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

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Message-ID: <e80adfa5-b51a-a1ae-f582-58eaa4a5be68@os.amperecomputing.com>
Date:   Tue, 28 Feb 2023 08:23:19 +0530
From:   Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.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 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/

> 
>>
>> 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.

> 
>> +
>> +	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