linux-kernel - Re: [PATCH v1 2/2] PCI: Allow user to request power management of conventional and hotplug bridges

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Message-ID: <7778.1519082904@turing-police.cc.vt.edu>
Date:   Mon, 19 Feb 2018 18:28:24 -0500
From:   valdis.kletnieks@...edu
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-pci@...r.kernel.org, Mathias Nyman <mathias.nyman@...el.com>,
        linux-pm@...r.kernel.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
        Peter Wu <peter@...ensteyn.nl>,
        Qipeng Zha <qipeng.zha@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andreas Noever <andreas.noever@...il.com>,
        Dave Airlie <airlied@...il.com>, Qi Zheng <qi.zheng@...el.com>
Subject: Re: [PATCH v1 2/2] PCI: Allow user to request power management of conventional and hotplug bridges

On Mon, 19 Feb 2018 17:14:06 -0600, Bjorn Helgaas said:

> Change "pcie_port_pm=force" to enable power management of conventional PCI
> bridges and hotplug bridges as well as PCIe ports.  As with the previous
> PCIe port-only behavior, this is not expected to work in all systems.

This part says the behavior changes - which is itself a Bad Idea unless you
have a deprecation cut-over across several releases.  The general rule is that
you're not allowed to break somebody's kernel without a lot of warning.
Remember that there's probably a lot of embedded systems that hardcode their
boot cmdline and changing the behavior can result in a failed boot - which can
be a royal bitch to debug if the embedded system doesn't have a console.....

In addition, it doesn't match the actual patch, which documents the boot
parameter as being removed, rather than the behavior changed:

> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1d1d53f85ddd..4660105ec851 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt

> @@ -3143,10 +3147,6 @@
>  		compat	Treat PCIe ports as PCI-to-PCI bridges, disable the PCIe
>  			ports driver.
>
> -	pcie_port_pm=	[PCIE] PCIe port power management handling:
> -		off	Disable power management of all PCIe ports
> -		force	Forcibly enable power management of all PCIe ports
> -

And *that* doesn't match the rest of the patch, which never touches the handling
of that parameter, either changing it or removing it.

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