linux-kernel - Re: [PATCH 09/15] x86, amd: add accessor for number of cores per compute unit

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Message-ID: <55E5CD1C.4070502@roeck-us.net>
Date:	Tue, 01 Sep 2015 09:06:52 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Borislav Petkov <bp@...en8.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Huang Rui <ray.huang@....com>,
	Borislav Petkov <bp@...e.de>, Jean Delvare <jdelvare@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andreas Herrmann <herrmann.der.user@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org,
	x86@...nel.org,
	Andreas Herrmann <herrmann.der.user@...glemail.com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>
Subject: Re: [PATCH 09/15] x86, amd: add accessor for number of cores per
 compute unit

On 09/01/2015 08:56 AM, Borislav Petkov wrote:
> On Mon, Aug 31, 2015 at 02:24:08PM -0700, Guenter Roeck wrote:
>> That is a matter of driver implementation. Very commonly hwmon drivers
>> implement value caching, where data from the device is only read at
>> minimum intervals, and the cached values are reported if the information
>> is polled too rapidly.
>
> I don't see how we can use cached values in that case as they'd be
> basically a lie as to how much power the CU/processor has consumed. I
> guess it would be fairer to the user to warn instead and not issue any
> values or to simply delay the read to a min timeout...
>

This is all a matter of ABI description. I personally don't see the
harm in caching values for up to <n> milliseconds and documenting that
readings may reflect power consumption from <n> milliseconds ago.
However, I don't mind any other solution either if that is considered
unacceptable, as long as that solution doesn't break the ABI.

Guenter

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