Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754139AbaBQVmf (ORCPT ); Mon, 17 Feb 2014 16:42:35 -0500 Received: from perceval.ideasonboard.com ([95.142.166.194]:36838 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751154AbaBQVme (ORCPT ); Mon, 17 Feb 2014 16:42:34 -0500 From: Laurent Pinchart To: Magnus Damm Cc: Mark Rutland , Laurent Pinchart , "linux-sh@vger.kernel.org" , Thomas Gleixner , Daniel Lezcano , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 23/27] clocksource: sh_cmt: Add DT support Date: Mon, 17 Feb 2014 22:43:41 +0100 Message-ID: <3240866.imZR2oMAtf@avalon> User-Agent: KMail/4.11.5 (Linux/3.10.25-gentoo; KDE/4.11.5; x86_64; ; ) In-Reply-To: References: <1392339605-20691-1-git-send-email-laurent.pinchart+renesas@ideasonboard.com> <35710038.vGLtzuIoLe@avalon> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/