linux-kernel - Re: [PATCH v3 03/15] cxl/pci: Introduce PCIe helper functions pcie_is_cxl() and pcie_is_cxl

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Message-ID: <ZzYbHZvU_RFXZuk0@wunner.de>
Date: Thu, 14 Nov 2024 16:45:33 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Terry Bowman <terry.bowman@....com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, nifan.cxl@...il.com, ming4.li@...el.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
Subject: Re: [PATCH v3 03/15] cxl/pci: Introduce PCIe helper functions
 pcie_is_cxl() and pcie_is_cxl_port()

On Wed, Nov 13, 2024 at 03:54:17PM -0600, Terry Bowman wrote:
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5038,6 +5038,20 @@ static u16 cxl_port_dvsec(struct pci_dev *dev)
>  					 PCI_DVSEC_CXL_PORT);
>  }
>  
> +bool pcie_is_cxl_port(struct pci_dev *dev)
> +{
> +	if (!pcie_is_cxl(dev))
> +		return false;
> +
> +	if ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
> +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_UPSTREAM) &&
> +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM))
> +		return false;
> +
> +	return cxl_port_dvsec(dev);
> +}
> +EXPORT_SYMBOL_GPL(pcie_is_cxl_port);

This doesn't need to be exported because the only caller introduced
in this series is in drivers/pci/pcie/aer.c (in patch 05/15), which
is dependent on CONFIG_PCIEAER, which is bool not tristate.

The "!pcie_is_cxl(dev)" check at the top of the function is identical
to the return value "cxl_port_dvsec(dev)".  This looks redundant.
However one cannot call pci_pcie_type() without first checking
pci_is_pcie().  So I'm wondering if the "!pcie_is_cxl(dev)" check
is actually erroneous and supposed to be "!pci_is_pcie(dev)"?
That would make more sense to me.

Alternatively, just return true instead of "cxl_port_dvsec(dev)".
That would probably be the simplest solution here.

> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -443,6 +443,7 @@ struct pci_dev {
>  	unsigned int	is_hotplug_bridge:1;
>  	unsigned int	shpc_managed:1;		/* SHPC owned by shpchp */
>  	unsigned int	is_thunderbolt:1;	/* Thunderbolt controller */
> +	unsigned int	is_cxl:1;               /* CXL alternate protocol */

I suspect the audience consists mostly of CXL-unaware PCI developers,
so spelling out Compute Express Link here (and omitting "alternate
protocol" if it doesn't fit) might be more appropriate.

Thanks,

Lukas