linux-kernel - Re: [PATCH v7 03/17] CXL/PCI: Introduce PCIe helper functions pcie_is_cxl() and pcie_is_cxl

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Message-ID: <67abd04519e67_2d1e294f2@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 11 Feb 2025 14:33:41 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Terry Bowman <terry.bowman@....com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	<nifan.cxl@...il.com>, <dave@...olabs.net>, <jonathan.cameron@...wei.com>,
	<dave.jiang@...el.com>, <alison.schofield@...el.com>,
	<vishal.l.verma@...el.com>, <dan.j.williams@...el.com>,
	<bhelgaas@...gle.com>, <mahesh@...ux.ibm.com>, <ira.weiny@...el.com>,
	<oohall@...il.com>, <Benjamin.Cheatham@....com>, <rrichter@....com>,
	<nathan.fontenot@....com>, <Smita.KoralahalliChannabasappa@....com>,
	<lukas@...ner.de>, <ming.li@...omail.com>,
	<PradeepVineshReddy.Kodamati@....com>
Subject: Re: [PATCH v7 03/17] CXL/PCI: Introduce PCIe helper functions
 pcie_is_cxl() and pcie_is_cxl_port()

Terry Bowman wrote:
> CXL and AER drivers need the ability to identify CXL devices and CXL port
> devices.
> 
> First, add set_pcie_cxl() with logic checking for CXL Flexbus DVSEC
> presence. The CXL Flexbus DVSEC presence is used because it is required
> for all the CXL PCIe devices.[1]
> 
> Add boolean 'struct pci_dev::is_cxl' with the purpose to cache the CXL
> Flexbus presence.
> 
> Add pcie_is_cxl() as a macro to return 'struct pci_dev::is_cxl'.
> 
> Add pcie_is_cxl_port() to check if a device is a CXL Root Port, CXL
> Upstream Switch Port, or CXL Downstream Switch Port. Also, verify the
> CXL Extensions DVSEC for Ports is present.[1]
> 
> [1] CXL 3.1 Spec, 8.1.1 PCIe Designated Vendor-Specific Extended
>     Capability (DVSEC) ID Assignment, Table 8-2
> 
> Signed-off-by: Terry Bowman <terry.bowman@....com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> Reviewed-by: Fan Ni <fan.ni@...sung.com>
> ---
>  drivers/pci/pci.c             | 13 +++++++++++++
>  drivers/pci/probe.c           | 10 ++++++++++
>  include/linux/pci.h           |  5 +++++
>  include/uapi/linux/pci_regs.h |  3 ++-
>  4 files changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 869d204a70a3..a2d8b41dd043 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5032,6 +5032,19 @@ static u16 cxl_port_dvsec(struct pci_dev *dev)
>  					 PCI_DVSEC_CXL_PORT);
>  }
>  
> +inline bool pcie_is_cxl(struct pci_dev *pci_dev)
> +{
> +	return pci_dev->is_cxl;
> +}
> +
> +bool pcie_is_cxl_port(struct pci_dev *dev)
> +{
> +	if (!pcie_is_cxl(dev))
> +		return false;
> +
> +	return (cxl_port_dvsec(dev) > 0);

At first I was concerned that this adds a capability list walk during
error handling, but patch 17 takes pcie_is_cxl_port() out of the
handles_cxl_errors() path.

It is still used in the aer_probe() path which means enumeration can
potentially race a CXL link up event.

I think this is fine for now because the CXL core has the same top-down
vs bottom-up race, and the CXL SBR code also shares the same race
problem.

A follow-on change needs to arrange for cxl_port_probe() to
enable/disable internal errors, because that path knows that a link has
been negotiated with an endpoint and that the CXL link details should be
stable.

> +}
> +
>  static bool cxl_sbr_masked(struct pci_dev *dev)
>  {
>  	u16 dvsec, reg;
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index b6536ed599c3..7737b9ce7a83 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1676,6 +1676,14 @@ static void set_pcie_thunderbolt(struct pci_dev *dev)
>  		dev->is_thunderbolt = 1;
>  }
>  
> +static void set_pcie_cxl(struct pci_dev *dev)
> +{
> +	u16 dvsec = pci_find_dvsec_capability(dev, PCI_VENDOR_ID_CXL,
> +					      PCI_DVSEC_CXL_FLEXBUS);
> +	if (dvsec)
> +		dev->is_cxl = 1;
> +}

Similar race problem here as it is premature to check for this DVSEC on
disconnected ports.

For now, lets add a comment to include/uapi/linux/pci_regs.h along the
lines of:

diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
index 3445c4970e4d..32df7abdd23c 100644
--- a/include/uapi/linux/pci_regs.h
+++ b/include/uapi/linux/pci_regs.h
@@ -1208,7 +1208,13 @@
 #define PCI_DOE_DATA_OBJECT_DISC_RSP_3_PROTOCOL		0x00ff0000
 #define PCI_DOE_DATA_OBJECT_DISC_RSP_3_NEXT_INDEX	0xff000000
 
-/* Compute Express Link (CXL r3.1, sec 8.1.5) */
+/*
+ * Compute Express Link (CXL r3.1, sec 8.1)
+ *
+ * Note that CXL DVSEC id 3 and 7 to be ignored when the CXL link state
+ * is "disconnected" (CXL r3.1, sec 9.12.3). Re-enumerate these
+ * registers on downstream link-up events.
+ */
 #define PCI_DVSEC_CXL_PORT				3
 #define PCI_DVSEC_CXL_PORT_CTL				0x0c
 #define PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR		0x00000001

...to at least remind our future selves that there is work to do here to
make the implementation robust against hot-plug scenarios.

With that you can add:

Reviewed-by: Dan Williams <dan.j.williams@...el.com>