Subject: Re: [PATCH v2 2/2] PCI/DPC: Allow Native DPC Host Bridges to use DPC

From: "Kuppuswamy, Sathyanarayanan" <[email-masked]>
Date: Mon, 27 Apr 2020 08:43:03 -0700

On 4/27/20 8:15 AM, Derrick, Jonathan wrote:
> Hi Sathyanarayanan,
> 
> On Sat, 2020-04-25 at 13:46 -0700, Kuppuswamy, Sathyanarayanan wrote:
>>
>> On 4/23/20 8:11 AM, Derrick, Jonathan wrote:
>>> Hi Sathyanarayanan,
>>>
>>> On Wed, 2020-04-22 at 15:50 -0700, Kuppuswamy, Sathyanarayanan wrote:
>>>> On 4/20/20 2:37 PM, Jon Derrick wrote:
>>>>> The existing portdrv model prevents DPC services without either OS
>>>>> control (_OSC) granted to AER services, a Host Bridge requesting Native
>>>>> AER, or using one of the 'pcie_ports=' parameters of 'native' or
>>>>> 'dpc-native'.
>>>>>
>>>>> The DPC port service driver itself will also fail to probe if the kernel
>>>>> assumes the port is using Firmware-First AER. It's a reasonable
>>>>> expectation that a port using Firmware-First AER will also be using
>>>>> Firmware-First DPC, however if a Host Bridge requests Native DPC, the
>>>>> DPC driver should allow it and not fail to bind due to AER capability
>>>>> settings.
>>>>>
>>>>> Host Bridges which request Native DPC port services will also likely
>>>>> request Native AER, however it shouldn't be a requirement. This patch
>>>>> allows ports on those Host Bridges to have DPC port services.
>>>>>
>>>>> This will avoid the unlikely situation where the port is Firmware-First
>>>>> AER and Native DPC, and a BIOS or switch firmware preconfiguration of
>>>>> the DPC trigger could result in unhandled DPC events.
>>>>>
>>>>> Signed-off-by: Jon Derrick <[email-masked]>
>>>>> ---
>>>>>     drivers/pci/pcie/dpc.c          | 3 ++-
>>>>>     drivers/pci/pcie/portdrv_core.c | 3 ++-
>>>>>     2 files changed, 4 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
>>>>> index 7621704..3f3106f 100644
>>>>> --- a/drivers/pci/pcie/dpc.c
>>>>> +++ b/drivers/pci/pcie/dpc.c
>>>>> @@ -284,7 +284,8 @@ static int dpc_probe(struct pcie_device *dev)
>>>>>     	int status;
>>>>>     	u16 ctl, cap;
>>>>>     
>>>>> -	if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native)
>>>>> +	if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native &&
>>>>> +	    !pci_find_host_bridge(pdev->bus)->native_dpc)
>>>> Why do it in probe as well ? if host->native_dpc is not set then the
>>>> device DPC probe it self won't happen right ?
>>>
>>> Portdrv only enables the interrupt and allows the probe to occur.
>>
>> Please check the following snippet of code (from portdrv_core.c).
>>
>> IIUC, pcie_device_init() will not be called if PCIE_PORT_SERVICE_DPC is
>> not set in capabilities. Your change in portdrv_core.c already
>> selectively enables the PCIE_PORT_SERVICE_DPC service based on
>> native_dpc value.
>>
> That's right. So pcie_device_init registers the port service driver
> allowing the services enumeration to occur.
> 
>> So IMO, adding native_dpc check in dpc_probe() is redundant.
>>
>> int pcie_port_device_register(struct pci_dev *dev)
>> 	/* Allocate child services if any */
>> 	status = -ENODEV;
>> 	nr_service = 0;
>> 	for (i = 0; i < PCIE_PORT_DEVICE_MAXSERVICES; i++) {
>> 		int service = 1 << i;
>> 		if (!(capabilities & service))
>> 			continue;
>> 		if (!pcie_device_init(dev, service, irqs[i]))
>> 			nr_service++;
>> 	}
>>
> This is the tricky part
> There's still a check in dpc_probe for AER FFS or pcie_ports=dpc-
> native:
> 
> if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native)
> 	return -ENOTSUPP;
> 
> One option is to move that to get_port_device_capability and remove the
> dpc_probe check
Yes, its better to group them together in get_port_device_capability().

But it should be done in a separate patch.
> 
>>> The probe itself will still fail if there's a mixed-mode _OSC
>>> negotiated AER & DPC, due to pcie_aer_get_firmware_first returning 1
>>> for AER and no check for DPC.
>>>
>>> I don't know if such a platform will exist, but the kernel is already
>>> wired for 'dpc-native' so it makes sense to extend it for this..
>>>
>>> This transform might be more readable:
>>> 	if (pcie_aer_get_firmware_first(pdev) &&
>>> 	    !(pcie_ports_dpc_native || hb->native_dpc))
>>>
>>>
>>>
>>>>>     		return -ENOTSUPP;
>>>>>     
>>>>>     	status = devm_request_threaded_irq(device, dev->irq, dpc_irq,
>>>>> diff --git a/drivers/pci/pcie/portdrv_core.c b/drivers/pci/pcie/portdrv_core.c
>>>>> index 50a9522..f2139a1 100644
>>>>> --- a/drivers/pci/pcie/portdrv_core.c
>>>>> +++ b/drivers/pci/pcie/portdrv_core.c
>>>>> @@ -256,7 +256,8 @@ static int get_port_device_capability(struct pci_dev *dev)
>>>>>     	 */
>>>>>     	if (pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DPC) &&
>>>>>     	    pci_aer_available() &&
>>>>> -	    (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
>>>>> +	    (pcie_ports_dpc_native || host->native_dpc ||
>>>>> +	     (services & PCIE_PORT_SERVICE_AER)))
>>>>>     		services |= PCIE_PORT_SERVICE_DPC;
>>>>>     
>>>>>     	if (pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM ||
>>>>>