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Date:   Thu, 7 Jul 2022 10:58:56 +0100
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Subject: Re: [PATCH v3 03/15] iommu: Always register bus notifiers
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To:     "Tian, Kevin" <kevin.tian@intel.com>,
        "joro@8bytes.org" <joro@8bytes.org>
Cc:     "will@kernel.org" <will@kernel.org>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
        "suravee.suthikulpanit@amd.com" <suravee.suthikulpanit@amd.com>,
        "vasant.hegde@amd.com" <vasant.hegde@amd.com>,
        "mjrosato@linux.ibm.com" <mjrosato@linux.ibm.com>,
        "gerald.schaefer@linux.ibm.com" <gerald.schaefer@linux.ibm.com>,
        "schnelle@linux.ibm.com" <schnelle@linux.ibm.com>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <cover.1657034827.git.robin.murphy@arm.com>
 <8c380309f264cd0dfc73ba2ec060adc9515af2f2.1657034828.git.robin.murphy@arm.com>
 <BN9PR11MB5276B6689D88D3D5D57915908C839@BN9PR11MB5276.namprd11.prod.outlook.com>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <BN9PR11MB5276B6689D88D3D5D57915908C839@BN9PR11MB5276.namprd11.prod.outlook.com>
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On 2022-07-07 07:31, Tian, Kevin wrote:
>> From: Robin Murphy <robin.murphy@arm.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.

No, because at that point the instance is only known internally to the 
driver. As long as it isn't erroneously returned from 
->probe_device(dev), dev->iommu will remain NULL and the rest of the 
core code works as expected.

Thanks,
Robin.