Subject: Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early
 in boot
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com,
        john.stultz@linaro.org, sboyd@codeaurora.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org,
        tglx@linutronix.de, hpa@zytor.com, douly.fnst@cn.fujitsu.com,
        Peter Zijlstra <peterz@infradead.org>,
        Dou Liyang <douly.fnst@cn.fujitsu.com>
References: <1504116205-355281-1-git-send-email-pasha.tatashin@oracle.com>
 <1504116205-355281-2-git-send-email-pasha.tatashin@oracle.com>
 <20170927144506.GU20805@n2100.armlinux.org.uk>
From: Pasha Tatashin <pasha.tatashin@oracle.com>
Message-ID: <28c575e4-daa1-a959-e9a5-20f05033a6f0@oracle.com>
Date: Wed, 27 Sep 2017 13:10:35 -0400
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Hi Russell,

This might be so for ARM, and in fact if you look at my SPARC 
implementation, I simply made source clock initialize early, so regular 
sched_clock() is used. As on SPARC, we use either %tick or %stick 
registers with frequency determined via OpenFrimware. But, on x86 there 
are dozen ways clock sources are setup, and some of them available quiet 
late in boot because of various dependencies. So, my early clock 
initialization for x86 (and expendable to other platforms with unstable 
clocks) is to make it available when TSC is available, which is 
determined by already existing kernel functionality in 
simple_udelay_calibration().

My goal was not to introduce any regressions to the already complex (in 
terms of number of branches and loads) sched_clock_cpu(), therefore I 
added a new function and avoided any extra branches through out the life 
of the system. I could mitigate some of that by using static branches, 
but imo the current approach is better.

Pasha