linux-kernel - Re: [PATCH 02/13] iommu: Move bus setup to IOMMU device registration

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Message-ID: <7f693a3a-bbab-237a-af9a-4395e501a5b2@arm.com>
Date:   Tue, 19 Apr 2022 08:20:22 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>, joro@...tes.org,
        will@...nel.org
Cc:     iommu@...ts.linux-foundation.org, sven@...npeter.dev,
        robdclark@...il.com, m.szyprowski@...sung.com,
        yong.wu@...iatek.com, mjrosato@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, zhang.lyra@...il.com,
        thierry.reding@...il.com, vdumpa@...dia.com,
        jean-philippe@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/13] iommu: Move bus setup to IOMMU device registration

On 2022-04-19 00:37, Lu Baolu wrote:
> On 2022/4/19 6:09, Robin Murphy wrote:
>> On 2022-04-16 01:04, Lu Baolu wrote:
>>> On 2022/4/14 20:42, Robin Murphy wrote:
>>>> @@ -1883,27 +1900,12 @@ static int iommu_bus_init(struct bus_type *bus)
>>>>    */
>>>>   int bus_set_iommu(struct bus_type *bus, const struct iommu_ops *ops)
>>>>   {
>>>> -    int err;
>>>> -
>>>> -    if (ops == NULL) {
>>>> -        bus->iommu_ops = NULL;
>>>> -        return 0;
>>>> -    }
>>>> -
>>>> -    if (bus->iommu_ops != NULL)
>>>> +    if (bus->iommu_ops && ops && bus->iommu_ops != ops)
>>>>           return -EBUSY;
>>>>       bus->iommu_ops = ops;
>>>
>>> Do we still need to keep above lines in bus_set_iommu()?
>>
>> It preserves the existing behaviour until each callsite and its 
>> associated error handling are removed later on, which seems like as 
>> good a thing to do as any. Since I'm already relaxing 
>> iommu_device_register() to a warn-but-continue behaviour while it 
>> keeps the bus ops on life-support internally, I figured not changing 
>> too much at once would make it easier to bisect any potential issues 
>> arising from this first step.
> 
> Fair enough. Thank you for the explanation.
> 
> Do you have a public tree that I could pull these patches and try them
> on an Intel hardware? Or perhaps you have done this? I like the whole
> idea of this series, but it's better to try it with a real hardware.

I haven't bothered with separate branches since there's so many 
different pieces in-flight, but my complete (unstable) development 
branch can be found here:

https://gitlab.arm.com/linux-arm/linux-rm/-/commits/iommu/bus

For now I'd recommend winding the head back to "iommu: Clean up 
bus_set_iommu()" for testing - some of the patches above that have 
already been posted and picked up by their respective subsystems, but 
others are incomplete and barely compile-tested. I'll probably rearrange 
it later this week to better reflect what's happened so far.

Cheers,
Robin.