linux-kernel - Re: [RFC] PCI: Change default MPS behavior

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Message-ID: <20161207191504.GA22478@localhost.localdomain>
Date:   Wed, 7 Dec 2016 14:15:04 -0500
From:   Keith Busch <keith.busch@...el.com>
To:     Jon Mason <jon.mason@...adcom.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] PCI: Change default MPS behavior

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.