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[2001:1c00:c0c:fe00:fc7e:fd47:85c1:1ab3]) by smtp.gmail.com with ESMTPSA id h13sm10309833wml.45.2020.02.23.05.55.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 23 Feb 2020 05:55:05 -0800 (PST) Subject: Re: [PATCH v3 3/3] x86/tsc_msr: Make MSR derived TSC frequency more accurate To: Thomas Gleixner , Andy Shevchenko , Ingo Molnar Cc: Vipul Kumar , Vipul Kumar , Daniel Lezcano , Srikanth Krishnakar , Cedric Hombourger , Len Brown , Rahul Tanwar , Tony Luck , Gayatri Kammela , x86@kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20200207205456.113758-1-hdegoede@redhat.com> <20200207205456.113758-3-hdegoede@redhat.com> <87eev67xkc.fsf@nanos.tec.linutronix.de> From: Hans de Goede Message-ID: <36b1e1bb-cbb0-88c0-dab2-aa5e14233e63@redhat.com> Date: Sun, 23 Feb 2020 14:55:03 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <87eev67xkc.fsf@nanos.tec.linutronix.de> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 2/8/20 1:05 AM, Thomas Gleixner wrote: > Hans, > > Hans de Goede writes: >> @@ -120,11 +180,23 @@ unsigned long cpu_khz_from_msr(void) >> rdmsr(MSR_FSB_FREQ, lo, hi); >> index = lo & freq_desc->mask; >> >> - /* Map CPU reference clock freq ID(0-7) to CPU reference clock freq(KHz) */ >> - freq = freq_desc->freqs[index]; >> - >> - /* TSC frequency = maximum resolved freq * maximum resolved bus ratio */ >> - res = freq * ratio; >> + /* >> + * Note this also catches cases where the index points to an unpopulated >> + * part of muldiv, in that case the else will set freq and res to 0. >> + */ >> + if (freq_desc->muldiv[index].divider) { >> + freq = DIV_ROUND_CLOSEST(TSC_REFERENCE_KHZ * >> + freq_desc->muldiv[index].multiplier, >> + freq_desc->muldiv[index].divider); >> + /* Multiply by ratio before the divide for better accuracy */ >> + res = DIV_ROUND_CLOSEST(TSC_REFERENCE_KHZ * >> + freq_desc->muldiv[index].multiplier * >> + ratio, >> + freq_desc->muldiv[index].divider); > > What about: > > struct muldiv *md = &freq_desc->muldiv[index]; > > if (md->divider) { > tscref = TSC_REFERENCE_KHZ * md->multiplier; > freq = DIV_ROUND_CLOSEST(tscref, md->divider); > /* > * Multiplying by ratio before the division has better > * accuracy than just calculating freq * ratio > */ > res = DIV_ROUND_CLOSEST(tscref * ratio, md->divider); > > Hmm? That indeed looks nicer, I've prepared (and tested) a v4 with the suggested change, I'll send out v4 right after this email. Regards, Hans