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Subject: Re: [PATCH v3 8/8] iommu/arm-smmu-v3: Make default domain type of
 HiSilicon PTT device to identity
To:     Robin Murphy <robin.murphy@arm.com>, Will Deacon <will@kernel.org>
References: <20220124131118.17887-1-yangyicong@hisilicon.com>
 <20220124131118.17887-9-yangyicong@hisilicon.com>
 <e58888c1-5448-77c7-7f6c-f5db999a888f@huawei.com>
 <20220215130044.GA7154@willie-the-truck>
 <9018a1d9-4d42-3a99-dbc6-c55139abcb1e@arm.com>
 <20220215134232.GA7592@willie-the-truck>
 <161e5005-ea12-fde4-0e31-ec871d2fe591@arm.com>
From:   Yicong Yang <yangyicong@huawei.com>
Message-ID: <ab91cc5b-625e-c90b-7ab8-a0144a9b1a92@huawei.com>
Date:   Wed, 16 Feb 2022 17:35:24 +0800
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On 2022/2/15 22:29, Robin Murphy wrote:
> On 2022-02-15 13:42, Will Deacon wrote:
>> On Tue, Feb 15, 2022 at 01:30:26PM +0000, Robin Murphy wrote:
>>> On 2022-02-15 13:00, Will Deacon wrote:
>>>> On Mon, Feb 14, 2022 at 08:55:20PM +0800, Yicong Yang wrote:
>>>>> On 2022/1/24 21:11, Yicong Yang wrote:
>>>>>> The DMA of HiSilicon PTT device can only work with identical
>>>>>> mapping. So add a quirk for the device to force the domain
>>>>>> passthrough.
>>>>>>
>>>>>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
>>>>>> ---
>>>>>>    drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 16 ++++++++++++++++
>>>>>>    1 file changed, 16 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>>>> index 6dc6d8b6b368..6f67a2b1dd27 100644
>>>>>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>>>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>>>>> @@ -2838,6 +2838,21 @@ static int arm_smmu_dev_disable_feature(struct device *dev,
>>>>>>        }
>>>>>>    }
>>>>>> +#define IS_HISI_PTT_DEVICE(pdev)    ((pdev)->vendor == PCI_VENDOR_ID_HUAWEI && \
>>>>>> +                     (pdev)->device == 0xa12e)
>>>>>> +
>>>>>> +static int arm_smmu_def_domain_type(struct device *dev)
>>>>>> +{
>>>>>> +    if (dev_is_pci(dev)) {
>>>>>> +        struct pci_dev *pdev = to_pci_dev(dev);
>>>>>> +
>>>>>> +        if (IS_HISI_PTT_DEVICE(pdev))
>>>>>> +            return IOMMU_DOMAIN_IDENTITY;
>>>>>> +    }
>>>>>> +
>>>>>> +    return 0;
>>>>>> +}
>>>>>> +
>>>>>>    static struct iommu_ops arm_smmu_ops = {
>>>>>>        .capable        = arm_smmu_capable,
>>>>>>        .domain_alloc        = arm_smmu_domain_alloc,
>>>>>> @@ -2863,6 +2878,7 @@ static struct iommu_ops arm_smmu_ops = {
>>>>>>        .sva_unbind        = arm_smmu_sva_unbind,
>>>>>>        .sva_get_pasid        = arm_smmu_sva_get_pasid,
>>>>>>        .page_response        = arm_smmu_page_response,
>>>>>> +    .def_domain_type    = arm_smmu_def_domain_type,
>>>>>>        .pgsize_bitmap        = -1UL, /* Restricted during device attach */
>>>>>>        .owner            = THIS_MODULE,
>>>>>>    };
>>>>>>
>>>>>
>>>>> Is this quirk ok with the SMMU v3 driver? Just want to confirm that I'm on the
>>>>> right way to dealing with the issue of our device.
>>>>
>>>> I don't think the quirk should be in the SMMUv3 driver. Assumedly, you would
>>>> have the exact same problem if you stuck the PTT device behind a different
>>>> type of IOMMU, and so the quirk should be handled by a higher level of the
>>>> stack.
>>>
>>> Conceptually, yes, but I'm inclined to be pragmatic here. Default domain
>>> quirks could only move out as far as the other end of the call from
>>> iommu_get_def_domain_type() - it's not like we could rely on some flag in a
>>> driver which may not even be loaded yet, let alone matched to the device.
>>> And even then there's an equal and opposite argument for why the core code
>>> should have to maintain a list of platform-specific quirks rather than code
>>> specific to the relevant platforms. The fact is that a HiSilicon RCiEP is
>>> not going to end up behind anything other than a HiSilicon IOMMU, and if
>>> those ever stop being SMMUv3 *and* such a quirk still exists we can worry
>>> about it then.
>>

That's true that this RCiEP only appears behind the HiSilicon's IOMMU which using
SMMU v3 driver.

>> Perhaps, but you know that by adding this hook it's only a matter of time
>> before we get random compatible string matches in there, so I'd rather keep
>> the flood gates closed as long as we can.
>>
>> Given that this is a PCI device, why can't we have a PCI quirk for devices
>> which require an identity mapping and then handle that in the IOMMU core?
> 

As Robin mentioned below, not only PCI devices but some platform devices also want
to passthrough the IOMMU. I noticed there're already some fields describe the device's
DMA information in struct device, so follow your point can it go there if we're going
to make it more generic?

Anyway if we're going to make all these quirks in a more generic place, I'll willing
to add this device there and have a test.

> Oh, don't think I *like* having quirks in the driver, it just seems like the least-worst choice from a bad bunch. All of the default domain quirks so far (including this one) exist for integrated devices and/or dodgy firmware setups such that they are platform-specific, so there is no technical reason for trying to split *some* of them off into a generic mechanism when the driver-based platform-specific mechanism still needs to exist anyway (some of them do depend on driver state as well).
> 
> Feel free to test the waters with a patch punting qcom_smmu_def_domain_type() to core code, but I think you'll struggle to find a reason to give in the commit message other than "I don't like it".
> 
>>> Ugly as it is, this is the status quo. I don't recall anyone ever arguing
>>> that the equivalent quirks for Intel integrated graphics should be made
>>> generic ;)
>>
>> I don't know anything about Intel integrated graphics. Have they solved this
>> problem in a better way, or could they equally make use of a generic quirk?
> 
> See intel-iommu's device_def_domain_type() implementation. The shape of it may seem quite familiar...
> 

Yes Intel's IOMMU passthrough some PCI devices in this way and this patch imitates that.
https://github.com/torvalds/linux/blob/master/drivers/iommu/intel/iommu.c#L2959

btw Will, much appreciated if you could have a look at the perf and PMU part of this driver.:)

Thanks,
Yicong