linux-kernel - Re: [RESEND v13 02/25] PCI/CXL: Introduce pcie_is

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Message-ID: <930fcd64-92b2-462c-8301-6c753f41c498@amd.com>
Date: Wed, 19 Nov 2025 09:55:56 -0600
From: "Bowman, Terry" <terry.bowman@....com>
To: dan.j.williams@...el.com, dave@...olabs.net, jonathan.cameron@...wei.com,
 dave.jiang@...el.com, alison.schofield@...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,
 lukas@...ner.de, Benjamin.Cheatham@....com,
 sathyanarayanan.kuppuswamy@...ux.intel.com, linux-cxl@...r.kernel.org,
 alucerop@....com, ira.weiny@...el.com
Cc: linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [RESEND v13 02/25] PCI/CXL: Introduce pcie_is_cxl()



On 11/18/2025 9:19 PM, dan.j.williams@...el.com wrote:
> Terry Bowman wrote:
>> CXL and AER drivers need the ability to identify CXL devices.
>>
>> Introduce set_pcie_cxl() with logic checking for CXL.mem or CXL.cache
>> status in the CXL Flexbus DVSEC status register. 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
>> CXL.cache and CXl.mem status.
>>
>> In the case the device is an EP or USP, call set_pcie_cxl() on behalf of
>> the parent downstream device. Once a device is created there is
>> possibilty the parent training or CXL state was updated as well. This
>> will make certain the correct parent CXL state is cached.
>>
>> Add function pcie_is_cxl() to return 'struct pci_dev::is_cxl'.
>>
>> [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: Ira Weiny <ira.weiny@...el.com>
>> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>> Reviewed-by: Dave Jiang <dave.jiang@...el.com>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
>> Reviewed-by: Alejandro Lucero <alucerop@....com>
>> Reviewed-by: Ben Cheatham <benjamin.cheatham@....com>
>>
>> ---
>>
>> Changes in v12->v13:
>> - Add Ben's "reviewed-by"
>>
>> Changes in v11->v12:
>> - Add review-by for Alejandro
>> - Add comment in set_pcie_cxl() explaining why updating parent status.
>>
>> Changes in v10->v11:
>> - Amend set_pcie_cxl() to check for Upstream Port's and EP's parent
>>   downstream port by calling set_pcie_cxl(). (Dan)
>> - Retitle patch: 'Add' -> 'Introduce'
>> - Add check for CXL.mem and CXL.cache (Alejandro, Dan)
> [..]
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 0ce98e18b5a8..63124651f865 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -1709,6 +1709,33 @@ static void set_pcie_thunderbolt(struct pci_dev *dev)
>>  		dev->is_thunderbolt = 1;
>>  }
>>  
>> +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;
> Why are downstream ports excluded?
I thought we only need to check the upstream 'parent' if dev is an EP 
or USP as those are the only PCIe types in CXL that interface directly 
to the upstream dport device. And its the upstream dport device that must 
be checked to ensure it has the correct is_cxl setting. 

Do I need to update is_cxl for USP in the case of DSP-USP topology? 

Terry
>> +
>> +	/*
>> +	 * Update parent's CXL state because alternate protocol training
>> +	 * may have changed
>> +	 */
>> +	parent = pci_upstream_bridge(dev);
> This parent is a downstream port...


Terry