Date:	Mon, 17 Feb 2014 22:43:41 +0100
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Magnus Damm <magnus.damm@...il.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 23/27] clocksource: sh_cmt: Add DT support

Hi Magnus,

On Monday 17 February 2014 10:48:55 Magnus Damm wrote:
> On Mon, Feb 17, 2014 at 10:45 AM, Laurent Pinchart wrote:
> > On Saturday 15 February 2014 02:22:00 Magnus Damm wrote:
> >> On Sat, Feb 15, 2014 at 1:12 AM, Laurent Pinchart wrote:
> >> > On Saturday 15 February 2014 01:01:30 Magnus Damm wrote:
> >> >> On Sat, Feb 15, 2014 at 12:53 AM, Laurent Pinchart wrote:
> >> >> > On Friday 14 February 2014 10:58:22 Mark Rutland wrote:
> >> >> >> On Fri, Feb 14, 2014 at 01:00:01AM +0000, Laurent Pinchart wrote:
> >> >> >> > +Channels Optional Properties:
> >> >> >> > +
> >> >> >> > +  - clock-source-rating: rating of the timer as a clock source
> >> >> >> > device.
> >> >> >> > +  - clock-event-rating: rating of the timer as a clock event
> >> >> >> > device.
> >> >> >> 
> >> >> >> This feels like a leak of Linux internals. Why do you need this?
> >> >> > 
> >> >> > You're right, it is. The clock source and clock event ratings are
> >> >> > currently configured through platform data, I'll need to find a way
> >> >> > to compute them in the driver instead.
> >> >> 
> >> >> That would be very good!
> >> > 
> >> > Any pointer would be appreciated :-) How did you compute the various
> >> > ratings used in platform data all over the place ?
> >> 
> >> Historically we used the rating to select between CMT and TMU. For
> >> clock sources I suppose you also have the jiffy rating to consider.
> >> And for the SMP parts we have ARM IP for TWD and arch timers that have
> >> their ratings too. So you need to check all the timers on a particular
> >> system and consider what you want to have operating by default. The
> >> ARM IP timers should be preferred if available. For clock sources the
> >> rule is probably the higher resolution the better.
> >> 
> >> >> > There's still one piece of Linux-specific data I need though, as I
> >> >> > need to specify for each channel whether to use it as a clock source
> >> >> > device, a clock event device, both of them or none. That's
> >> >> > configuration information that needs to be provided somehow.
> >> >> 
> >> >> I think you can decide clock source or clock event assignment based on
> >> >> number of channels available. If you have only a single channel then
> >> >> both clock event and clock source need to be supported. Otherwise use
> >> >> one channel for clock source and the rest for clock events.
> >> > 
> >> > That won't match the current situation. Look at CMT0 in r8a7790 for
> >> > instance. There's two hardware channels available, and we only use the
> >> > first one, for clock events only.
> >> 
> >> You are correct. The reason for that is that the CMT driver today is
> >> optimized for combined clock event and clock source operation.
> >> 
> >> Historically the hardware it initially was written for (sh-mobile on
> >> the SH arch) only had a single timer channel so combined operation was
> >> required for tickless to work. But since you're asking how to allocate
> >> channels then I propose checking numbers of channels available and go
> >> from there. With that the r8a7790 support can only get better. =)
> >> 
> >> >> This is probably out of scope for this DT conversion, but it would be
> >> >> neat if you somehow could specify the CPU affinity for a channel to
> >> >> tie a clock event to an individual CPU core. This would make a a per-
> >> >> cpu timer unless I'm mistaken. But that's more of a software policy
> >> >> than anything else.
> >> > 
> >> > Yes, that's a configuration that needs to be specified somewhere. I
> >> > don't know where though.
> >> 
> >> As long as you have per-channel interrupts described in DT you can
> >> probably handle this in a generic way in the driver.
> > 
> > But how do we decide whether to use a single timer channel or one channel
> > per CPU ? Will the kernel use one clock event device per CPU
> > automatically ? I have to confess I have no idea how this works.
> 
> I guess that's the tricky bit about timer support, it is a mix of
> hardware description and software configuration. So it sounds to me
> that we need some kind of software configuration interface. But it can
> probably be considered when/if we add such kind of support to the
> driver. Probably out of scope for now.
> 
> Regardless it seems to me that the hardware description in DT doesn't
> need to care about this.

I'll revisit that later. Per-CPU timers is not a high priority for now, so 
I'll just return -EAGAIN :-)

Nonetheless, specifying which timer channel to use as a clock source and which 
channel to use as a clock event device might need to be specified in DT (or 
somewhere else, but I'm not sure what other options we have here).

-- 
Regards,

Laurent Pinchart
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