linux-kernel - Re: [rfc]pwm: add xilinx pwm driver

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Message-ID: <16896948.U7hTsgDub4@wuerfel>
Date:	Thu, 15 May 2014 18:30:13 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	monstr@...str.eu
Cc:	Bart Tanghe <bart.tanghe@...masmore.be>, thierry.reding@...il.com,
	michal.simek@...inx.com, robh+dt@...nel.org, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, rob@...dley.net, grant.likely@...aro.org,
	linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [rfc]pwm: add xilinx pwm driver

On Thursday 15 May 2014 15:56:03 Michal Simek wrote:
> IP is configurable as is normal for us.
> You can select IP with just one timer.
> It means register locations for specific timer are fixed.
> http://www.xilinx.com/support/documentation/ip_documentation/xps_timer.pdf
> 
> timer0 - offset 0x0
> timer1 - offset 0x10 (doesn't need to be synthesized)
> 
> There is one interrupt for both timers.
> 
> Timers can be as timers (up/down count/ reload with or without IRQs)
> But then one options is to use both timers and generate PWM signal.
> From full ip description in DT you can see xlnx,gen0-assert = <1>;
> which can suggest that this IP can output PMW signal.
> (We can also detect if PWM0 signal is connected just to be sure
> that PWM can be enabled).
> 
> There is also capture trigger mode where external signal start/stop
> timer counting.
> 
> It means there are 3 modes - timer, capture and PWM.
> Timer (clocksource, clockevent) requires specific handling,
> PWM has own subsystem and not sure if there is any subsystem for
> capture mode. Is there any?

I don't think so. Possibly somewhere in IIO.

> Not every timer configuration is suitable for all things
> but fully configured timer can be used in all 3 modes.
> 
> 
> I think that make sense to register and map all timers in the system
> and classify them with flags (like can't be used for capture or PMW
> if there are not outputs connected) and then use the best
> timer for clocksource and clockevent. The best candidate is IP
> with the lowest IRQ number in dual timer mode but in general
> whatever timer can be used that's why we can't probably avoid
> timer specification in DT.
> In spite of this smells because you are saying via DT
> to Linux which timer should be used for what purpose.

We had discussions about this in the past, but I don't remember the
outcome of that.

	Arnd
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