linux-kernel - Re: [PATCH v2 2/2] powercap/rapl: reduce ipi calls

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Date:	Wed, 13 Jan 2016 13:54:43 -0800
From:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:	Borislav Petkov <bp@...en8.de>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	X86 Kernel <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 2/2] powercap/rapl: reduce ipi calls

On Wed, 2016-01-13 at 22:26 +0100, Borislav Petkov wrote:

[Cut]

> 
> 	rmwmsrl_safe_on_cpu(policy->cpu, MSR_IA32_PERF_CTL,
> 			    INTEL_PERF_CTL_MASK,
> 			    (u32)sfi_cpufreq_array[next_perf_state].ctr
> l_val & INTEL_PERF_CTL_MASK);
> 
> Yikes!
> 
> So yes, it can work but it is ugly, hard to parse and use, not
> generic
> enough, etc, etc.
> 
> So thanks, but no thanks.
> 
I agree, in some cases it will not make much sense to use read-
modify_write calls, the user may decide whether it makes sense or not.
But such interface is not new to Linux kernel:

regmap_update_bits(), which is referenced for 346 times.

Are you saying that any such calls are not useful?

Thanks,
Srinivas