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: <ffd740e5-235a-4b74-8bf9-91331b619a7f@amd.com>
Date: Thu, 14 Nov 2024 10:45:39 -0600
From: "Bowman, Terry" <terry.bowman@....com>
To: Lukas Wunner <lukas@...ner.de>
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()

Hi Lukas,

I added comments below.

On 11/14/2024 9:45 AM, Lukas Wunner wrote:
> 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.
Ok.
> 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.

I see pcie_is_cxl(dev) is different than cxl_port_dvsec(dev).They check different DVSECs.[1] CXL flexbus DVSEC presence is cached in pci_dev::is_cxl and returned by pcie_is_cxl().
This is used for indicating CXL device.

cxl_port_dvsec(dev) returns boolean based on presence of CXL port DVSEC to 
indicate a CXL port device.

I don't believe they are redundant if you consider you can have a CXL device that 
is not a CXL port device. 

[1] - CXL 3.1, 8.1.1 Specification, PCIe Designated Vendor-Specific Extended Capability (DVSEC) ID Assignment
> 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
Ok.

Regards,
Terry