[PATCH v2] Revert "PCI: layerscape: Change default error response behavior"

[Easwar Hariharan]

> This reverts commit 84d897d69938
> ("PCI: layerscape: Change default error response behavior").
> 
> The Layerscape PCIe driver programs the DesignWare
> AMBA_ERROR_RESPONSE_DEFAULT register at offset 0x8d0 with 0x9401.
> 
> For AXI, the documented default with AMBA_ERROR_RESPONSE_GLOBAL cleared
> returns OKAY with all-ones data for errors on non-posted requests.
> Setting AMBA_ERROR_RESPONSE_GLOBAL causes an AXI error response instead,
> with AMBA_ERROR_RESPONSE_MAP selecting SLVERR or DECERR for individual
> PCIe completion errors.
> 
> The value 0x9401 sets AMBA_ERROR_RESPONSE_GLOBAL and maps Unsupported
> Request, Completer Abort, and Completion Timeout to SLVERR.
> 
> A PCI configuration access can race with the link going down after
> dw_pcie_other_conf_map_bus() checks the link but before
> pci_generic_config_read() performs the MMIO access. If the transaction
> results in a Completion Timeout, the Layerscape error response setting
> propagates it to AXI as SLVERR, resulting in an asynchronous SError and
> kernel panic.
> 

Could you bring back the kernel panic backtrace from v1?

With that,

Reviewed-by: Easwar Hariharan <easwar.hariharan at linux.microsoft.com>

> Reproduce the race by obtaining the configuration-space mapping while
> the link is up, then disabling the link from another CPU immediately
> before performing the MMIO configuration read.
> 
> With the error response override enabled, the overlapping configuration
> read results in an asynchronous SError and kernel panic. With the
> DesignWare default behavior restored, the same test returns 0xffffffff.
> AER reports a non-fatal Completion Timeout, and no SError or kernel
> panic occurs.
> 
> Restore the documented DesignWare default error response behavior so a
> failed non-posted PCIe transaction does not become a fatal AXI error.
> 
> Fixes: 84d897d69938 ("PCI: layerscape: Change default error response behavior")
> Cc: stable at vger.kernel.org
> Signed-off-by: Jeff Barnes <jeffbarnes at linux.microsoft.com>
> 
> Changes in v2:
> - Change the subject to identify this as a revert, as requested by
>   Frank Li.
> - Document the DesignWare AMBA_ERROR_RESPONSE_DEFAULT behavior and
>   decode the 0x9401 setting used by Layerscape.
> - Clarify that the setting propagates Completion Timeout as AXI SLVERR
>   rather than causing the underlying PCIe transaction failure.
>
> ---
> drivers/pci/controller/dwc/pci-layerscape.c | 12 ------------
> 1 file changed, 12 deletions(-)