PCIe machine check - "Access to an illegal configuration space"

[sopgenorth]

On our system (a P2010 or P1014 configured in RC mode), we will occasionally get a machine check exception during reboot tests of startup initialization. 

The machine check is marked as being a BUS_RBERR, and occurs on an lwz instruction to PCIe IO space. 

- PEX_CONFIG_ADDR = 0x8001_0000 (EP's PCIe vendor ID read)
- PEX_CONFIG_DATA = 0x4c10_8888
- PEX_ERR_DR = 0x8002_0000
      - bit 0 - multiple errors (this seems to always be on regularly during "good" operation)
      -  bit 14 - Access to an illegal configuration space from PEX_CONFIG_ADDR
- PEX_ERR_CAP_STAT = 0x0000_0041
      - bit 25 - transaction originated from PEX_CONFIG_ADDR/PEX_CONFIG_DATA
      - bit 31 - error capture valid

What situations can cause the PCIe peripheral to generate this "Access to an illegal configuration space from PEX_CONFIG_ADDR"? Everything that we're doing seems valid and works 97% of the time. It's as if there is some sort of race condition that we're not aware of. 

[sopgenorth]

Huh, it looks like the LDD (Link down detected) bit is set in the PEX_PME_MES_DR register. 

I think this would explain why access to the PCIe access would be considered illegal. Beyond some intermittent hardware issue, I can't think of a reason why this would occur. 

[yipingwang]

Hello Seth Opgenorth,

The Link Down event is indicated in the PEXx_PEX_PME_MES_DR[LDD] and could be signaled by the corresponding interrupt, when PEXx_PEX_PME_MES_DISR[LDDD]=0 and PEXx_PEX_PME_MES_IER[LDDIE]=1.

In normal case it is needed to poll the PEX_LTSSM_STAT until the L0 state is reported. After reset or when recovering from a link down condition, external transactions should not to attempted until the link has successfully trained. Software can poll the LTSSM state status register (PEX_LTSSM_STAT) to check the status of link training before issuing external requests.

For details, please refer to LTSSM State Status Register(LTSSM_State_Status_Register) in the Reference Manual.

Have a great day,
TIC

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