linux-kernel - Re: [PATCH v2] pci: Don't set RCB bit in LNKCTL if the upstream bridge hasn't

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Date:   Mon, 31 Oct 2016 15:41:44 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Yinghai Lu <yinghai@...nel.org>,
        Hannes Reinecke <hare@...e.de>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, agraf@...e.de
Subject: Re: [PATCH v2] pci: Don't set RCB bit in LNKCTL if the upstream
 bridge hasn't

On Wed, Oct 26, 2016 at 04:30:16PM +0200, Johannes Thumshirn wrote:
> The Read Completion Boundary bit must only be set on a device or endpoint if
> it is set on the upstream bridge.

Please include the spec reference and a stable tag (if you think
that's relevant) in the next version.

I know you have an internal SuSE bugzilla that isn't useful here.  I
would like a public bugzilla at bugzilla.kernel.org with an attached
"lspci -vv" output and dmesg log.  You can exclude things that are
proprietary or otherwise secret, but this is a fairly subtle area
related to MPS and MRRS.  We have known issue there, and if/when we
get around to addressing them, I'd like to have public documentation
about pitfalls we need to avoid.

It would really be helpful if this changelog could include an outline
of what's going wrong from the hardware point of view when the
endpoint's RCB is set incorrectly.  I've read the spec, but I'm not
enough of a hardware person to develop intution about what's failing
here.  A concrete scenario would help a lot, e.g.,

  - Root Port RCB is 64 bytes
  - Endpoint RCB is 128 bytes (incorrect per spec, since Root Port's
    RDB is only 64 bytes)
  - Endpoint issues Memory Read Request for 128 bytes, 128-byte
    aligned
  - Root Port responds to Memory Read Request with two 64-byte
    Completions
  - Endpoint (as the Receiver) has RCB of 128, so it is expecting a
    single Completion and sees two Completions as a violation of the
    "Completions for Requests which do not cross the naturally aligned
    address boundaries at integer multiples of RCB bytes must include
    all data specified in the Request" rule in 2.3.1.1, and treats
    this as a Malformed TLP.

If that's really the scenario, we should be able to see this in the
lspci output, e.g., something like:

  # lspci -vv
  # modprobe mlx4
  # lspci -vv

should show no error in the first lspci, but something in the second.

It seems plausible that this is exposed by a BIOS bug.  I'd like to
include specifics about the adapter, the machine, and BIOS version in
the changelog to help connect this fix with the machine known to have
the problem.

> Fixes: 7a1562d4f ("PCI: Apply _HPX Link Control settings to all devices with a link")
> Signed-off-by: Johannes Thumshirn <jthumshirn@...e.de>
> Reviewed-by: Hannes Reinecke <hare@...e.com>
> ---
>  drivers/pci/probe.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> Changes from v1:
> * Check if we have a upstream bridge to not trip on a NULL pointer
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index ab00267..716c5cf 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1439,6 +1439,19 @@ static void program_hpp_type1(struct pci_dev *dev, struct hpp_type1 *hpp)
>  		dev_warn(&dev->dev, "PCI-X settings not supported\n");
>  }
>  
> +static bool pcie_get_upstream_rcb(struct pci_dev *dev)
> +{
> +	struct pci_dev *bridge = pci_upstream_bridge(dev);
> +	u16 lnkctl;
> +
> +	if (!bridge)
> +		return false;
> +
> +	pcie_capability_read_word(bridge, PCI_EXP_LNKCTL, &lnkctl);
> +
> +	return lnkctl & PCI_EXP_LNKCTL_RCB;
> +}
> +
>  static void program_hpp_type2(struct pci_dev *dev, struct hpp_type2 *hpp)
>  {
>  	int pos;
> @@ -1468,9 +1481,21 @@ static void program_hpp_type2(struct pci_dev *dev, struct hpp_type2 *hpp)
>  			~hpp->pci_exp_devctl_and, hpp->pci_exp_devctl_or);
>  
>  	/* Initialize Link Control Register */
> -	if (pcie_cap_has_lnkctl(dev))
> +	if (pcie_cap_has_lnkctl(dev)) {
> +		bool us_rcb;
> +		u16 clear;
> +		u16 set;
> +
> +		us_rcb = pcie_get_upstream_rcb(dev);

Per spec (PCIe r3.0, sec 7.8.7), RCB is applicable only to Root Ports,
Endpoints, and Bridges.  This is somewhat ambiguous: the "Bridge"
definition in "Terms and Acronyms" seems to include Switch Ports, but
the text in 7.8.7 seems to exclude Switch Ports.

In any case, 7.8.7 is clear that we should set this bit only if the
RCB in the upstream *Root Port* (not the nearest upstream switch port
as we're using here) is set.

> +
> +		clear = ~hpp->pci_exp_lnkctl_and;
> +		set = hpp->pci_exp_lnkctl_or;
> +		if (!us_rcb)
> +			set &= ~PCI_EXP_LNKCTL_RCB;
> +
>  		pcie_capability_clear_and_set_word(dev, PCI_EXP_LNKCTL,
> -			~hpp->pci_exp_lnkctl_and, hpp->pci_exp_lnkctl_or);
> +						  clear, set);
> +	}
>  
>  	/* Find Advanced Error Reporting Enhanced Capability */
>  	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ERR);
> -- 
> 1.8.5.6
> 
> --
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