linux-kernel - Re: [PATCH 1/5] PCI/DPC: Ignore devices with no AER Capability

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Date:   Sat, 28 Nov 2020 20:32:32 -0800
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     ashok.raj@...el.com, knsathya@...nel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Olof Johansson <olof@...om.net>
Subject: Re: [PATCH 1/5] PCI/DPC: Ignore devices with no AER Capability



On 11/28/20 3:25 PM, Bjorn Helgaas wrote:
> On Sat, Nov 28, 2020 at 01:56:23PM -0800, Kuppuswamy, Sathyanarayanan wrote:
>> On 11/28/20 1:53 PM, Bjorn Helgaas wrote:
>>> On Sat, Nov 28, 2020 at 01:49:46PM -0800, Kuppuswamy, Sathyanarayanan wrote:
>>>> On 11/28/20 12:24 PM, Bjorn Helgaas wrote:
>>>>> On Wed, Nov 25, 2020 at 06:01:57PM -0800, Kuppuswamy, Sathyanarayanan wrote:
>>>>>> On 11/25/20 5:18 PM, Bjorn Helgaas wrote:
>>>>>>> From: Bjorn Helgaas <bhelgaas@...gle.com>
>>>>>>>
>>>>>>> Downstream Ports may support DPC regardless of whether they support AER
>>>>>>> (see PCIe r5.0, sec 6.2.10.2).  Previously, if the user booted with
>>>>>>> "pcie_ports=dpc-native", it was possible for dpc_probe() to succeed even if
>>>>>>> the device had no AER Capability, but dpc_get_aer_uncorrect_severity()
>>>>>>> depends on the AER Capability.
>>>>>>>
>>>>>>> dpc_probe() previously failed if:
>>>>>>>
>>>>>>>       !pcie_aer_is_native(pdev) && !pcie_ports_dpc_native
>>>>>>>       !(pcie_aer_is_native() || pcie_ports_dpc_native)    # by De Morgan's law
>>>>>>>
>>>>>>> so it succeeded if:
>>>>>>>
>>>>>>>       pcie_aer_is_native() || pcie_ports_dpc_native
>>>>>>>
>>>>>>> Fail dpc_probe() if the device has no AER Capability.
>>>>>>>
>>>>>>> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>>>>>>> Cc: Olof Johansson <olof@...om.net>
>>>>>>> ---
>>>>>>>      drivers/pci/pcie/dpc.c | 3 +++
>>>>>>>      1 file changed, 3 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
>>>>>>> index e05aba86a317..ed0dbc43d018 100644
>>>>>>> --- a/drivers/pci/pcie/dpc.c
>>>>>>> +++ b/drivers/pci/pcie/dpc.c
>>>>>>> @@ -287,6 +287,9 @@ static int dpc_probe(struct pcie_device *dev)
>>>>>>>      	int status;
>>>>>>>      	u16 ctl, cap;
>>>>>>> +	if (!pdev->aer_cap)
>>>>>>> +		return -ENOTSUPP;
>>>>>> Don't we check aer_cap support in drivers/pci/pcie/portdrv_core.c ?
>>>>>>
>>>>>> We don't enable DPC service, if AER service is not enabled. And AER
>>>>>> service is only enabled if AER capability is supported.
>>>>>>
>>>>>> So dpc_probe() should not happen if AER capability is not supported?
>>>>>
>>>>> I don't think that's always true.  If I'm reading this right, we have
>>>>> this:
>>>>>
>>>>>      get_port_device_capability(...)
>>>>>      {
>>>>>      #ifdef CONFIG_PCIEAER
>>>>>        if (dev->aer_cap && ...)
>>>>>          services |= PCIE_PORT_SERVICE_AER;
>>>>>      #endif
>>>>>
>>>>>        if (pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DPC) &&
>>>>>            pci_aer_available() &&
>>>>>            (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
>>>>>          services |= PCIE_PORT_SERVICE_DPC;
>>>>>      }
>>>>>
>>>>> and in the case where:
>>>>>
>>>>>      - CONFIG_PCIEAER=y
>>>>>      - booted with "pcie_ports=dpc-native" (pcie_ports_dpc_native is true)
>>>>>      - "dev" has no AER capability
>>>>>      - "dev" has DPC capability
>>>>>
>>>>> I think we do enable PCIE_PORT_SERVICE_DPC.
>>>> Got it. But further looking into it, I am wondering whether
>>>> we should keep this dependency? Currently we just use it to
>>>> dump the error information. Do we need to create dependency
>>>> between DPC and AER (which is functionality not dependent) just
>>>> to see more details about the error?
>>>
>>> That's a good question, but I don't really want to get into the actual
>>> operation of the AER and DPC drivers in this series, so maybe
>>> something we should explore later.
> 
>> In that case, can you move this check to
>> drivers/pci/pcie/portdrv_core.c?  I don't see the point of
>> distributed checks in both get_port_device_capability() and
>> dpc_probe().
> 
> I totally agree that these distributed checks are terrible, but my
> long-term hope is to get rid of portdrv and handle these "services"
> more like we handle other capabilities.  For example, maybe we can
> squash dpc_probe() into pci_dpc_init(), so I'd actually like to move
> things from get_port_device_capability() into dpc_probe().
Removing the service driver model will be a major overhaul. It would
affect even the error recovery drivers. You can find motivation
for service drivers in Documentation/PCI/pciebus-howto.rst.

But till we fix this part, I recommend grouping all dependency checks
to one place (either dpc_probe() or portdrv service driver).
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer