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Message-ID: <47a88e5f-ee56-9c4b-e134-e018cd6dc49e@linux.intel.com>
Date: Mon, 27 Feb 2023 19:08:17 -0800
From: Sathyanarayanan Kuppuswamy
<sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
joro@...tes.org, bhelgaas@...gle.com, robin.murphy@....com,
will@...nel.org
Cc: jean-philippe@...aro.org, darren@...amperecomputing.com,
scott@...amperecomputing.com
Subject: Re: [PATCH 1/2] PCI/ATS: Add a helper function to configure ATS STU
of a PF.
On 2/27/23 6:53 PM, Ganapatrao Kulkarni wrote:
>
>
> On 28-02-2023 12:59 am, Sathyanarayanan Kuppuswamy wrote:
>> Hi,
>>
>> On 2/27/23 5:21 AM, Ganapatrao Kulkarni wrote:
>>> As per PCI specification (PCI Express Base Specification Revision
>>> 6.0, Section 10.5) both PF and VFs of a PCI EP are permitted to be enabled
>>> independently for ATS capability, however the STU(Smallest Translation
>>> Unit) is shared between PF and VFs. For VFs, it is hardwired to Zero and
>>> the associated PF's value applies to VFs.
>>>
>>> In the current code, the STU is being configured while enabling the PF ATS.
>>> Hence, it is not able to enable ATS for VFs, if it is not enabled on the
>>> associated PF already.>
>>> Adding a function pci_ats_stu_configure(), which can be called to
>>> configure the STU during PF enumeration.
>>> Latter enumerations of VFs can successfully enable ATS independently.
>>
>> Why not enable ATS in PF before enabling it in VF? Just updating STU of
>> PF and not enabling it seem odd.
>
> More details are in PATCH 0/2 and 2/2.
>
> Also, This was discussed at
> https://lore.kernel.org/linux-arm-kernel/20230221154624.GA3701506@bhelgaas/T/
>
I agree with Bjorn's comments. It is incorrect to directly configure PF
registers from VF enable function. It is a buggy fix.
My question is, why not ensure PF ATS is configured and enabled before enabling
ATS for VF. Anyway, PF device will be enumerated before VF, right?
>>
>>>
>>> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
>>> ---
>>> drivers/pci/ats.c | 32 ++++++++++++++++++++++++++++++--
>>> include/linux/pci-ats.h | 1 +
>>> 2 files changed, 31 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>>> index f9cc2e10b676..70e1982efdb4 100644
>>> --- a/drivers/pci/ats.c
>>> +++ b/drivers/pci/ats.c
>>> @@ -46,6 +46,34 @@ bool pci_ats_supported(struct pci_dev *dev)
>>> }
>>> EXPORT_SYMBOL_GPL(pci_ats_supported);
>>> +/**
>>> + * pci_ats_stu_configure - Configure STU of a PF.
>>> + * @dev: the PCI device
>>> + * @ps: the IOMMU page shift
>>> + *
>>> + * Returns 0 on success, or negative on failure.
>>> + */
>>> +int pci_ats_stu_configure(struct pci_dev *dev, int ps)
>>> +{
>>> + u16 ctrl;
>>> +
>>> + if (dev->ats_enabled || dev->is_virtfn)
>>> + return 0;
>>> +
>>> + if (!pci_ats_supported(dev))
>>> + return -EINVAL;
>>> +
>>> + if (ps < PCI_ATS_MIN_STU)
>>> + return -EINVAL;
>>> +
>>> + dev->ats_stu = ps;
>>> + ctrl = PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
>>> + pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
>>
>> If you just want to update the STU, don't overwrite other fields.
>
> Can be read modify write, but felt not necessary, since all other fields are at default value zero.
It may not always be true. If there is a case where AMAD and AMAE attribute values are
configured independently, then your change can overwrite it.
IMO, since your intention is to update STU, I recommend just updating it.
>
>>
>>> +
>>> + return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(pci_ats_stu_configure);
>>> +
>>> /**
>>> * pci_enable_ats - enable the ATS capability
>>> * @dev: the PCI device
>>> @@ -68,8 +96,8 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
>>> return -EINVAL;
>>> /*
>>> - * Note that enabling ATS on a VF fails unless it's already enabled
>>> - * with the same STU on the PF.
>>> + * Note that enabling ATS on a VF fails unless it's already
>>> + * configured with the same STU on the PF.
>>> */
>>> ctrl = PCI_ATS_CTRL_ENABLE;
>>> if (dev->is_virtfn) {
>>> diff --git a/include/linux/pci-ats.h b/include/linux/pci-ats.h
>>> index df54cd5b15db..9b40eb555124 100644
>>> --- a/include/linux/pci-ats.h
>>> +++ b/include/linux/pci-ats.h
>>> @@ -8,6 +8,7 @@
>>> /* Address Translation Service */
>>> bool pci_ats_supported(struct pci_dev *dev);
>>> int pci_enable_ats(struct pci_dev *dev, int ps);
>>> +int pci_ats_stu_configure(struct pci_dev *dev, int ps);
>>
>> What about dummy declaration for !CONFIG_PCI_ATS case?
>
> Thanks, I overlooked else case.
>>
>>> void pci_disable_ats(struct pci_dev *dev);
>>> int pci_ats_queue_depth(struct pci_dev *dev);
>>> int pci_ats_page_aligned(struct pci_dev *dev);
>>
>
>
> Thanks,
> Ganapat
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer
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