Message-ID: <c7a68d5f-3b95-723f-f384-223fcbd6a877@os.amperecomputing.com>
Date:   Tue, 14 Mar 2023 15:38:16 +0530
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Subject: Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS
 STU of a PF
Content-Language: en-US
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev,
        joro@8bytes.org, bhelgaas@google.com, robin.murphy@arm.com,
        will@kernel.org, jean-philippe@linaro.org,
        darren@os.amperecomputing.com, scott@os.amperecomputing.com
References: <20230313211201.GA1540091@bhelgaas>
 <6339c50a-8dfe-f3a2-63d7-504abd4e62f0@linux.intel.com>
From:   Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
In-Reply-To: <6339c50a-8dfe-f3a2-63d7-504abd4e62f0@linux.intel.com>
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On 14-03-2023 04:00 am, Sathyanarayanan Kuppuswamy wrote:
> Hi Kulkarni,
> 
> On 3/13/23 2:12 PM, Bjorn Helgaas wrote:
>> On Mon, Feb 27, 2023 at 08:21:36PM -0800, 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.
>>
>> s/STU(Smallest/STU (Smallest/ (add space before paren)
>> s/Adding a function pci_ats_stu_configure()/Add pci_ats_stu_configure()/
>> s/Latter/Subsequent/
>>
>> Add blank line between paragraphs (it looks like "Latter enumerations"
>> is intended to start a new paragraph).
>>
>>> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
>>
>> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
>>
>> Given an ack for the IOMMU patch, I'd be happy to merge both (and I
>> can do the commit log tweaks); just let me know.
>>
>> One comment/question below.
>>
>>> ---
>>>   drivers/pci/ats.c       | 33 +++++++++++++++++++++++++++++++--
>>>   include/linux/pci-ats.h |  3 +++
>>>   2 files changed, 34 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>>> index f9cc2e10b676..1611bfa1d5da 100644
>>> --- a/drivers/pci/ats.c
>>> +++ b/drivers/pci/ats.c
>>> @@ -46,6 +46,35 @@ 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;
>>
>> I might return an error for the VF case on the assumption that it's
>> likely an error in the caller.  I guess one could argue that it
>> simplifies the caller if it doesn't have to check for PF vs VF.  But
>> the fact that STU is shared between PF and VFs is an important part of
>> understanding how ATS works, so the caller should be aware of the
>> distinction anyway.
> 
> I have already asked this question. But let me repeat it.
> 
> We don't have any checks for the PF case here. That means you can re-configure
> the STU as many times as you want until ATS is enabled in PF. So, if there are
> active VFs which uses this STU, can PF re-configure the STU at will?
> 

IMO, Since STU is shared, programming it multiple times is not expected 
from callers code do it, however we can add below check to allow to 
program STU once from a PF.

diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
index 1611bfa1d5da..f7bb01068e18 100644
--- a/drivers/pci/ats.c
+++ b/drivers/pci/ats.c
@@ -60,6 +60,10 @@ int pci_ats_stu_configure(struct pci_dev *dev, int ps)
         if (dev->ats_enabled || dev->is_virtfn)
                 return 0;

+       /* Configured already */
+       if (dev->ats_stu)
+               return 0;
+
         if (!pci_ats_supported(dev))
                 return -EINVAL;
>>
>>> +
>>> +	if (!pci_ats_supported(dev))
>>> +		return -EINVAL;
>>> +
>>> +	if (ps < PCI_ATS_MIN_STU)
>>> +		return -EINVAL;
>>> +
>>> +	dev->ats_stu = ps;
>>> +	pci_read_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, &ctrl);
>>> +	ctrl |= PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
>>> +	pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
>>> +
>>> +	return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(pci_ats_stu_configure);
>>> +
>>>   /**
>>>    * pci_enable_ats - enable the ATS capability
>>>    * @dev: the PCI device
>>> @@ -68,8 +97,8 @@ 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..7d62a92aaf23 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);
>>>   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);
>>> @@ -16,6 +17,8 @@ static inline bool pci_ats_supported(struct pci_dev *d)
>>>   { return false; }
>>>   static inline int pci_enable_ats(struct pci_dev *d, int ps)
>>>   { return -ENODEV; }
>>> +static inline int pci_ats_stu_configure(struct pci_dev *d, int ps)
>>> +{ return -ENODEV; }
>>>   static inline void pci_disable_ats(struct pci_dev *d) { }
>>>   static inline int pci_ats_queue_depth(struct pci_dev *d)
>>>   { return -ENODEV; }
>>> -- 
>>> 2.38.1
>>>
>>>
>>> _______________________________________________
>>> linux-arm-kernel mailing list
>>> linux-arm-kernel@lists.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

Thanks,
Ganapat