linux-kernel - Re: [PATCH] PCI: cadence: Set the AFS bit in Device Capabilities 2 Register

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Message-ID: <20230802161953.GA60043@bhelgaas>
Date:   Wed, 2 Aug 2023 11:19:53 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Achal Verma <a-verma1@...com>
Cc:     Vignesh Raghavendra <vigneshr@...com>,
        Tom Joseph <tjoseph@...ence.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczy_ski <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-omap@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: cadence: Set the AFS bit in Device Capabilities 2
 Register

In subject, "Advertise ARI Forwarding Supported".

It's not obvious that "AFS" refers to ARI Forwarding Supported, and
the bit name is enough; we don't need to know that it's in Dev Cap 2.
"Advertise" shows that we're just *advertising* the functionality, not
*enabling* it.

On Wed, Aug 02, 2023 at 04:00:59PM +0530, Achal Verma wrote:
> J7 PCIe Root Complex has ARI Forwarding Support, means supporting
> forwarding of TLPs addressed to functions with function number greater than
> 7 but some PCIe instances on J7 have this bit cleared which results in
> failure of forwarding of TLPs destined for function number > 7.
> Setting the AFS bit in Device Capabilities 2 Register explicitly, resolves
> the issue and leads to successful access to function number > 7.

s/AFS/ARI Forwarding Supported/

> Some observations:
> 1. J7200-EVB has single PCIe instance(PCIe1) for which ARIFwd bit is not
>    set. Enumeration gracefully fails for funciton number greater than 7 but
>    later read/write access to these funcitons results in a crash.

By "ARIFwd bit" here, I assume you mean PCI_EXP_DEVCAP2_ARI in the Root
Port?  Maybe you can use the #define to make this more greppable.

s/funciton/function/ (twice)

If we don't enumerate function numbers greater than 7, we shouldn't
have pci_dev structs for them, so why are there later read/write
config accesses to them?

If the Root Port doesn't advertise ARI Forwarding Supported,
bridge->ari_enabled will not be set, and we shouldn't even try to
enumerate functions greater than 7.  So it's not that enumeration
*fails*; it just doesn't happen at all.

> 2. On J721E-EVB, PCIe1 instance has ARIFwd bit set while it is cleared for
>    PCIe0 instance. This issue combined with errata i2086
>    (Unsupported Request (UR) Response Results in External Abort) results in
>    SERROR while scanning multi-function endpoint device.

Is the SERROR when scanning under PCIe0 or under PCIe1?

I'm not clear on what's happening here:

  1) Root Port advertises PCI_EXP_DEVCAP2_ARI, we set
     bridge->ari_enabled and scan functions > 7, we do a config read
     to function 8, get a UR response (as expected during enumeration)
     and that results in SERROR?

  2) Root Port *doesn't* advertise PCI_EXP_DEVCAP2_ARI, we don't set
     bridge->ari_enabled, so we don't try config read to function 8,
     and something blows up later?

  3) Something else?

> @@ -507,6 +507,7 @@ int cdns_pcie_host_setup(struct cdns_pcie_rc *rc)
>  	struct cdns_pcie *pcie;
>  	struct resource *res;
>  	int ret;
> +	u32 pcie_cap2;
>  
>  	bridge = pci_host_bridge_from_priv(rc);
>  	if (!bridge)
> @@ -536,6 +537,12 @@ int cdns_pcie_host_setup(struct cdns_pcie_rc *rc)
>  	if (rc->quirk_detect_quiet_flag)
>  		cdns_pcie_detect_quiet_min_delay_set(&rc->pcie);
>  
> +	if (rc->set_afs_bit) {
> +		pcie_cap2 = cdns_pcie_rp_readl(pcie, CDNS_PCIE_RP_CAP_OFFSET + PCI_EXP_DEVCAP2);
> +		pcie_cap2 |= PCI_EXP_DEVCAP2_ARI;
> +		cdns_pcie_rp_writel(pcie, CDNS_PCIE_RP_CAP_OFFSET + PCI_EXP_DEVCAP2, pcie_cap2);
> +	}

This seems like a j721e defect; why does the workaround need to be in
the generic cadence code?

Bjorn