Re: [Linux-kernel-mentees] [RFC PATCH 00/35] Move all PCIBIOS* definitions into arch/x86

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Bjorn Helgaas <helgaas@kernel.org>,
	David Laight <David.Laight@ACULAB.COM>
Cc: linux-pci <linux-pci@vger.kernel.org>,
	Paul Mackerras <paulus@samba.org>,
	sparclinux <sparclinux@vger.kernel.org>,
	Toan Le <toan@os.amperecomputing.com>,
	Christoph Hellwig <hch@lst.de>,
	Marek Vasut <marek.vasut+renesas@gmail.com>,
	Rob Herring <robh@kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	Kevin Hilman <khilman@baylibre.com>,
	Russell King <linux@armlinux.org.uk>,
	Ley Foon Tan <ley.foon.tan@intel.com>,
	Greg Ungerer <gerg@linux-m68k.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
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	Philipp Zabel <p.zabel@pengutronix.de>,
	"Saheed O. Bolarinwa" <refactormyself@gmail.com>,
	'Oliver O'Halloran' <oohall@gmail.com>,
	Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Heiner Kallweit <hkallweit1@gmail.com>
Subject: Re: [Linux-kernel-mentees] [RFC PATCH 00/35] Move all PCIBIOS* definitions into arch/x86
Date: Thu, 16 Jul 2020 08:49:21 +1000	[thread overview]
Message-ID: <5d4b3a716f85017c17c52a85915fba9e19509e81.camel@kernel.crashing.org> (raw)
In-Reply-To: <20200715221230.GA563957@bjorn-Precision-5520>

On Wed, 2020-07-15 at 17:12 -0500, Bjorn Helgaas wrote:
> > I've 'played' with PCIe error handling - without much success.
> > What might be useful is for a driver that has just read ~0u to
> > be able to ask 'has there been an error signalled for this device?'.
> 
> In many cases a driver will know that ~0 is not a valid value for the
> register it's reading.  But if ~0 *could* be valid, an interface like
> you suggest could be useful.  I don't think we have anything like that
> today, but maybe we could.  It would certainly be nice if the PCI core
> noticed, logged, and cleared errors.  We have some of that for AER,
> but that's an optional feature, and support for the error bits in the
> garden-variety PCI_STATUS register is pretty haphazard.  As you note
> below, this sort of SERR/PERR reporting is frequently hard-wired in
> ways that takes it out of our purview.

We do have pci_channel_state (via pci_channel_offline()) which covers
the cases where the underlying error handling (such as EEH or unplug)
results in the device being offlined though this tend to be
asynchronous so it might take a few ~0's before you get it.

It's typically used to break potentially infinite loops in some
drivers.

There is no interface to check whether *an* error happened though for
the most cases it will be captured in the status register, which is
harvested (and cleared ?) by some EDAC drivers iirc... 

All this lacks coordination, I agree.

Cheers,
Ben.

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