Date: Tue, 25 Jun 2024 09:29:45 -0500
From: Terry Bowman <Terry.Bowman@....com>
To: Dan Williams <dan.j.williams@...el.com>, ira.weiny@...el.com,
 dave@...olabs.net, dave.jiang@...el.com, alison.schofield@...el.com,
 ming4.li@...el.com, vishal.l.verma@...el.com, jim.harris@...sung.com,
 ilpo.jarvinen@...ux.intel.com, ardb@...nel.org,
 sathyanarayanan.kuppuswamy@...ux.intel.com, linux-cxl@...r.kernel.org,
 linux-kernel@...r.kernel.org, Yazen.Ghannam@....com, Robert.Richter@....com
Subject: Re: [RFC PATCH 0/9] Add RAS support for CXL root ports, CXL
 downstream switch ports, and CXL upstream switch ports



On 6/24/24 15:51, Dan Williams wrote:
> Terry Bowman wrote:
>> Hi Dan,
>>
>> I added responses below.
>>
>> On 6/21/24 14:04, Dan Williams wrote:
>>> Terry Bowman wrote:
>>>> This patchset provides RAS logging for CXL root ports, CXL downstream
>>>> switch ports, and CXL upstream switch ports. This includes changes to
>>>> use a portdrv notifier chain to communicate CXL AER/RAS errors to a
>>>> cxl_pci callback.
>>>>
>>>> The first 3 patches prepare for and add an atomic notifier chain to the
>>>> portdrv driver. The portdrv's notifier chain reports the port device's
>>>> AER internal errors to the registered callback(s). The preparation changes
>>>> include a portdrv update to call the uncorrectable handler for PCIe root
>>>> ports and PCIe downstream switch ports. Also, the AER correctable error
>>>> (CE) status is made available to the AER CE handler.
>>>>
>>>> The next 4 patches are in preparation for adding an atomic notification
>>>> callback in the cxl_pci driver. This is for receiving AER internal error
>>>> events from the portdrv notifier chain. Preparation includes adding RAS
>>>> register block mapping, adding trace functions for logging, and
>>>> refactoring cxl_pci RAS functions for reuse.
>>>>
>>>> The final 2 patches enable the AER internal error interrupts.
>>> [..] 
>>>>
>>>> Solutions Considered (1-4):
>>>>   Below are solutions that were considered. Solution #4 is
>>>>   implemented in this patchset. 
>>> [..]
>>>>  2.) Update the AER driver to call cxl_pci driver's error handler before
>>>>  calling pci_aer_handle_error()
>>>>
>>>>  This is similar to the existing RCH port error approach in aer.c.
>>>>  In this solution the AER driver searches for a downstream CXL endpoint
>>>>  to 'handle' detected CXL port protocol errors.
>>>>
>>>>  This is a good solution to consider if the one presented in this patchset
>>>>  is not acceptable. I was initially reluctant to this approach because it
>>>>  adds more CXL coupling to the AER driver. But, I think this solution
>>>>  would technically work. I believe Ming was working towards this
>>>>  solution.
>>>
>>> I feel like the coupling is warranted because these things *are* PCIe
>>> and CXL ports, but it means solving the interrupt distribution problem.
>>>
>>
>> I understand the service driver interrupt issue but it is not clear how it 
>> applies to the CXL port error handling. Can you help me understand how the 
>> interrupt issue affects CXL port AER UIE/CIE handling in the AER driver.
> 
> Just the case of the AER MSI/-X vector being multiplexed with other CXL
> functionality on the same device. If the CXL interrupt vector is to be
> enabled later then it means MSI/-X vector enabling needs to be dynamic.
> 
> ...but yeah, not a problem now as we are only talking about PCIe AER
> events and not multiplexing yet. I.e. that problem can be solved later.
> 
>>
>>
>>>>   3.) Refactor portdrv
>>>>   The portdrv refactoring solution is to change the portdrv service drivers
>>>>   into PCIe auxiliary drivers. With this change the facility drivers can be
>>>>   associated with a PCIe driver instead fixed bound to the portdrv driver.
>>>>
>>>>   In this case the CXL port functionality would be added either as a CXL
>>>>   auxiliary driver or as a CXL specific port driver
>>>>   (PCI_CLASS_BRIDGE_PCI_NORMAL).
>>>>
>>>>   This solution has challenges in the interrupt allocation by separate
>>>>   auxiliary drivers and in binding of a specific driver. Binding is
>>>>   currently based on PCIe class and would require extending the binding
>>>>   logic to support multiple drivers for the same class.
>>>>
>>>>   Jonathan Cameron is working towards this solution by initially solving
>>>>   for the PMU service driver.[1] It is using the auxiliary bus to associate
>>>>   what were service drivers with the portdrv driver. Using a CXL auxiliary
>>>>   for handling CXL port RAS errors would result in RAS logic called from
>>>>   the cxl_pci and CXL auxiliary drivers. This may need a library driver.
>>>
>>> I don't think auxiliary bus is a fundamental step forward from pcie
>>> portdrv, it's just a s/pcie_port_bus_type/auxiliary_bus_type/ rename,
>>> but with all the same problems around how to distribute interrupt
>>> services to different interested parties.
>>>
>>> So I think notifiers are interesting from the perspective of a software
>>> hack to enable interrupt distribution. However, given that dynamic MSI-X
>>> support is within reach I am interested in exploring that path and
>>> mandating that archs that want to handle CXL protocol errors natively
>>> need to enable dynamic MSI-X. Otherwise, those platforms should disclaim
>>> native protocol error handling support via CXL _OSC.
>>>
>>> In other words, I expect native dynamic MSI-X support is more
>>> maintainable in the sense of keeping all the code in one notification
>>> domain.
>>>
>>>>   4.) Using a portdrv notifier chain/callback for CIE/UIE
>>>>   (Implemented in this patchset)
>>>>
>>>>   This solution uses a portdrv atomic chain notifier and a cxl_pci
>>>>   callback to handle and log CXL port RAS errors.
>>>
>>> Oh, I will need to look that the cxl_pci tie in for this, I was
>>> expecting cxl_pci only gets involved in the RCH case because the port
>>> and the endpoint are one in the same object. in the VH case I would only
>>> expect cxl_pci to get involved for its own observed protocol errors, not
>>> those reported upstream from that endpoint.
>>>
>>
>> The CXL port error handling needs a place to live with few options at the moment.
>> Where do you want the CXL port error handlers to reside? 
> 
> I need to go understand exactly why cxl_pci is involved in this current
> proposal, but I was thinking it is probably more natural for cxl_port to
> have error handlers.

Ok. I agree, cxl_port is a better location for the handlers.

Regards,
Terry

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