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Date:   Tue, 30 Apr 2019 21:12:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Alex G <mr.nuke.me@...il.com>, Lukas Wunner <lukas@...ner.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Alexandru Gagniuc <alex_gagniuc@...lteam.com>,
        Keith Busch <keith.busch@...el.com>,
        Shyam Iyer <Shyam_Iyer@...l.com>,
        Sinan Kaya <okaya@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "PCI/LINK: Report degraded links via link
 bandwidth notification"

On Tue, Apr 30, 2019 at 12:18:13PM -0600, Keith Busch wrote:
> On Tue, Apr 30, 2019 at 12:05:09PM -0600, Keith Busch wrote:
> > On Tue, Apr 30, 2019 at 11:11:51AM -0500, Bjorn Helgaas wrote:
> > > > I'm not convinced a revert is the best call.
> > > 
> > > I have very limited options at this stage of the release, but I'd be
> > > glad to hear suggestions.  My concern is that if we release v5.1
> > > as-is, we'll spend a lot of energy on those false positives.
> > 
> > May be too late now if the revert is queued up, but I think this feature
> > should have been a default 'false' Kconfig bool rather than always on.

Since this feature currently just adds a message in dmesg, which we
don't really consider a stable API, I think a Kconfig switch is a
reasonable option.

If you send me a signed-off-by for the following patch, I can apply it:

commit 302b77157e66
Author: Keith Busch <kbusch@...nel.org>
Date:   Tue Apr 30 12:18:13 2019 -0600

    PCI/LINK: Add Kconfig option (default off)
    
    e8303bb7a75c ("PCI/LINK: Report degraded links via link bandwidth
    notification") added dmesg logging whenever a link changes speed or width
    to a state that is considered degraded.  Unfortunately, it cannot
    differentiate signal integrity-related link changes from those
    intentionally initiated by an endpoint driver, including drivers that may
    live in userspace or VMs when making use of vfio-pci.  Some GPU drivers
    actively manage the link state to save power, which generates a stream of
    messages like this:
    
      vfio-pci 0000:07:00.0: 32.000 Gb/s available PCIe bandwidth, limited by 2.5 GT/s x16 link at 0000:00:02.0 (capable of 64.000 Gb/s with 5 GT/s x16 link)
    
    Since we can't distinguish the intentional changes from the signal
    integrity issues, leave the reporting turned off by default.  Add a Kconfig
    option to turn it on if desired.
    
    Fixes: e8303bb7a75c ("PCI/LINK: Report degraded links via link bandwidth
    notification")

diff --git a/drivers/pci/pcie/Kconfig b/drivers/pci/pcie/Kconfig
index 5cbdbca904ac..4a094f0d2856 100644
--- a/drivers/pci/pcie/Kconfig
+++ b/drivers/pci/pcie/Kconfig
@@ -142,3 +142,12 @@ config PCIE_PTM
 
 	  This is only useful if you have devices that support PTM, but it
 	  is safe to enable even if you don't.
+
+config PCIE_BW
+	bool "PCI Express Bandwidth Change Notification"
+	default n
+	depends on PCIEPORTBUS
+	help
+	  This enables PCI Express Bandwidth Change Notification.  If
+	  you know link width or rate changes occur only to correct
+	  unreliable links, you may answer Y.
diff --git a/drivers/pci/pcie/Makefile b/drivers/pci/pcie/Makefile
index f1d7bc1e5efa..d356a5bdb158 100644
--- a/drivers/pci/pcie/Makefile
+++ b/drivers/pci/pcie/Makefile
@@ -3,7 +3,6 @@
 # Makefile for PCI Express features and port driver
 
 pcieportdrv-y			:= portdrv_core.o portdrv_pci.o err.o
-pcieportdrv-y			+= bw_notification.o
 
 obj-$(CONFIG_PCIEPORTBUS)	+= pcieportdrv.o
 
@@ -13,3 +12,4 @@ obj-$(CONFIG_PCIEAER_INJECT)	+= aer_inject.o
 obj-$(CONFIG_PCIE_PME)		+= pme.o
 obj-$(CONFIG_PCIE_DPC)		+= dpc.o
 obj-$(CONFIG_PCIE_PTM)		+= ptm.o
+obj-$(CONFIG_PCIE_BW)		:= bw_notification.o

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