linux-kernel - Re: [PATCH (sh-2.6) 1/4] clksource: Generic timer infrastructure

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Date:	Tue, 1 Mar 2011 17:43:19 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Stuart Menefy <stuart.menefy@...com>
Cc:	Peppe CAVALLARO <peppe.cavallaro@...com>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>
Subject: Re: [PATCH (sh-2.6) 1/4] clksource: Generic timer infrastructure

On Tuesday 01 March 2011, Stuart Menefy wrote:
> On 24/02/11 17:20, Arnd Bergmann wrote:
>
> > Why is this code useful to you? In the scenarios I've seen, the
> > board can always assign a timer to a specific device in a fixed
> > way that can be describe in a board file or device tree.
> 
> What we were trying to do was separate the code which actually manipulates
> the timer hardware from the code which wants that timer service. In this
> case it was a network device driver which is used on multiple SoC devices,
> while the timer hardware tends to differ from device to device.

Right. It certainly makes sense to have an well-defined interface between
the user and the provider of a timer interrupt.

> The other user of this code which we have is an OProfile driver, which
> with this change can now be independent of the hardware it is running on,
> while the previous version manipulated the timer hardware directly.

Ok.

> > Also, what is the difference between this and clkdev?
> 
> clkdev can be used to find a struct clk, which is fine if you just want to
> read the time. In this instance we want to get interrupts from the timer
> hardware, which isn't supported by the clk infrastructure.

(adding Russell to Cc)

Is this something that could sensibly be added to clk/clkdev?

> If anything this duplicates clockevents. The main reason for not using
> clockevents was that nobody else does! Currently clockevents are
> used strictly for time keeping within the kernel, and most architectures
> only register those which are intended to be used for this purpose.
> We felt a bit nervous about adding code to register all the device's timers
> as clockevents, and having the network device driver pick up one of those
> for its own use.

I see. Using a clock_event_device for anything but the system timer tick
is currently not supported, so it certainly would not be straightforward.

I think you need a bit of both, clkdev and clockevent. I think the
options you have are:

1. copy the clkdev code to make it possible to associate a device with
a periodic timer.
2. extend the clkdev/clk code to handle periodic interrupts and reuse
the infrastructure there.
3. extend the clockevent code to make it possible for regular device
drivers to use a clockevent source.

I have no idea which makes the most sense (or if there are other ideas).
Maybe Russell, Thomas or John can comment.

	Arnd
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