linux-kernel - Re: [PATCH] pci-error-recover: doc cleanup

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Date:   Fri, 9 Dec 2016 09:11:23 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Linas Vepstas <linasvepstas@...il.com>
Cc:     Cao jin <caoj.fnst@...fujitsu.com>,
        Jonathan Corbet <corbet@....net>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] pci-error-recover: doc cleanup

On Fri, 9 Dec 2016 14:44:25 +0800
Linas Vepstas <linasvepstas@...il.com> wrote:

> On Fri, Dec 9, 2016 at 2:37 PM, Cao jin <caoj.fnst@...fujitsu.com> wrote:
> >
> >
> > On 12/09/2016 02:24 PM, Linas Vepstas wrote:  
> >> I suppose I'm confused, but I recall that link resets are non-fatal.
> >> Fatal errors typically require that the the pci adapter be completely
> >> reset, any adapter firmware to be reloaded from scratch, the device
> >> driver has to kill all device state and start from scratch. Its huge.
> >> If the fatal error is on pci device that is under a block device
> >> holding a file system, then (usually) there is no way to recover,
> >> because the block layer (and file system) cannot deal with a block
> >> device that disappeared and then reappeared some few seconds later.
> >> (maybe some future zfs or lvm or btrfs might be able to deal with
> >> this, but not today)
> >>
> >> By contrast, link resets are far more gentle: the device driver might
> >> have to discard some half-full FIFO's, or cancel some in-flight
> >> commands, but can otherwise gracefully recover without telling the
> >> higher layers that there were any problems.
> >>
> >> --linas
> >>  
> >
> > I am little confused too, even not sure if we are talking the same
> > *fatal error*, I am talking the fatal error defined in PCI Express spec,
> > chapter 6.2.2.2.1:
> >
> > Fatal errors are uncorrectable error conditions which render the
> > particular Link and related hardware unreliable. For Fatal errors, a
> > reset of the components on the Link may be required to return to
> > reliable operation. Platform handling of Fatal errors, and any efforts
> > to limit the effects of these errors, is platform implementation specific.
> >
> > Link reset means set *secondary bus reset* bit in pci bridge config
> > space, can reset the link and device simultaneously, is the strongest
> > kind of reset as I know.  
> 
> OK, well, its been far too many years, and I don't have the PCI spec
> at my fingertips.
> Isn't there a link reset that can be performed, without forcing a device reset?
> 
> The intent was that some PCI link errors are due to vibration,
> ground-bounce, humidity, etc. and that these errors can be detected
> and do not corrupt the device state or the device driver state.  Since
> they are not associated with data corruption (or rather, the
> corruption is local to the link), these can be recovered by reseting
> just the link, without resetting the whole adapter. They may require
> reseting some device-driver state, but not all of it.
> 
> However, this was all decided before the PCI-E spec was written, so
> maybe the newer PCI-E specs now say something different.

Perhaps you're thinking of link retraining?  That sort of error would
be considered correctable, not fatal.  Fatal errors are uncorrected
errors and a bigger hammer is needed to deal with them, such as a link
reset.  Thanks,

Alex