From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: [PATCH] [RFC] clocksource: improve GENERIC_CLOCKEVENTS dependency Date: Tue, 12 Sep 2017 10:10:13 +0100 Message-ID: <20170912091012.GD20805@n2100.armlinux.org.uk> References: <20170905150504.1720954-1-arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-m68k-owner@vger.kernel.org List-Id: linux-m68k@vger.kernel.org To: Linus Walleij Cc: Arnd Bergmann , Daniel Lezcano , Thomas Gleixner , Randy Dunlap , Geert Uytterhoeven , "linux-m68k@lists.linux-m68k.org" , linux-ia64@vger.kernel.org, Tony Luck , Fenghua Yu , Ding Tianhong , Mark Rutland , Marc Zyngier , Vineet Gupta , "linux-kernel@vger.kernel.org" On Tue, Sep 12, 2017 at 10:09:51AM +0200, Linus Walleij wrote: > For ARM we now have two subarchs not using generic clockevents: > RISC PC and EBSA110. > > I think Russell stated these two cannot be converted to generic clockevents > because of hardware limitations I guess, no timer interrupt, simply, which > means no clockevents, or unreliable or not granular enough timers. > > IIUC the SA110 does not contain the built-in SoC goodies of the SA1100 > so it needs external timer blocks, and those two machines don't have > good enough timers. That's hardly surprising because SA1100 is a SoC, SA110 is just a CPU, containing no peripherals at all. EBSA110 only has one usable timer, which must be programmed to produce a regular timer tick to the OS: it's no good trying to double up the clocksource and a periodic clockevent onto one counter register - the clock source will see the same timer value +/- interrupt latency, and in any case it won't wrap in a power-of-2 manner. This breaks the assumptions behind the clocksource and timekeeping code, which are that we have a timer that wraps in a power-of-2 manner, and which takes much longer than the desired period to wrap. I think RiscPC may be convertable as there are two timers, and I think the second timer is unused (so could be programmed to the requirements of a clocksource) but is there much reason to bother given the EBSA110? I think there isn't. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up