Re: [PATCH v18 10/11] PCI/DPC: Add Error Disconnect Recover (EDR) support

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: bhelgaas@google.com, 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
Date: Thu, 11 Apr 2024 12:16:06 -0700	[thread overview]
Message-ID: <d1f40ee3-ffac-4277-a5b5-6f3d678dff6b@linux.intel.com> (raw)
In-Reply-To: <20240411180757.GA2190937@bhelgaas>

On 4/11/24 11:07 AM, Bjorn Helgaas wrote:
> 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
>> +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);
>> +        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);
> This has been upstream for a while, just a follow-up question: this
> _DSM function was defined by the ECN with Rev 5.  The ECN was
> incorporated into the PCI Firmware spec r3.3 with slightly different
> behavior as Rev 6.
>
> The main differences are:
>
>   ECN
>     - Rev 5
>     - Arg3 is an Integer
>     - Return is 0 (DPC disabled) or 1 (DPC enabled)
>
>   r3.3 spec
>     - Rev 6
>     - Arg3 is a Package of one Integer
>     - Return is 0 (DPC disabled, Hot-Plug Surprise may be set), 1 (DPC
>       enabled, Hot-Plug Surprise may be cleared), or 2 (failure)
>
> So the question is whether this actually implements Rev 5 or Rev 6?
> It looks like this builds a *package* for Arg3 (which would correspond
> to Rev 6), but we're evaluating Rev 5, which specified an Integer.
>
> The meaning of the Arg3 values is basically the same, so I don't see
> an issue there, but it looks like if a platform implemented Rev 5
> according to the ECN to take a bare Integer, this might not work
> correctly.

I think it implements rev 6. The version number needs to be updated.

If you would like, I can submit a patch to fix it.

>
>> +	if (!obj)
>> +		return 0;
>> +
>> +	if (obj->type != ACPI_TYPE_INTEGER) {
>> +		pci_err(pdev, FW_BUG "Enable DPC _DSM returned non integer\n");
>> +		status = -EIO;
>> +	}
>> +
>> +	if (obj->integer.value != 1) {
>> +		pci_err(pdev, "Enable DPC _DSM failed to enable DPC\n");
>> +		status = -EIO;
>> +	}
>> +
>> +	ACPI_FREE(obj);
>> +
>> +	return status;
>> +}

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer