linux-kernel - Re: [PATCH v5 03/16] CXL/PCI: Introduce PCIe helper functions pcie_is_cxl() and pcie_is_cxl

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Message-ID: <94ea539f-763b-42ae-bf5c-6013edf2f188@amd.com>
Date: Tue, 14 Jan 2025 17:39:40 -0600
From: "Bowman, Terry" <terry.bowman@....com>
To: Ira Weiny <ira.weiny@...el.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,
 oohall@...il.com, Benjamin.Cheatham@....com, rrichter@....com,
 nathan.fontenot@....com, Smita.KoralahalliChannabasappa@....com,
 lukas@...ner.de, ming.li@...omail.com, PradeepVineshReddy.Kodamati@....com,
 alucerop@....com
Subject: Re: [PATCH v5 03/16] CXL/PCI: Introduce PCIe helper functions
 pcie_is_cxl() and pcie_is_cxl_port()




On 1/14/2025 5:33 PM, Ira Weiny wrote:
> Bowman, Terry wrote:
>>
>>
>> On 1/13/2025 5:49 PM, Ira Weiny wrote:
>>> 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           |  4 ++++
>>>>  include/uapi/linux/pci_regs.h |  3 ++-
>>>>  4 files changed, 29 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index 661f98c6c63a..9319c62e3488 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -5036,10 +5036,23 @@ static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
>>>>  
>>>>  static u16 cxl_port_dvsec(struct pci_dev *dev)
>>>>  {
>>>> +	if (!pcie_is_cxl(dev))
>>>> +		return 0;
>>>> +
>>>>  	return pci_find_dvsec_capability(dev, PCI_VENDOR_ID_CXL,
>>>>  					 PCI_DVSEC_CXL_PORT);
>>>>  }
>>>>  
>>>> +bool pcie_is_cxl_port(struct pci_dev *dev)
>>>> +{
>>>> +	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);
>>> Returning bool from a function which returns u16 is odd and I don't think
>>> it should be coded this way.  I don't think it is wrong right now but this
>>> really ought to code the pcie_is_cxl() here and leave cxl_port_dvsec()
>>> alone.  Calling cxl_port_dvsec(), checking for if the dvsec exists, and
>>> returning bool.
>> Hi Ira,
>>
>> Thanks for reviewing. Is this what you are looking for here:
>>
>> +bool pcie_is_cxl_port(struct pci_dev *dev)
>> +{
>> +	return (cxl_port_dvsec(dev) > 0);
> With the type checks, yes that is more clear.
>
> Ira
>
> [snip]
Since sending the above I made update to be:

static u16 cxl_port_dvsec(struct pci_dev *dev)
{
        return pci_find_dvsec_capability(dev, PCI_VENDOR_ID_CXL,
                                         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 *pci_dev)
{
        if (!pcie_is_cxl(pci_dev))
                return false;

        return (cxl_port_dvsec(pci_dev) > 0);
}

I can change if you see anything is needed.

Regards,
Terry