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: <f8fb0737-3450-4dcf-87a1-3b9f03ec94f2@amd.com>
Date: Fri, 15 Nov 2024 07:54:37 -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()



On 11/15/2024 2:47 AM, Lukas Wunner wrote:
> On Thu, Nov 14, 2024 at 11:07:26AM -0600, Bowman, Terry wrote:
>>> Can you have a CXL port that is not a CXL device?
>>>
>>> If not, it would seem to me that checking for Flexbus DVSEC presence
>>> *is* redundant.  Or do you anticipate broken devices which lack the
>>> Flexbus DVSEC and that you explicitly want to exclude?
>> No, the CXL port device is always a CXL device per spec.
>>
>> This was added to short-circuit the function by returning immediately
>> if the device is _not_ a CXL device. Otherwise for PCIe Port devices,
>> the CXL Port DVSEC will be searched. I was trying to avoid the unnecessary
>> CXL port DVSEC search unless the other criteria are met.
>> And I expect most cases will not be a CXL device.
>>
>> I will remove the "if (!pcie_is_cxl(dev))" block as you suggested.
> Ah, this is meant as a speed-up.  Actually that makes sense,
> so feel free to keep it.
>
> If you do remove it, I think you'll have to move the cxl_port_dvsec()
> invocation up in the function, in front of the pci_pcie_type() checks.
> The latter require that one first checks that the device is PCIe.
> That's done implicitly by cxl_port_dvsec() because it returns 0 in
> the non-PCIe case.  (Due to the "if (dev->cfg_size <= PCI_CFG_SPACE_SIZE)"
> check in pci_find_next_ext_capability().)
>
> Another idea would be to put a "if (!pcie_is_cxl(dev)) return 0;" speed-up
> in cxl_port_dvsec() so that the other caller benefits from it as well.
>
> Thanks,
>
> Lukas
Ok, I'll look at adding the same pcie_is_cxl() call and check in cxl_port_devsec().

Regards,
Terry