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

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Message-ID: <20180530045557.GA26682@kroah.com>
Date:   Wed, 30 May 2018 06:55:57 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Jonathan Corbet <corbet@....net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Christoffer Dall <cdall@...aro.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Thymo van Beers <thymovanbeers@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Dongdong Liu <liudongdong3@...wei.com>,
        Frederick Lawler <fred@...dlawl.com>,
        Oza Pawandeep <poza@...eaurora.org>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: Add pci=safemode option

On Tue, May 29, 2018 at 09:41:33PM -0700, Sinan Kaya wrote:
> On 5/29/2018 9:31 PM, Greg Kroah-Hartman wrote:
> > On Tue, May 29, 2018 at 11:19:41PM -0400, Sinan Kaya wrote:
> >> Adding pci=safemode kernel command line parameter to turn off all PCI
> >> Express service driver as well as all optional PCIe features such as LTR,
> >> Extended tags, Relaxed Ordering etc.
> >>
> >> Also setting MPS configuration to PCIE_BUS_SAFE so that MPS and MRRS can be
> >> reconfigured with by the kernel in case BIOS hands off a broken
> >> configuration.
> > 
> > Why not fix the BIOS?  That's what sane platforms do :)
> > 
> >>
> >> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> >> ---
> >>  Documentation/admin-guide/kernel-parameters.txt | 2 ++
> >>  drivers/pci/pci.c                               | 7 +++++++
> >>  drivers/pci/pci.h                               | 2 ++
> >>  drivers/pci/pcie/portdrv_core.c                 | 2 +-
> >>  drivers/pci/probe.c                             | 6 ++++++
> >>  5 files changed, 18 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> >> index 641ec9c..247adbb 100644
> >> --- a/Documentation/admin-guide/kernel-parameters.txt
> >> +++ b/Documentation/admin-guide/kernel-parameters.txt
> >> @@ -3153,6 +3153,8 @@
> >>  		noari		do not use PCIe ARI.
> >>  		noats		[PCIE, Intel-IOMMU, AMD-IOMMU]
> >>  				do not use PCIe ATS (and IOMMU device IOTLB).
> >> +		safemode	turns of all optinal PCI features. Useful
> >> +				for bringup/troubleshooting.
> > 
> > s/optinal/optional/ ?
> 
> sure.
> 
> > 
> > And you should explain what exactly in PCI is "optional".  Who defines
> > this and where is that list and what can go wrong if those options are
> > not enabled?
> 
> 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.
> 
> I can add a few words describing them. The goal of this option is to keep
> base PCI features with MSI only. Things like PME, AER, ASPM, Extended
> Tags, LTR, Relaxed Ordering, SRIOV are all considered optional. safemode
> is certainly not intended for production environments. 

Ok, then you should say that here, or somewhere, so that people know
this.  Otherwise people will see that "hey this lets my hardware boot!"
and then never change it :(

> I can taint the kernel as a suggestion.

I would not worry about that.

> I defined minimum as just booting a device and to be able to do DMA traffic
> only with 0 optimization

Ok, again, just document this really well, so that people do not have
questions and start wondering why their devices barely seem to work.

thanks,

greg k-h