linux-kernel - RE: [PATCH v3 03/15] iommu: Always register bus notifiers

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Date:   Thu, 7 Jul 2022 06:31:31 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        "joro@...tes.org" <joro@...tes.org>
CC:     "will@...nel.org" <will@...nel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "vasant.hegde@....com" <vasant.hegde@....com>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "gerald.schaefer@...ux.ibm.com" <gerald.schaefer@...ux.ibm.com>,
        "schnelle@...ux.ibm.com" <schnelle@...ux.ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 03/15] iommu: Always register bus notifiers

> From: Robin Murphy <robin.murphy@....com>
> Sent: Wednesday, July 6, 2022 1:08 AM
> 
> The number of bus types that the IOMMU subsystem deals with is small and
> manageable, so pull that list into core code as a first step towards
> cleaning up all the boilerplate bus-awareness from drivers. Calling
> iommu_probe_device() before bus->iommu_ops is set will simply return
> -ENODEV and not break the notifier call chain, so there should be no
> harm in proactively registering all our bus notifiers at init time.
> 

Suppose we miss a check on iommu ops in iommu_release_device():

	if (!dev->iommu) <<<<<<<
		return;

	iommu_device_unlink(dev->iommu->iommu_dev, dev);

	ops = dev_iommu_ops(dev);
	ops->release_device(dev);

following the rationale in patch01 a device could be removed when
it's associated with a known but not registered instance.