linux-kernel - Re: arm64 iommu groups issue

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Date:   Thu, 13 Feb 2020 15:49:52 +0000
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, Marc Zyngier <maz@...nel.org>,
        "Will Deacon" <will@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Guohanjun (Hanjun Guo)" <guohanjun@...wei.com>
CC:     iommu <iommu@...ts.linux-foundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linuxarm <linuxarm@...wei.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Saravana Kannan" <saravanak@...gle.com>
Subject: Re: arm64 iommu groups issue

>>
>> The underlying issue is that, for historical reasons, OF/IORT-based
>> IOMMU drivers have ended up with group allocation being tied to endpoint
>> driver probing via the dma_configure() mechanism (long story short,
>> driver probe is the only thing which can be delayed in order to wait for
>> a specific IOMMU instance to be ready).However, in the meantime, the
>> IOMMU API internals have evolved sufficiently that I think there's a way
>> to really put things right - I have the spark of an idea which I'll try
>> to sketch out ASAP...
>>
> 
> OK, great.

Hi Robin,

I was wondering if you have had a chance to consider this problem again?

One simple idea could be to introduce a device link between the endpoint 
device and its parent bridge to ensure that they probe in order, as 
expected in pci_device_group():

Subject: [PATCH] PCI: Add device link to ensure endpoint device driver 
probes after bridge

It is required to ensure that a device driver for an endpoint will probe
after the parent port driver, so add a device link for this.

---
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 512cb4312ddd..4b832ad25b20 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2383,6 +2383,7 @@ static void pci_set_msi_domain(struct pci_dev *dev)
  void pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
  {
  	int ret;
+	struct device *parent;

  	pci_configure_device(dev);

@@ -2420,6 +2421,10 @@ void pci_device_add(struct pci_dev *dev, struct 
pci_bus *bus)
  	/* Set up MSI IRQ domain */
  	pci_set_msi_domain(dev);

+	parent = dev->dev.parent;
+	if (parent && parent->bus == &pci_bus_type)
+		device_link_add(&dev->dev, parent, DL_FLAG_AUTOPROBE_CONSUMER);
+
  	/* Notifier could use PCI capabilities */
  	dev->match_driver = false;
  	ret = device_add(&dev->dev);
-- 

This would work, but the problem is that if the port driver fails in 
probing - and not just for -EPROBE_DEFER - then the child device will 
never probe. This very thing happens on my dev board. However we could 
expand the device links API to cover this sort of scenario.

As for alternatives, it looks pretty difficult to me to disassociate the 
group allocation from the dma_configure path.

Let me know.

Thanks,
John