linux-kernel - Re: [PATCH v2 1/3] PCI/DPC: Request DPC only if also requesting AER

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Message-ID: <b81b1e5b-81fa-4df0-926d-1d75323cf47b@linux.intel.com>
Date: Sun, 25 Feb 2024 11:46:07 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Matthew W Carlis <mattc@...estorage.com>,
 Keith Busch <kbusch@...nel.org>, Lukas Wunner <lukas@...ner.de>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Jesse Brandeburg <jesse.brandeburg@...el.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] PCI/DPC: Request DPC only if also requesting AER


On 2/22/24 2:15 PM, Bjorn Helgaas wrote:
> From: Bjorn Helgaas <bhelgaas@...gle.com>
>
> When booting with "pci=noaer", we don't request control of AER, but we
> previously *did* request control of DPC, as in the dmesg log attached at
> the bugzilla below:
>
>   Command line: ... pci=noaer
>   acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM Segments MSI EDR HPX-Type3]
>   acpi PNP0A08:00: _OSC: OS now controls [PCIeHotplug SHPCHotplug PME PCIeCapability LTR DPC]
>
> That's illegal per PCI Firmware Spec, r3.3, sec 4.5.1, table 4-5, which
> says:
>
>   If the operating system sets this bit [OSC_PCI_EXPRESS_DPC_CONTROL], it
>   must also set bit 7 of the Support field (indicating support for Error
>   Disconnect Recover notifications) and bits 3 and 4 of the Control field
>   (requesting control of PCI Express Advanced Error Reporting and the PCI
>   Express Capability Structure).

IIUC, this dependency is discussed in sec 4.5.2.4. "Dependencies Between
_OSC Control Bits".

Because handling of Downstream Port Containment has a dependency on Advanced Error
Reporting, the operating system is required to request control over Advanced Error Reporting (bit 3
of the Control field) while requesting control over Downstream Port Containment Configuration
(bit 7 of the Control field). If the operating system attempts to claim control of Downstream Port
Containment Configuration without also claiming control over Advanced Error Reporting, firmware
is required to refuse control of the feature being illegally claimed and mask the corresponding bit.
Firmware is required to maintain ownership of Advanced Error Reporting if it retains ownership of
Downstream Port Containment Configuration.
If the operating system sets bit 7 of the Control field, it must set bit 7 of the Support field, indicating
support for the Error Disconnect Recover event.

>
> Request DPC control only if we have also requested AER control.
>
> Fixes: ac1c8e35a326 ("PCI/DPC: Add Error Disconnect Recover (EDR) support")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218491#c12
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: <stable@...r.kernel.org>	# v5.7+
> Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Cc: Matthew W Carlis <mattc@...estorage.com>
> Cc: Keith Busch <kbusch@...nel.org>
> Cc: Lukas Wunner <lukas@...ner.de>
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Jesse Brandeburg <jesse.brandeburg@...el.com>
> ---
Code wise it looks fine to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>  drivers/acpi/pci_root.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
> index 58b89b8d950e..efc292b6214e 100644
> --- a/drivers/acpi/pci_root.c
> +++ b/drivers/acpi/pci_root.c
> @@ -518,17 +518,19 @@ static u32 calculate_control(void)
>  	if (IS_ENABLED(CONFIG_HOTPLUG_PCI_SHPC))
>  		control |= OSC_PCI_SHPC_NATIVE_HP_CONTROL;
>  
> -	if (pci_aer_available())
> +	if (pci_aer_available()) {
>  		control |= OSC_PCI_EXPRESS_AER_CONTROL;
>  
> -	/*
> -	 * Per the Downstream Port Containment Related Enhancements ECN to
> -	 * the PCI Firmware Spec, r3.2, sec 4.5.1, table 4-5,
> -	 * OSC_PCI_EXPRESS_DPC_CONTROL indicates the OS supports both DPC
> -	 * and EDR.
> -	 */
> -	if (IS_ENABLED(CONFIG_PCIE_DPC) && IS_ENABLED(CONFIG_PCIE_EDR))
> -		control |= OSC_PCI_EXPRESS_DPC_CONTROL;
> +		/*
> +		 * Per PCI Firmware Spec, r3.3, sec 4.5.1, table 4-5, the
> +		 * OS can request DPC control only if it has advertised
> +		 * OSC_PCI_EDR_SUPPORT and requested both
> +		 * OSC_PCI_EXPRESS_CAPABILITY_CONTROL and
> +		 * OSC_PCI_EXPRESS_AER_CONTROL.
> +		 */
> +		if (IS_ENABLED(CONFIG_PCIE_DPC))
> +			control |= OSC_PCI_EXPRESS_DPC_CONTROL;
> +	}
>  
>  	return control;
>  }

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer