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Date: Tue, 25 Aug 2020 09:20:33 +0300 From: Artem Bityutskiy <dedekind1@...il.com> To: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org> Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>, LKML <linux-kernel@...r.kernel.org>, Doug Smythies <dsmythies@...us.net> Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs On Mon, 2020-08-24 at 19:42 +0200, Rafael J. Wysocki wrote: > From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com> > > Make the energy_performance_preference policy attribute in sysfs > always return the last EPP value written to it instead of the one > currently in the HWP Request MSR to avoid possible confusion when > the performance scaling algorithm is used in the active mode with > HWP enabled (in which case the EPP is forced to 0 regardless of > what value it has been set to via sysfs). Why is this a good idea, I wonder. If there was a prior discussion, please, point to it. The general approach to changing settings via sysfs is often like this: 1. Write new value. 2. Read it back and verify that it is the same. Because there is no better way to verify that the kernel "accepted" the value. Let's say I write 'balanced' to energy_performance_preference. I read it back, and it contains 'balanced', so I am happy, I trust the kernel changed EPP to "balanced". If the kernel, in fact, uses something else, I want to know about it and have my script fail. Why caching the value and making my script _think_ it succeeded is a good idea. In other words, in my usage scenarios at list, I prefer kernel telling the true EPP value, not some "cached, but not used" value. Artem.
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