linux-kernel - Re: [PATCH] PCI/AER: Enable SERR# forwarding in non ACPI flow

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Date:   Tue, 31 Jul 2018 17:47:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        bhelgaas@...gle.com
Subject: Re: [PATCH] PCI/AER: Enable SERR# forwarding in non ACPI flow

On Thu, Jul 12, 2018 at 08:15:19PM +0530, Bharat Kumar Gogada wrote:
> Currently PCI_BRIDGE_CTL_SERR is being enabled only in
> ACPI flow.
> This bit is required for forwarding errors reported
> by EP devices to upstream device.
> This patch enables SERR# for Type-1 PCI device.

This does seem broken.

Figure 6-3 in PCIe r4.0, sec 6.2.6, would be a helpful reference to
include in the commit log.

Semi-related question: there are about 40 drivers that call
pci_enable_pcie_error_reporting() and
pci_disable_pcie_error_reporting().  I see that the PCI core
calls pci_enable_pcie_error_reporting() for Root Ports and Switch
Ports in this path:

  aer_probe                     # for root ports only
    aer_enable_rootport
      set_downstream_devices_error_reporting
        set_device_error_reporting
          if (ROOT_PORT || UPSTREAM || DOWNSTREAM)
            pci_enable_pcie_error_reporting
        pci_walk_bus(..., set_device_error_reporting)

But the core doesn't call pci_enable_pcie_error_reporting() for
endpoints.  I wonder why not.  Could we?  And then remove the calls
from those drivers?  If PCI_EXP_AER_FLAGS should only be set if the
driver is prepared, the pci_driver.err_handler would be a good hint.
But I suspect we could do something sensible and at least report
errors even if the driver doesn't have err_handler callbacks.

On MIPS Octeon, it looks like pcibios_plat_dev_init() does already set
PCI_EXP_AER_FLAGS for every device.

But this question is obviously far beyond the scope of this current
patch.

> Signed-off-by: Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
> ---
>  drivers/pci/pcie/aer.c |   23 +++++++++++++++++++++++
>  1 files changed, 23 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
> index a2e8838..943e084 100644
> --- a/drivers/pci/pcie/aer.c
> +++ b/drivers/pci/pcie/aer.c
> @@ -343,6 +343,19 @@ int pci_enable_pcie_error_reporting(struct pci_dev *dev)
>  	if (!dev->aer_cap)
>  		return -EIO;
>  
> +	if (!IS_ENABLED(CONFIG_ACPI) &&
> +	    dev->hdr_type == PCI_HEADER_TYPE_BRIDGE) {

I think this test needs to be refined a little bit.  If the kernel
happens to be built with CONFIG_ACPI=y but the current platform
doesn't support ACPI, we still want to set PCI_BRIDGE_CTL_SERR,
don't we?

> +		u16 control;
> +
> +		/*
> +		 * A Type-1 PCI bridge will not forward ERR_ messages coming
> +		 * from an endpoint if SERR# forwarding is not enabled.
> +		 */
> +		pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &control);
> +		control |= PCI_BRIDGE_CTL_SERR;
> +		pci_write_config_word(dev, PCI_BRIDGE_CONTROL, control);
> +	}
> +
>  	return pcie_capability_set_word(dev, PCI_EXP_DEVCTL, PCI_EXP_AER_FLAGS);
>  }
>  EXPORT_SYMBOL_GPL(pci_enable_pcie_error_reporting);
> @@ -352,6 +365,16 @@ int pci_disable_pcie_error_reporting(struct pci_dev *dev)
>  	if (pcie_aer_get_firmware_first(dev))
>  		return -EIO;
>  
> +	if (!IS_ENABLED(CONFIG_ACPI) &&
> +	    dev->hdr_type == PCI_HEADER_TYPE_BRIDGE) {
> +		u16 control;
> +
> +		/* Clear SERR Forwarding */
> +		pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &control);
> +		control &= ~PCI_BRIDGE_CTL_SERR;
> +		pci_write_config_word(dev, PCI_BRIDGE_CONTROL, control);
> +	}
> +
>  	return pcie_capability_clear_word(dev, PCI_EXP_DEVCTL,
>  					  PCI_EXP_AER_FLAGS);
>  }
> -- 
> 1.7.1
>