linux-kernel - Re: [PATCH V5 3/3] PCI: Mask and unmask hotplug interrupts during reset

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Date:   Tue, 3 Jul 2018 16:12:55 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     okaya@...eaurora.org
Cc:     linux-pci@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Oza Pawandeep <poza@...eaurora.org>,
        Keith Busch <keith.busch@...el.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V5 3/3] PCI: Mask and unmask hotplug interrupts during
 reset

On Tue, Jul 03, 2018 at 07:30:28AM -0400, okaya@...eaurora.org wrote:
> On 2018-07-03 04:34, Lukas Wunner wrote:
> >On Mon, Jul 02, 2018 at 06:52:47PM -0400, Sinan Kaya wrote:
> >>If a bridge supports hotplug and observes a PCIe fatal error, the
> >>following
> >>events happen:
> >>
> >>1. AER driver removes the devices from PCI tree on fatal error
> >>2. AER driver brings down the link by issuing a secondary bus reset
> >>waits
> >>for the link to come up.
> >>3. Hotplug driver observes a link down interrupt
> >>4. Hotplug driver tries to remove the devices waiting for the rescan
> >>lock
> >>but devices are already removed by the AER driver and AER driver is
> >>waiting
> >>for the link to come back up.
> >>5. AER driver tries to re-enumerate devices after polling for the link
> >>state to go up.
> >>6. Hotplug driver obtains the lock and tries to remove the devices
> >>again.
> >>
> >>If a bridge is a hotplug capable bridge, mask hotplug interrupts before
> >>the
> >>reset and unmask afterwards.
> >
> >Would it work for you if you just amended the AER driver to skip
> >removal and re-enumeration of devices if the port is a hotplug bridge?
> >Just check for is_hotplug_bridge in struct pci_dev.
> 
> The reason why we want to remove devices before secondary bus reset is to
> quiesce pcie bus traffic before issuing a reset.
> 
> Skipping this step might cause transactions to be lost in the middle of the
> reset as there will be active traffic flowing and drivers will suddenly
> start reading ffs.

Interesting, I think that merits a code comment.

FWIW, macOS has a "PCI pause" callback to quiesce a device:
https://opensource.apple.com/source/IOPCIFamily/IOPCIFamily-239.1.2/pause.rtf

They're using it to reconfigure a device's BAR and bus number
at runtime (sic!), e.g. if mmio windows need to be moved around
on Thunderbolt hotplug if there's insufficient space:

	"During pause reconfiguration, the following may be changed:
	 - device BAR registers
	 - the devices bus number
	 - registry properties reflecting these values ("ranges",
	   "assigned-addresses", "reg")
	 - device MSI block values for address and value, but not the
	   number of MSIs allocated"

Conceptually, "PCI pause" is similar to putting the device in a suspend
state.  I'm wondering if suspending the devices below the bridge would
make more sense than removing them in the AER driver.

Lukas