Re: [PATCH 1/2] PCI/ATS: Add a helper function to configure ATS STU of a PF.

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

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@os.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