From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter De Schrijver Subject: Re: [RFC PATCH 3/3] clk: tegra: Implement Tegra124 shared/cbus clks Date: Thu, 15 May 2014 13:17:00 +0300 Message-ID: <20140515101700.GH15168@tbergstrom-lnx.Nvidia.com> References: <1399990023-30318-1-git-send-email-pdeschrijver@nvidia.com> <1399990023-30318-4-git-send-email-pdeschrijver@nvidia.com> <53725FED.7050303@wwwdotorg.org> <20140514142739.GA8612@ulmo> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-tegra-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Andrew Bresticker Cc: Thierry Reding , Stephen Warren , Mike Turquette , Prashant Gaikwad , Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Arnd Bergmann , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: devicetree@vger.kernel.org On Wed, May 14, 2014 at 07:58:26PM +0200, Andrew Bresticker wrote: > On Wed, May 14, 2014 at 7:27 AM, Thierry Reding > wrote: > > As for shared clocks I'm only aware of one use-case, namely EMC scaling. > > Using clocks for that doesn't seem like the best option to me. While it > > can probably fix the immediate issue of choosing an appropriate > > frequency for the EMC clock it isn't a complete solution for the problem > > that we're trying to solve. From what I understand EMC scaling is one > > part of ensuring quality of service. The current implementations of that > > seems to abuse clocks (essentially one X.emc clock per X clock) to > > signal the amount of memory bandwidth required by any given device. But > > there are other parts to the puzzle. Latency allowance is one. The value > > programmed to the latency allowance registers for example depends on the > > EMC frequency. > > > > Has anyone ever looked into using a different framework to model all of > > these requirements? PM QoS looks like it might fit, but if none of the > > existing frameworks have what we need, perhaps something new can be > > created. > > On Exynos we use devfreq, though in that case we monitor performance > counters to determine how internal buses should be scaled - not sure > if Tegra SoCs have similar counters that could be used for this > purpose. It seems like EMC scaling would fit nicely within the PM QoS > framework, perhaps with a new PM_QOS_MEMORY_THROUGHPUT class. We do have counters, however, counters are reactive which is a problem for some isochronous clients (eg. display). Counters also don't solve the latency problem. Cheers, Peter.