Re: WARNING: CPU: 4 PID: 863 at include/drm/drm_crtc.h:1577 drm_helper_choose_encoder_dpms+0x88/0x90() - evildoer found and neutralized

[Joerg Roedel <joro@8bytes.org>]

On Thu, Oct 01, 2015 at 01:00:44AM +0800, Jiang Liu wrote:
> Thanks Joerg, that makes sense. If some driver tries to binding to the
> IOMMU device, it will trigger the scenario as you described. For
> example,  Xen backend driver will try to probe all PCI devices
> if enabled. I will do more investigation tomorrow.

Not only that, the probe code looks like this in __pci_device_probe:

                error = -ENODEV;

                id = pci_match_device(drv, pci_dev);
                if (id)
                        error = pci_call_probe(drv, pci_dev, id);
                if (error >= 0)
                        error = 0;

The pci_match_device() function will always return NULL for the iommu
pci_dev, because no driver matches the ids of it. So the function
returns -ENODEV, which will be handled in the caller (pci_device_probe):


        error = pcibios_alloc_irq(pci_dev);
        if (error < 0)
                return error;

        pci_dev_get(pci_dev);
        error = __pci_device_probe(drv, pci_dev);
        if (error) {
                pcibios_free_irq(pci_dev);
                pci_dev_put(pci_dev);
        }

For the IOMMU pci_dev a pcibios-irq will be allocated (if there is one,
like on Boris' system) and because __pci_device_probe returns -ENODEV it
will be freed again with pcibios_free_irq().

The pcibios_free_irq() function will set dev->irq = 0, which overwrites
the value that pci_enable_msi() wrote there. So later in suspend/resume
code the msi-handling part tries to fetch the irq-descriptor for the
wrong irq (which is NULL) and causes the crash.

The issue got introduced because with your changes pci_enable_msi() is
only allowed after a pci-device was successfully probed by the driver.
But this assumption is not true, as the AMD IOMMU driver does not
register as a pci-driver.

Registering a pci-driver would actually be harmful, because a device can
be forcibly unbound from its driver, which would be pretty bad for an
IOMMU in the running system.

So the right fix is to allow pci_enable_msi() for pci-devices not
registered against a driver. The fix I sent Boris has issues (I think it
leaks pcibios irqs when MSI is in use), but was thinking about fixing it
in pci_device_probe by not allocating a pcibios-irq when MSI is already
active. What do you think?

Regards,

	Joerg