Date:   Mon, 13 Mar 2023 16:12:01 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
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,
        sathyanarayanan.kuppuswamy@linux.intel.com,
        darren@os.amperecomputing.com, scott@os.amperecomputing.com
Subject: Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS
 STU of a PF
Message-ID: <20230313211201.GA1540091@bhelgaas>
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In-Reply-To: <20230228042137.1941024-2-gankulkarni@os.amperecomputing.com>
Precedence: bulk

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.

> +
> +	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
> 
> 
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