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Message-ID: <6339c50a-8dfe-f3a2-63d7-504abd4e62f0@linux.intel.com>
Date: Mon, 13 Mar 2023 15:30:30 -0700
From: Sathyanarayanan Kuppuswamy
<sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Bjorn Helgaas <helgaas@...nel.org>,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Cc: 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, jean-philippe@...aro.org,
darren@...amperecomputing.com, scott@...amperecomputing.com
Subject: Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS
STU of a PF
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@...amperecomputing.com>
>
> Acked-by: Bjorn Helgaas <bhelgaas@...gle.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?
>
>> +
>> + 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@...ts.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer
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