linux-kernel - Re: [PATCH v3 4/8] pciehp: Don't disable the link permanently, during removal

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Date:	Mon, 13 Jan 2014 10:38:44 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Rajat Jain <rajatxjain@...il.com>
Cc:	Rajat Jain <rajatjain@...iper.net>,
	Rajat Jain <rajatjain.linux@...il.com>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yijing Wang <wangyijing@...wei.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Guenter Roeck <groeck@...iper.net>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH v3 4/8] pciehp: Don't disable the link permanently, during removal

On Mon, Jan 13, 2014 at 1:30 AM, Rajat Jain <rajatxjain@...il.com> wrote:

> Yinghai: I am trying to understand what exactly is this platform bug
> and how to add a quirk such that this platform remains unaffected. Can
> you please help me by suggesting how to decide if this is _the_
> platform that has the bug (the pcie repeater).
>
> Bjorn: It seems to be that identification of this platform will be out
> PCI code (since the bug seems to be in a pcie repeater chip which is
> not a PCI device visible to SW). So even if we find a way to identify
> this platform (e.g DMI) , I doubt if you'd want me to add that in the
> pciehp code (which is platform independent so far). At best, the only
> way out I can see is to provide a knob from the pciehp, that can be
> use by the platform code to either enable or disable the link state
> hotplug. It could go back towards using a module parameter like
> pciehp_use_link_events. Please suggest.
>
> The only other way I can think of, is that I can remove the debug
> message altogether (Link up / Link down). (Or the user can change the
> verbosity).

I think it's perfectly fine to add a DMI-based quirk in pciehp.  Yes,
it's a bit ugly, but that's just the nature of working around hardware
defects.  Identify the platform, emit a diagnostic ("disabling link
state because platform may be buggy"), enable the workaround.  That
seems better than requiring the user to figure out what hardware he
has and whether it has a defect.

I would also be OK with adding a pciehp module parameter to explicitly
enable or disable the workaround if that seems necessary.  I just want
the common case of correctly working hardware to work without any
switches.

Removing or rate-limiting the link up/down debug message is also fine with me.

Bjorn
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