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Date:   Tue, 24 Mar 2020 16:37:10 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     sathyanarayanan.kuppuswamy@linux.intel.com
Cc:     linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        ashok.raj@intel.com, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH v18 10/11] PCI/DPC: Add Error Disconnect Recover (EDR)
 support
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On Mon, Mar 23, 2020 at 05:26:07PM -0700, sathyanarayanan.kuppuswamy@linux.intel.com wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> 
> Error Disconnect Recover (EDR) is a feature that allows ACPI firmware to
> notify OSPM that a device has been disconnected due to an error condition
> (ACPI v6.3, sec 5.6.6).  OSPM advertises its support for EDR on PCI devices
> via _OSC (see [1], sec 4.5.1, table 4-4).  The OSPM EDR notify handler
> should invalidate software state associated with disconnected devices and
> may attempt to recover them.  OSPM communicates the status of recovery to
> the firmware via _OST (sec 6.3.5.2).
> 
> For PCIe, firmware may use Downstream Port Containment (DPC) to support
> EDR.  Per [1], sec 4.5.1, table 4-6, even if firmware has retained control
> of DPC, OSPM may read/write DPC control and status registers during the EDR
> notification processing window, i.e., from the time it receives an EDR
> notification until it clears the DPC Trigger Status.
> 
> Note that per [1], sec 4.5.1 and 4.5.2.4,
> 
>   1. If the OS supports EDR, it should advertise that to firmware by
>      setting OSC_PCI_EDR_SUPPORT in _OSC Support.
> 
>   2. If the OS sets OSC_PCI_EXPRESS_DPC_CONTROL in _OSC Control to request
>      control of the DPC capability, it must also set OSC_PCI_EDR_SUPPORT in
>      _OSC Support.
> 
> Add an EDR notify handler to attempt recovery.
> 
> [1] Downstream Port Containment Related Enhancements ECN, Jan 28, 2019,
>     affecting PCI Firmware Specification, Rev. 3.2
>     https://members.pcisig.com/wg/PCI-SIG/document/12888
> Link: https://lore.kernel.org/r/9ae1d3285beeb81bbf85571a89b8f3d4451eae8f.1583286655.git.sathyanarayanan.kuppuswamy@linux.intel.com
> Link: https://lore.kernel.org/r/246aa05acca8f0a7e6d20a65ab05af0027f60118.1583286655.git.sathyanarayanan.kuppuswamy@linux.intel.com
> [bhelgaas: squash add/enable patches into one]
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Len Brown <lenb@kernel.org>
> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>

> +static int acpi_enable_dpc(struct pci_dev *pdev)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(&pdev->dev);
> +	union acpi_object *obj, argv4, req;
> +	int status;
> +
> +	/*
> +	 * Some firmware implementations will return default values for
> +	 * unsupported _DSM calls. So checking acpi_evaluate_dsm() return
> +	 * value for NULL condition is not a complete method for finding
> +	 * whether given _DSM function is supported or not. So use
> +	 * explicit func 0 call to find whether given _DSM function is
> +	 * supported or not.
> +	 */
> +        status = acpi_check_dsm(adev->handle, &pci_acpi_dsm_guid, 5,
> +				1ULL << EDR_PORT_DPC_ENABLE_DSM);

This is really ugly.  What's the story on this firmware?  It sounds
defective to me.

Or is everybody that uses _DSM supposed to check before evaluating it?
E.g.,

  if (!acpi_check_dsm(...))
    return -EINVAL;

  obj = acpi_evaluate_dsm(...);

If everybody is supposed to do this, it seems like the check part
should be moved into acpi_evaluate_dsm().

> +        if (!status)
> +                return 0;
> +
> +	status = 0;
> +	req.type = ACPI_TYPE_INTEGER;
> +	req.integer.value = 1;
> +
> +	argv4.type = ACPI_TYPE_PACKAGE;
> +	argv4.package.count = 1;
> +	argv4.package.elements = &req;
> +
> +	/*
> +	 * Per Downstream Port Containment Related Enhancements ECN to PCI
> +	 * Firmware Specification r3.2, sec 4.6.12, EDR_PORT_DPC_ENABLE_DSM is
> +	 * optional.  Return success if it's not implemented.
> +	 */
> +	obj = acpi_evaluate_dsm(adev->handle, &pci_acpi_dsm_guid, 5,
> +				EDR_PORT_DPC_ENABLE_DSM, &argv4);
> +	if (!obj)
> +		return 0;