Re: [PATCH] PCI: Mark Intel bridge on SuperMicro Atom C3xxx motherboards to avoid bus reset

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 24 May 2019 17:31:18 +0200
Maik Broemme <mbroemme@libmpq.org> wrote:

> The Intel PCI bridge on SuperMicro Atom C3xxx motherboards do not
> successfully complete a bus reset when used with certain child devices.

What are these 'certain child devices'?  We can't really regression
test to know if/when the problem might be resolved if we don't know
what to test.  Do these devices reset properly in other systems?  Are
there any devices that can do a bus reset properly on this system?  We'd
really only want to blacklist bus reset on this root port(?) if this is
a systemic problem with the root port, which is not clearly proven
here.  Thanks,

Alex

> After the reset, config accesses to the child may fail. If assigning
> such device via VFIO it will immediately fail with:
> 
>   vfio-pci 0000:01:00.0: Failed to return from FLR
>   vfio-pci 0000:01:00.0: timed out waiting for pending transaction;
>   performing function level reset anyway
> 
> Device will disappear from PCI device list:
> 
>   !!! Unknown header type 7f
>   Kernel driver in use: vfio-pci
>   Kernel modules: ddbridge
> 
> The attached patch will mark the root port as incapable of doing a
> bus level reset. After that all my tested devices survive a VFIO
> assignment and several VM reboot cycles.
> 
> Signed-off-by: Maik Broemme <mbroemme@libmpq.org>
> ---
>  drivers/pci/quirks.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index 0f16acc323c6..86cd42872708 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -3433,6 +3433,13 @@ DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATHEROS, 0x0034, quirk_no_bus_reset);
>   */
>  DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_CAVIUM, 0xa100, quirk_no_bus_reset);
>  
> +/*
> + * Root port on some SuperMicro Atom C3xxx motherboards do not successfully
> + * complete a bus reset when used with certain child devices. After the
> + * reset, config accesses to the child may fail.
> + */
> +DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, 0x19a4, quirk_no_bus_reset);
> +
>  static void quirk_no_pm_reset(struct pci_dev *dev)
>  {
>  	/*