && (cur < old || cur > next))) {
> > > > +		/*
> > > > +		 * Clear the CPCS bit in the status register to avoid
> > > > +		 * generating a spurious interrupt next time a valid
> > > > +		 * timer event is configured.
> > > > +		 */
> > > > +		old = readl(tc.base + ATMEL_TC_SR(tc.channels[0]));
> > > > +		return -ETIME;
> > > > +	}
> > > 
> > > Aarg. Doesn;t that timer block have a simple count down and fire mode?
> > > These compare equal timers suck.
> > 
> > It only counts up...
> 
> Have you tried to play with that waveform stuff?
> 

There are only a count up and count up then down modes. As the counter
value is in a read only register, the only configurable starting value
is 0 so it will always start by counting up. I'm pretty sure the up/down
mode will not help us.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com