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    SubjectRe: [PATCH] pcie: Add quirk for the Arm Neoverse N1SDP platform
    On Thu, 12 Dec 2019 21:07:24 +0000
    Andrew Murray <andrew.murray@arm.com> wrote:

    Hi,

    > On Tue, Dec 10, 2019 at 08:41:15AM -0600, Bjorn Helgaas wrote:
    > > On Mon, Dec 09, 2019 at 04:06:38PM +0000, Andre Przywara wrote:

    [ ... ]

    > > Even ECAM compliance is not really minor -- if this controller were
    > > fully compliant with the spec, you would need ZERO Linux changes to
    > > support it. Every quirk like this means additional maintenance
    > > burden, and it's not just a one-time thing. It means old kernels that
    > > *should* "just work" on your system will not work unless somebody
    > > backports the quirk.
    >
    > With regards to URs resulting in unwanted aborts or similar - this seems
    > to be a very common theme amongst ARM PCI controller drivers. For example
    > both ARM32 imx6 and ARM32 keystone have fault handlers to handle an abort
    > and fabricate a 0xffffffff read value.
    >
    > The ARM32 rcar driver, whilst it doesn't appear to produce an abort, does
    > read the PCI_STATUS register after making a config read to determine if
    > any aborts have happened - in which case it reports
    > PCIBIOS_DEVICE_NOT_FOUND.
    >
    > And as recently reported [1], the rockchip driver also appears to produce
    > aborts.
    >
    > I suspect that this ARM64 controller driver won't be the last either. Thus
    > any solution here may form the basis of copy-cat solutions for subsequent
    > controllers.

    Well, I think Bjorn is aware of them, but was actually hoping that those broken controllers would go away at some point ;-)
    And just to make this clear: I would categorise this issue as an integration bug, which just can't be fixed in hardware or firmware easily. It was never meant to be this way. So I am not sure we should promote generic solutions here.

    > From my understanding of the issues, the ARM64 serrors are imprecise and
    > as a result there isn't a sensible way of using them to determine that a
    > read is a UR. So where there are no other solutions to suppress the
    > generation of an abort by the controller, the only solutions that seem to
    > exist are 1) pre-scan the devices in firmware and only talk to those devices
    > in Linux - a safe option but limiting - perhaps with side effects for CRS
    > and 2) the approach rcar takes in using the PCI_STATUS register - though
    > you'd end up having to mask the serror (PSTATE.A) for a limited period of
    > time - a risky option (you'll miss real serrors) - but with no side effects.
    >
    > (I don't know if option 2 is feasible in this case by the way).

    Interesting, we might evaluate this, but mostly out of curiosity or for debugging. I don't think it's really a better option.
    If there is a safe way of making this work in the majority of cases, that should be the way to go. Setting PSTATE.A sounds quite wacky to me.

    Thanks,
    Andre.

    > [1] https://lore.kernel.org/linux-pci/2a381384-9d47-a7e2-679c-780950cd862d@rock-chips.com/2-0001-WFT-PCI-rockchip-play-game-with-unsupported-request-.patch
    >
    > Thanks,
    >
    > Andrew Murray
    >
    > >
    > > > This allows the Arm Neoverse N1SDP board to boot Linux without crashing
    > > > and to access *any* devices (there are no platform devices except UART).

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