RE: [PATCH v3 4/6] cxl/pci: Add RCH downstream port error logging

[Dan Williams]

Terry Bowman wrote:
> RCH downstream port error logging is missing in the current CXL driver. The
> missing AER and RAS error logging is needed for communicating driver error
> details to userspace. Update the driver to include PCIe AER and CXL RAS
> error logging.
> 
> Add RCH downstream port error handling into the existing RCiEP handler.
> The downstream port error handler is added to the RCiEP error handler
> because the downstream port is implemented in a RCRB, is not PCI
> enumerable, and as a result is not directly accessible to the PCI AER
> root port driver. The AER root port driver calls the RCiEP handler for
> handling RCD errors and RCH downstream port protocol errors.
> 
> Update mem.c to include RAS and AER setup. This includes AER and RAS
> capability discovery and mapping for later use in the error handler.
> 
> Disable RCH downstream port's root port cmd interrupts.[1]
> 
> Update existing RCiEP correctable and uncorrectable handlers to also call
> the RCH handler. The RCH handler will read the RCH AER registers, check for
> error severity, and if an error exists will log using an existing kernel
> AER trace routine. The RCH handler will also log downstream port RAS errors
> if they exist.

I think this patch wants a lead in refactoring to move the existing
probe of the CXL RAS capability into the cxl_port driver so that the RCH
path and the VH path can be unified for register mapping and error
handling invocation. I do not see a compelling rationale to have 2
separate ways to map the RAS capability. The timing of when
cxl_setup_ras() is called looks problematic relative to when the first
error handler callback might happen.

For example what happens when an error fires after cxl_pci has
registered its error handlers, but before the component registers have
been mapped out of the RCRB?

This implies the need for a callback for cxl_pci to notify the cxl_port
driver of CXL errors to handle relative to a PCI AER event.