linux-kernel - Re: [PATCH v11 08/23] PCI/CXL: Introduce pcie_is

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Message-ID: <aMJGcDMBUPPAom4d@wunner.de>
Date: Thu, 11 Sep 2025 05:48:00 +0200
From: Lukas Wunner <lukas@...ner.de>
To: "Bowman, Terry" <terry.bowman@....com>
Cc: Alejandro Lucero Palau <alucerop@....com>, dave@...olabs.net,
	jonathan.cameron@...wei.com, dave.jiang@...el.com,
	alison.schofield@...el.com, dan.j.williams@...el.com,
	bhelgaas@...gle.com, shiju.jose@...wei.com, ming.li@...omail.com,
	Smita.KoralahalliChannabasappa@....com, rrichter@....com,
	dan.carpenter@...aro.org, PradeepVineshReddy.Kodamati@....com,
	Benjamin.Cheatham@....com,
	sathyanarayanan.kuppuswamy@...ux.intel.com,
	linux-cxl@...r.kernel.org, ira.weiny@...el.com,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v11 08/23] PCI/CXL: Introduce pcie_is_cxl()

On Wed, Sep 10, 2025 at 11:24:20AM -0500, Bowman, Terry wrote:
> On 8/28/2025 3:18 AM, Alejandro Lucero Palau wrote:
> > On 8/27/25 02:35, Terry Bowman wrote:
> >> +static void set_pcie_cxl(struct pci_dev *dev)
> >> +{
> >> +	struct pci_dev *parent;
> >> +	u16 dvsec = pci_find_dvsec_capability(dev, PCI_VENDOR_ID_CXL,
> >> +					      PCI_DVSEC_CXL_FLEXBUS_PORT);
> >> +	if (dvsec) {
> >> +		u16 cap;
> >> +
> >> +		pci_read_config_word(dev, dvsec + PCI_DVSEC_CXL_FLEXBUS_STATUS_OFFSET, &cap);
> >> +
> >> +		dev->is_cxl = FIELD_GET(PCI_DVSEC_CXL_FLEXBUS_STATUS_CACHE_MASK, cap) ||
> >> +			FIELD_GET(PCI_DVSEC_CXL_FLEXBUS_STATUS_MEM_MASK, cap);
> >> +	}
> >> +
> >> +	if (!pci_is_pcie(dev) ||
> >> +	    !(pci_pcie_type(dev) == PCI_EXP_TYPE_ENDPOINT ||
> >> +	      pci_pcie_type(dev) == PCI_EXP_TYPE_UPSTREAM))
> >> +		return;
> >> +
> >> +	parent = pci_upstream_bridge(dev);
> >> +	set_pcie_cxl(parent);
> >
> > This recursion is confusing to me.
> >
> > Is it not the parent already having this set from its own pci setup? Or 
> > maybe do we expect that to change after a reset and this is a sanity check?
> 
> Right. The upstream parent bus state is already set but could change after
> reset.

Please add a code comment as this is non-obvious at least to me.

Thanks,

Lukas