Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935724AbcLOJJA (ORCPT ); Thu, 15 Dec 2016 04:09:00 -0500 Received: from mga14.intel.com ([192.55.52.115]:33695 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935681AbcLOJIw (ORCPT ); Thu, 15 Dec 2016 04:08:52 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,351,1477983600"; d="scan'208";a="798245229" From: Jani Nikula To: Nicholas Mc Guire , Daniel Vetter Cc: Shashank Sharma , David Airlie , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Nicholas Mc Guire Subject: Re: [PATCH] drm/i915: use udelay for very short delays In-Reply-To: <1481776147-23041-1-git-send-email-hofrat@osadl.org> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo References: <1481776147-23041-1-git-send-email-hofrat@osadl.org> Date: Thu, 15 Dec 2016 11:08:49 +0200 Message-ID: <87wpf1pnj2.fsf@intel.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1681 Lines: 50 On Thu, 15 Dec 2016, Nicholas Mc Guire wrote: > Even on fast systems a 2 microsecond delay is most likely more efficient > as a busy-wait loop. The overhead of a hrtimer does not seem warranted - > change this to a udelay(2). Similar concerns as in [1]. We don't need the accuracy of udelay() here, so this boils down to which is the better use of CPU. We could probably relax the max delay more if that was helpful. But I'm not immediately sold on "is most likely more efficient" which sounds like a gut feeling. I'm sorry it's not clear in my other reply that I do appreciate addressing incorrect/silly use of usleep_range(); I'm just not (yet) convinced udelay() is the answer. BR, Jani. [1] http://lkml.kernel.org/r/8737hpr32a.fsf@intel.com > > Signed-off-by: Nicholas Mc Guire > --- > > Problem found by coccinelle: > > Patch was compile tested with: x86_64_defconfig (implies CONFIG_DRM_I915) > > Patch is against 4.9.0 (localversion-next is next-20161214) > > drivers/gpu/drm/i915/intel_dsi.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/i915/intel_dsi.c b/drivers/gpu/drm/i915/intel_dsi.c > index 5b72c50..19fe86b 100644 > --- a/drivers/gpu/drm/i915/intel_dsi.c > +++ b/drivers/gpu/drm/i915/intel_dsi.c > @@ -379,7 +379,7 @@ static void bxt_dsi_device_ready(struct intel_encoder *encoder) > val &= ~ULPS_STATE_MASK; > val |= (ULPS_STATE_ENTER | DEVICE_READY); > I915_WRITE(MIPI_DEVICE_READY(port), val); > - usleep_range(2, 3); > + udelay(2); > > /* 3. Exit ULPS */ > val = I915_READ(MIPI_DEVICE_READY(port)); -- Jani Nikula, Intel Open Source Technology Center