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In fact, > disabling only ReqRedir is enough to work around the issue. My guess would be your system has some kind of address alias going on? Assuming you are not facing an errata, ACS generally changes the routing of TLPs so if you have a DMA address that could go to two different places then messing with ACS will give you different behaviors. In specific when you turn all those ACS settings you cannot do P2P traffic anymore. If your system expects this for some reason then you must use the kernel command line option to disable acs. If you are just doing normal netdev stuff then it is doubtful that you are doing P2P at all, so I might guess a bug in the microchip ethernet driver doing a wild DMA? Stricter ACS settings cause it to AER and the device cannot recover? It will be hard to get the bottom of the defect without a PCI trace I don't know why your bisection landed on bcb8 - the intention was that pci_enable_acs() is always called, and I didn't notice an obvious reason why that wouldn't happen prior to bcb8.. It is called directly from pci_device_add() Maybe investigating that angle would be informative.. > I also read up on AER and I'm surprised that I don't see anything in dmesg > when the problem occurs, even through UERcvd+ start appearing on the root > context and AdvNonFatalErr+ appears on the switch. Though UE and AdvNonFatalErr sure are weird indications for an addressing error.. Is there some kind of special embedded system thing going on? Vendor messages over PCI perhaps? Jason