linux-kernel - Re: [PATCH v3 5/6] PCI: cadence: Add callback functions for RP and EP controller

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Message-ID: <94d08e9b-9718-4ee2-be9f-469177f7e4db@cixtech.com>
Date: Sun, 13 Apr 2025 00:02:39 +0800
From: Hans Zhang <hans.zhang@...tech.com>
To: Rob Herring <robh@...nel.org>
Cc: bhelgaas@...gle.com, lpieralisi@...nel.org, kw@...ux.com,
 manivannan.sadhasivam@...aro.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, Manikandan K Pillai <mpillai@...ence.com>
Subject: Re: [PATCH v3 5/6] PCI: cadence: Add callback functions for RP and EP
 controller



On 2025/4/12 23:45, Hans Zhang wrote:
>>> +     /*
>>> +      * Clear AXI link-down status
>>> +      */
>>
>> That is an odd thing to do in map_bus. Also, it is completely racy
>> because...
>>
>>> +     regval = cdns_pcie_hpa_readl(pcie, REG_BANK_AXI_SLAVE, 
>>> CDNS_PCIE_HPA_AT_LINKDOWN);
>>> +     cdns_pcie_hpa_writel(pcie, REG_BANK_AXI_SLAVE, 
>>> CDNS_PCIE_HPA_AT_LINKDOWN,
>>> +                          (regval & GENMASK(0, 0)));
>>> +
>>
>> What if the link goes down again here.
>>
> 
> Hi Rob,
> 
> Thanks your for reply. Compared to Synopsys PCIe IP, Cadence PCIe IP has 
> one more register - CDNS_PCIE_HPA_AT_LINKDOWN.  When the PCIe link 
> appears link down, the register -CDNS_PCIE_HPA_AT_LINKDOWN bit0 is set 
> to 1.  Then, ECAM cannot access config space. You need to clear 
> CDNS_PCIE_HPA_AT_LINKDOWN bit0 to continue the access.
> 
> In my opinion, this is where Cadence PCIe IP doesn't make sense.  As 
> Cadence users, we had no choice, and the chip had already been posted to 
> silicon.

Supplement: Prior to this series of patches, in the current linux master 
branch, Cadence first generation PCIe IP has the following code:

void __iomem *cdns_pci_map_bus(struct pci_bus *bus, unsigned int devfn,
			       int where)
{
	......
	/* Clear AXI link-down status */
	cdns_pcie_writel(pcie, CDNS_PCIE_AT_LINKDOWN, 0x0);
	......
}

It seems that all Cadence PCIe IPs have this problem.

> 
> Therefore, when we design the second-generation SOC, we will design an 
> SPI interrupt. When link down occurs, an SPI interrupt is generated. We 
> set CDNS_PCIE_HPA_AT_LINKDOWN bit0 to 0 in the interrupt function.
> 
> If there are other reasons, please Manikandan add.
> 
> 
> 
> In addition, by the way, ECAM is not accessible when the link is down. 
> For example, if the RP is set to hot reset, the hot reset cannot be 
> unreset. In this case, you need to use the APB to unreset. We mentioned 
> an RTL bug to Cadence that we currently can't fix with our first or 
> second generation chips. Cadence has not released RTL patch to us so far.
> 
> This software workaround approach will also later appear in the Cixtech 
> PCIe controller series patch.
> 
> 
> Best regards,
> Hans