From mboxrd@z Thu Jan 1 00:00:00 1970 From: Guenter Roeck Subject: Re: [PATCH 1/2] net: dsa: mv88e6xxx: sleep in _mv88e6xxx_stats_wait Date: Fri, 10 Jul 2015 11:36:23 -0700 Message-ID: <20150710183623.GB19854@roeck-us.net> References: <1436547449-26927-1-git-send-email-vivien.didelot@savoirfairelinux.com> <20150710171027.GB6585@groeck-UX31A> <1055594065.210272.1436552447158.JavaMail.zimbra@savoirfairelinux.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev , David , Andrew Lunn , linux-kernel , kernel To: Vivien Didelot Return-path: Received: from bh-25.webhostbox.net ([208.91.199.152]:59905 "EHLO bh-25.webhostbox.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933068AbbGJSg0 (ORCPT ); Fri, 10 Jul 2015 14:36:26 -0400 Content-Disposition: inline In-Reply-To: <1055594065.210272.1436552447158.JavaMail.zimbra@savoirfairelinux.com> Sender: netdev-owner@vger.kernel.org List-ID: Hi Vivien, On Fri, Jul 10, 2015 at 02:20:47PM -0400, Vivien Didelot wrote: > > > > is this really beneficial and/or needed ? > > Except using existing generic code, no. > > > It adds at least 1ms delay to a loop which did not have any delay at > > all unless the register read itself was sleeping. > > I must have missed where is the benefit from spin reading 10 times this > register, rather than sleeping 1ms between tests. Does this busy bit > behaves differently from the phy, atu, scratch, or vtu busy bits? > Benefit is reaction time, mostly. If the result isn't ready after the first spin, the new code path adds a mandatory 1-2ms delay. This could add up to a lot if that kind of retry is seen a lot. I don't now if there is a specific time limit for this busy bit, and/or if it behaves differently than the others in terms of timing. > > Is the original function seen to return a timeout error under some > > circumstances ? > > I didn't experience it myself, but I guess it may happen. In addition to > that, the current implementation doesn't check eventual read error. > That's why I saw a benefit in using _mv88e6xxx_wait(). Checking for a read error (or a timeout) is definitely a good thing. I could also imagine that, for example, a "clear statistics" request takes more time than currently supported. This is why I asked if you had seen a timeout with the old code. Personally I'd rather leave the wait loop alone and only introduce error checking unless there is a reason to introduce a sleep, but I'd like to hear Andrew's and/or Florian's opinion. Thanks, Guenter