linux-kernel - Re: [PATCH v2 02/16] arm64: dts: add sp804 timer node for Hi6220

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Date:	Mon, 4 Apr 2016 21:42:37 +0800
From:	Leo Yan <leo.yan@...aro.org>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Guodong Xu <guodong.xu@...aro.org>, Xu Wei <xuwei5@...ilicon.com>,
	Mark Rutland <mark.rutland@....com>,
	Rob Herring <robh@...nel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	XinWei Kong <kong.kongxinwei@...ilicon.com>
Subject: Re: [PATCH v2 02/16] arm64: dts: add sp804 timer node for Hi6220

On Mon, Apr 04, 2016 at 01:21:00PM +0200, Linus Walleij wrote:
> On Mon, Apr 4, 2016 at 3:43 AM, Leo Yan <leo.yan@...aro.org> wrote:
> > On Sun, Apr 03, 2016 at 09:23:42PM +0200, Linus Walleij wrote:
> >> On Sat, Apr 2, 2016 at 11:29 AM, Guodong Xu <guodong.xu@...aro.org> wrote:
> 
> >> By chance the code in the driver will allow just one clock and
> >> will then assume that both the bus to the timer and the timer
> >> itself is clocked from the same clock. But I highly doubt that this
> >> is the case.
> >
> > This patch has been sent out for review previously [1]; So I refered
> > other platforms and changed to only enable apb bus clock due I have
> > not found timer enabling bits in Hi6220's spec.
> 
> This is not about enabling/disabling the clock(s) to the timer.
> It doesn't matter if these clocks are always on.
> 
> It is about determining the *frequency* of the timers.
> 
> It is vital that the timer driver get the right frequency of the clock
> to the block from the clock implementation, and I do not think
> it is the same as the "apb_pclk".

Thanks for reminding. Fortunately, apb_pclk and timer's clock are
same, all of them's rate are 19.2MHz.

> The thing is that of course "any frequency" will
> work but what you will notice is that the timer runs very
> weirdly compared to wall-clock time unless the right clock
> yielding the right frequency has been specified here.

Compared sp804 timer counter with wall-clock (which is using ARM's
arch timer), and confirmed that sp804 timer's counter register is
decreasing with rate 19.2MHz.

Also have checked Hi6220's spec, there have no timer's dediated clock
enabling bits. This is the reason before I only registered one clock.
So according to you and Rob's comments, how about change as below?

		dual_timer0: timer@...08000 {
			compatible = "arm,sp804", "arm,primecell";
			reg = <0x0 0xf8008000 0x0 0x1000>;
			interrupts = <GIC_SPI 14 IRQ_TYPE_LEVEL_HIGH>,
				     <GIC_SPI 15 IRQ_TYPE_LEVEL_HIGH>;
			clocks = <&ao_ctrl HI6220_TIMER0_PCLK>,
				 <&ao_ctrl HI6220_TIMER0_PCLK>,
				 <&ao_ctrl HI6220_TIMER0_PCLK>;
			clock-names = "apb_pclk", "apb_pclk", "apb_pclk";
		};

Thanks,
Leo Yan