Re: [PATCH V6] PCI: rcar: Add L1 link state fix into data abort hook

[Marek Vasut <marek.vasut@gmail.com>]

On 7/26/21 7:49 PM, Bjorn Helgaas wrote:
> On Mon, Jul 26, 2021 at 04:47:54PM +0200, Geert Uytterhoeven wrote:
[...]
>>>> The R-Car PCIe controller is capable of handling L0s/L1 link states.
>>>> While the controller can enter and exit L0s link state, and exit L1
>>>> link state, without any additional action from the driver, to enter
>>>> L1 link state, the driver must complete the link state transition by
>>>> issuing additional commands to the controller.
>>>>
>>>> The problem is, this transition is not atomic. The controller sets
>>>> PMEL1RX bit in PMSR register upon reception of PM_ENTER_L1 DLLP from
>>>> the PCIe card, but then the controller enters some sort of inbetween
>>>> state. The driver must detect this condition and complete the link
>>>> state transition, by setting L1IATN bit in PMCTLR and waiting for
>>>> the link state transition to complete.
>>>>
>>>> If a PCIe access happens inside this window, where the controller
>>>> is between L0 and L1 link states, the access generates a fault and
>>>> the ARM 'imprecise external abort' handler is invoked.
>>>>
>>>> Just like other PCI controller drivers, here we hook the fault handler,
>>>> perform the fixup to help the controller enter L1 link state, and then
>>>> restart the instruction which triggered the fault. Since the controller
>>>> is in L1 link state now, the link can exit from L1 link state to L0 and
>>>> successfully complete the access.
>>>>
>>>> While it was suggested to disable L1 link state support completely on
>>>> the controller level, this would not prevent the L1 link state entry
>>>> initiated by the link partner. This happens e.g. in case a PCIe card
>>>> enters D3Hot state, which could be initiated from pci_set_power_state()
>>>> if the card indicates D3Hot support, which in turn means link must enter
>>>> L1 state. So instead, fix up the L1 link state after all.
>>>>
>>>> Note that this fixup is applicable only to Aarch32 R-Car controllers,
>>>> the Aarch64 R-Car perform the same fixup in TFA, see TFA commit [1]
>>>> 0969397f2 ("rcar_gen3: plat: Prevent PCIe hang during L1X config access")
>>>> [1] https://github.com/ARM-software/arm-trusted-firmware/commit/0969397f295621aa26b3d14b76dd397d22be58bf
>>>
>>> This patch is horribly ugly but it's working around a horrible
>>> hardware problem, and I don't have any better suggestions, so I guess
>>> we don't really have much choice.
>>>
>>> I do think the commit log is a bit glib:
>>>
>>>    - "The R-Car PCIe controller is capable of handling L0s/L1 link
>>>      states."  AFAICT every PCIe device is required to handle L0 and L1
>>>      without software assistance.  So saying R-Car is "capable" puts a
>>>      better face on this than seems warranted.
>>>
>>>      L0s doesn't seem relevant at all; at least it doesn't seem to play
>>>      a role in the patch.  There's no such thing as "returning to L0s"
>>>      as mentioned in the comment below; L0s is only reachable from L0.
>>>      Returns from L1 only go to L0 (PCIe r5.0, fig 5-1).
>>>
>>>    - "The problem is, this transition is not atomic."  I think the
>>>      *problem* is the hardware is broken in the first place.  This
>>>      transition is supposed to be invisible to software.
>>>
>>>    - "Just like other PCI controller drivers ..." suggests that this is
>>>      an ordinary situation that we shouldn't be concerned about.  This
>>>      patch may be the best we can do to work around a bad hardware
>>>      defect, but it's definitely not ordinary.
>>>
>>>      I think the other hook_fault_code() uses are for reporting
>>>      legitimate PCIe errors, which most controllers log and turn
>>>      into ~0 data responses without generating an abort or machine
>>>      check, not things caused by hardware defects, so they're not
>>>      really comparable.
>>>
>>> Has Renesas documented this as an erratum?  Will future devices
>>> require additions to rcar_pcie_abort_handler_of_match[]?
>>>
>>> It'd be nice if the commit log mentioned the user-visible effect of
>>> this problem.  I guess it does mention external aborts -- I assume you
>>> see those when downstream devices go to D3hot or when ASPM puts the
>>> link in L1?  And the abort results in a reboot?
>>>
>>> To be clear, I'm not objecting to the patch.  It's a hardware problem
>>> and we should work around it as best we can.
>>
>> Cool! So what's missing for this patch, which we have been polishing
>> for almost one year, to be applied, so innocent people can no longer
>> lock up an R-Car system just by inserting an ubiquitous Intel Ethernet
>> card, and suspending the system?
> 
> Nothing missing from my point of view, so if Lorenzo is OK with it,
> he'll apply it.  If I were applying it, I would make the commit log
> something like this:
> 
>    When the link is in L1, hardware should return it to L0
>    automatically whenever a transaction targets a component on the
>    other end of the link (PCIe r5.0, sec 5.2).
> 
>    The R-Car PCIe controller doesn't handle this transition correctly.
>    If the link is not in L0, an MMIO transaction targeting a downstream
>    device fails, and the controller reports an ARM imprecise external
>    abort.
> 
>    Work around this by hooking the abort handler so the driver can
>    detect this situation and help the hardware complete the link state
>    transition.
> 
>    When the R-Car controller receives a PM_ENTER_L1 DLLP from the
>    downstream component, it sets PMEL1RX bit in PMSR register, but then
>    the controller enters some sort of in-between state.  A subsequent
>    MMIO transaction will fail, resulting in the external abort.  The
>    abort handler detects this condition and completes the link state
>    transition by setting the L1IATN bit in PMCTLR and waiting for the
>    link state transition to complete.

OK, should I submit V7 and just copy-paste this commit message in, or 
wait for Lorenzo to provide clear direction ?

> I assume that on the PCIe side, there must be an error like
> Unsupported Request or Malformed TLP, and the R-Car controller is
> logging that and turning it into the ARM external abort?
> 
> I didn't see a clear response to Pali's question about what happens if
> there's no MMIO access, e.g., what if the downstream device initiates
> a DMA or MSI transaction?

If the link is in this state, the packet won't reach the root complex, 
so nothing happens. And I don't see a good way to fix that option.