linux-kernel - Re: [PATCH] PCI: Add pci=safemode option

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Date:   Wed, 30 May 2018 09:56:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     okaya@...eaurora.org
Cc:     Christoph Hellwig <hch@...radead.org>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-pci@...r.kernel.org, Keith Busch <keith.busch@...el.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Ingo Molnar <mingo@...nel.org>,
        Christoffer Dall <cdall@...aro.org>,
        Jonathan Corbet <corbet@....net>, timur@...eaurora.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dongdong Liu <liudongdong3@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Thymo van Beers <thymovanbeers@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederick Lawler <fred@...dlawl.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-msm@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        Oza Pawandeep <poza@...eaurora.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: Add pci=safemode option

On Wed, May 30, 2018 at 12:44:29AM -0700, okaya@...eaurora.org wrote:
> On 2018-05-30 00:37, Christoph Hellwig wrote:
> > On Tue, May 29, 2018 at 09:41:33PM -0700, Sinan Kaya wrote:
> > > Bjorn and I discussed the need for such a "safe" mode feature when you
> > > want to bring up PCI for a platform. You want to turn off everything
> > > as
> > > a starter and just stick to bare minimum.
> > 
> > Can we please make it a config option the instead of adding code
> > to every kernel?  Also maybe the bringup should be in the name
> > to make this more clear?
> 
> One other requirement was to have a runtime option rather than compile time
> option.
> 
> When someone reported a problem, we wanted to be able to say "use this
> option and see if system boots" without doing any bisects or recompilation.
> 
> This would be the first step in troubleshooting a system to see if
> fundamental features are working.
> 
> I don't mind changing the name
> Bjorn mentioned safe option. I made it safemode. I am looking at Bjorn for
> suggestions at this moment.

This *was* my idea, but I'm starting to think it was a bad idea.

I don't want people to use pci= parameters as the normal way to get a
system to boot.  That would be a huge support hassle (putting things
in release notes, diagnosing problems when people forget it, etc).

But the parameters *are* useful for debugging.  If we had a
"pci=safemode" and it avoided some problem, the next step would be to
narrow it down by using the more specific flags (pci=nomsi, pci=noari,
pci=no_ext_tags, etc).  So I think 95% of the value is in the specific
flags, and a "pci=safemode" might add a little bit of value but at the
cost of a small but nagging maintenance concern and code clutter.

Bjorn