From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Monjalon Subject: Re: [PATCH] Revert "timer: get TSC frequency from /proc/cpuinfo" Date: Wed, 09 Apr 2014 14:23:44 +0200 Message-ID: <1530885.SuHPsB8m1S@xps13> References: <1396875448-14778-1-git-send-email-bruce.richardson@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: dev-VfR2kkLFssw@public.gmane.org To: Bruce Richardson Return-path: In-Reply-To: <1396875448-14778-1-git-send-email-bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces-VfR2kkLFssw@public.gmane.org Sender: "dev" 2014-04-07 13:57, Bruce Richardson: > This reverts commit da6fd0759cbeb5fc14991a79e40105b9f6b99059. > > The use of cpuinfo to determine the frequency of the TSC is not > advisable and leads to incorrect results when power management is > in use. This is because, while the TSC frequency does not change > in modern cpus with constant_tsc support, the frequency of the core, > and hence the frequency of the core reported by cpuinfo *does* change. > > Depending on the current frequency of core 0 when an application is > started, the EAL can get a wildly incorrect value for the TSC freq. > Since frequency is scaled down for power saving, any incorrect value > is likely to be lower than the default, which means that any delay > loops inside the code which rely on the TSC will be shorter than > planned. This can cause issues (reported on the mailing list by a number > of people) where ports are not initialized correctly due to delays being > too short. > > Signed-off-by: Bruce Richardson I agree that using CPU frequency for TSC calibration is wrong with constant_tsc support. The goal was to have a precise calibration but the result is the reverse because of constant TSC. Acked and applied. Thank you Bruce for catching this. -- Thomas