linux-kernel - Re: [PATCH v3 04/15] iommu: Move bus setup to IOMMU device registration

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Date:   Thu, 7 Jul 2022 11:58:18 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     "Tian, Kevin" <kevin.tian@...el.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 04/15] iommu: Move bus setup to IOMMU device
 registration

On 2022-07-07 07:51, Tian, Kevin wrote:
>> From: Robin Murphy <robin.murphy@....com>
>> Sent: Wednesday, July 6, 2022 1:08 AM
>>
>> @@ -202,12 +210,32 @@ int iommu_device_register(struct iommu_device
>> *iommu,
>>   	spin_lock(&iommu_device_lock);
>>   	list_add_tail(&iommu->list, &iommu_device_list);
>>   	spin_unlock(&iommu_device_lock);
>> +
>> +	for (int i = 0; i < ARRAY_SIZE(iommu_buses); i++) {
>> +		struct bus_type *bus = iommu_buses[i];
>> +		int err;
>> +
>> +		if (bus->iommu_ops && bus->iommu_ops != ops) {
>> +			err = -EBUSY;
>> +		} else {
>> +			bus->iommu_ops = ops;
>> +			err = bus_iommu_probe(bus);
>> +		}
>> +		if (err) {
>> +			iommu_device_unregister(iommu);
>> +			return err;
>> +		}
>> +	}
>> +
> 
> Probably move above into a new function bus_iommu_probe_all():
> 
> 	/* probe all buses for devices associated with this iommu */
> 	err = bus_iommu_probe_all();
> 	if (err) {
> 		iommu_device_unregister(iommu);
> 		return err;
> 	}
> 
> Just  my personal preference on leaving logic in iommu_device_register()
> more relevant to the iommu instance itself.

On reflection I think it makes sense to pull the 
iommu_device_unregister() out of the loop anyway - I think that's really 
a left-over from between v1 and v2 when that error case briefly jumped 
to another cleanup loop, before I realised it was actually trivial for 
iommu_device_unregister() to clean up for itself.

However I now see I've also missed another opportunity, and the -EBUSY 
case should be hoisted out of the loop as well, since checking 
iommu_buses[0] is sufficient. Then it's hopefully much clearer that once 
the bus ops go away we'll be left with just a single extra line for the 
loop, as in iommu_device_unregister(). Does that sound reasonable?

> Apart from that:
> 
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>

Thanks! (and for the others as well)

Robin.