linux-kernel - Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS STU of a PF

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Message-ID: <eefaf1bc-586b-aa0f-8899-fcce0460b733@linux.intel.com>
Date:   Tue, 28 Feb 2023 08:30:24 -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 v2 1/2] PCI/ATS: Add a helper function to configure ATS
 STU of a PF



On 2/27/23 8:21 PM, 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.
> 
> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
> ---
>  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;

Is PF allowed to re-configure STU if there are other active
VF's which uses it?

> +
> +	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; }

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer