Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932995AbcLGTFN (ORCPT ); Wed, 7 Dec 2016 14:05:13 -0500 Received: from mga11.intel.com ([192.55.52.93]:44655 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753629AbcLGTFJ (ORCPT ); Wed, 7 Dec 2016 14:05:09 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,315,1477983600"; d="scan'208";a="1078844672" Date: Wed, 7 Dec 2016 14:15:04 -0500 From: Keith Busch To: Jon Mason Cc: Bjorn Helgaas , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] PCI: Change default MPS behavior Message-ID: <20161207191504.GA22478@localhost.localdomain> References: <1481070027-9096-1-git-send-email-jon.mason@broadcom.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1481070027-9096-1-git-send-email-jon.mason@broadcom.com> User-Agent: Mutt/1.7.0 (2016-08-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 819 Lines: 13 On Tue, Dec 06, 2016 at 07:20:27PM -0500, Jon Mason wrote: > Not all systems have a BIOS or firmware to preconfigure the PCIE MPS > prior to Linux booting. Without any firmware to pre-setup the MPS, the > PCIE_BUS_DEFAULT will simply set everything to 0 (128b). This behavior > causes these systems to have non-optimal MPS values. To get around this > issue, change the default value of pcie_bus_config to be PCIE_BUS_SAFE. > This configures all systems to use better values for the MPS, at the > expense of any potential HW errata that might not like being changed. Wouldn't it be better to provide the kernel parameter to override the default on such platforms? The PCIE_BUS_SAFE sets the MPS to the lowest common denominator below the root port, which may be less optimal than the default behavior for hot plug.