Re: [PATCH 5.2 123/413] PCI: Add missing link delays required by the PCIe spec

[Justin Forbes]

On Wed, Jul 24, 2019 at 3:31 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> [ Upstream commit c2bf1fc212f7e6f25ace1af8f0b3ac061ea48ba5 ]
>
> Currently Linux does not follow PCIe spec regarding the required delays
> after reset. A concrete example is a Thunderbolt add-in-card that
> consists of a PCIe switch and two PCIe endpoints:
>
>   +-1b.0-[01-6b]----00.0-[02-6b]--+-00.0-[03]----00.0 TBT controller
>                                   +-01.0-[04-36]-- DS hotplug port
>                                   +-02.0-[37]----00.0 xHCI controller
>                                   \-04.0-[38-6b]-- DS hotplug port
>
> The root port (1b.0) and the PCIe switch downstream ports are all PCIe
> gen3 so they support 8GT/s link speeds.
>
> We wait for the PCIe hierarchy to enter D3cold (runtime):
>
>   pcieport 0000:00:1b.0: power state changed by ACPI to D3cold
>
> When it wakes up from D3cold, according to the PCIe 4.0 section 5.8 the
> PCIe switch is put to reset and its power is re-applied. This means that
> we must follow the rules in PCIe 4.0 section 6.6.1.
>
> For the PCIe gen3 ports we are dealing with here, the following applies:
>
>   With a Downstream Port that supports Link speeds greater than 5.0
>   GT/s, software must wait a minimum of 100 ms after Link training
>   completes before sending a Configuration Request to the device
>   immediately below that Port. Software can determine when Link training
>   completes by polling the Data Link Layer Link Active bit or by setting
>   up an associated interrupt (see Section 6.7.3.3).
>
> Translating this into the above topology we would need to do this (DLLLA
> stands for Data Link Layer Link Active):
>
>   pcieport 0000:00:1b.0: wait for 100ms after DLLLA is set before access to 0000:01:00.0
>   pcieport 0000:02:00.0: wait for 100ms after DLLLA is set before access to 0000:03:00.0
>   pcieport 0000:02:02.0: wait for 100ms after DLLLA is set before access to 0000:37:00.0
>
> I've instrumented the kernel with additional logging so we can see the
> actual delays the kernel performs:
>
>   pcieport 0000:00:1b.0: power state changed by ACPI to D0
>   pcieport 0000:00:1b.0: waiting for D3cold delay of 100 ms
>   pcieport 0000:00:1b.0: waking up bus
>   pcieport 0000:00:1b.0: waiting for D3hot delay of 10 ms
>   pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
>   ...
>   pcieport 0000:00:1b.0: PME# disabled
>   pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:01:00.0: PME# disabled
>   pcieport 0000:02:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:02:00.0: PME# disabled
>   pcieport 0000:02:01.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:02:01.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
>   pcieport 0000:02:01.0: PME# disabled
>   pcieport 0000:02:02.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:02:02.0: PME# disabled
>   pcieport 0000:02:04.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:02:04.0: PME# disabled
>   pcieport 0000:02:01.0: PME# enabled
>   pcieport 0000:02:01.0: waiting for D3hot delay of 10 ms
>   pcieport 0000:02:04.0: PME# enabled
>   pcieport 0000:02:04.0: waiting for D3hot delay of 10 ms
>   thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)
>   ...
>   thunderbolt 0000:03:00.0: PME# disabled
>   xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
>   ...
>   xhci_hcd 0000:37:00.0: PME# disabled
>
> For the switch upstream port (01:00.0) we wait for 100ms but not taking
> into account the DLLLA requirement. We then wait 10ms for D3hot -> D0
> transition of the root port and the two downstream hotplug ports. This
> means that we deviate from what the spec requires.
>
> Performing the same check for system sleep (s2idle) transitions we can
> see following when resuming from s2idle:
>
>   pcieport 0000:00:1b.0: power state changed by ACPI to D0
>   pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
>   ...
>   pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   ...
>   pcieport 0000:02:02.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   pcieport 0000:02:02.0: restoring config space at offset 0x2c (was 0x0, writing 0x0)
>   pcieport 0000:02:01.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   pcieport 0000:02:04.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   pcieport 0000:02:02.0: restoring config space at offset 0x28 (was 0x0, writing 0x0)
>   pcieport 0000:02:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
>   pcieport 0000:02:02.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1fff1)
>   pcieport 0000:02:01.0: restoring config space at offset 0x2c (was 0x0, writing 0x60)
>   pcieport 0000:02:02.0: restoring config space at offset 0x20 (was 0x0, writing 0x73f073f0)
>   pcieport 0000:02:04.0: restoring config space at offset 0x2c (was 0x0, writing 0x60)
>   pcieport 0000:02:01.0: restoring config space at offset 0x28 (was 0x0, writing 0x60)
>   pcieport 0000:02:00.0: restoring config space at offset 0x2c (was 0x0, writing 0x0)
>   pcieport 0000:02:02.0: restoring config space at offset 0x1c (was 0x101, writing 0x1f1)
>   pcieport 0000:02:04.0: restoring config space at offset 0x28 (was 0x0, writing 0x60)
>   pcieport 0000:02:01.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1ff10001)
>   pcieport 0000:02:00.0: restoring config space at offset 0x28 (was 0x0, writing 0x0)
>   pcieport 0000:02:02.0: restoring config space at offset 0x18 (was 0x0, writing 0x373702)
>   pcieport 0000:02:04.0: restoring config space at offset 0x24 (was 0x10001, writing 0x49f12001)
>   pcieport 0000:02:01.0: restoring config space at offset 0x20 (was 0x0, writing 0x73e05c00)
>   pcieport 0000:02:00.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1fff1)
>   pcieport 0000:02:04.0: restoring config space at offset 0x20 (was 0x0, writing 0x89f07400)
>   pcieport 0000:02:01.0: restoring config space at offset 0x1c (was 0x101, writing 0x5151)
>   pcieport 0000:02:00.0: restoring config space at offset 0x20 (was 0x0, writing 0x8a008a00)
>   pcieport 0000:02:02.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
>   pcieport 0000:02:04.0: restoring config space at offset 0x1c (was 0x101, writing 0x6161)
>   pcieport 0000:02:01.0: restoring config space at offset 0x18 (was 0x0, writing 0x360402)
>   pcieport 0000:02:00.0: restoring config space at offset 0x1c (was 0x101, writing 0x1f1)
>   pcieport 0000:02:04.0: restoring config space at offset 0x18 (was 0x0, writing 0x6b3802)
>   pcieport 0000:02:02.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
>   pcieport 0000:02:00.0: restoring config space at offset 0x18 (was 0x0, writing 0x30302)
>   pcieport 0000:02:01.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
>   pcieport 0000:02:04.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
>   pcieport 0000:02:00.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
>   pcieport 0000:02:01.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
>   pcieport 0000:02:04.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
>   pcieport 0000:02:00.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
>   xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
>   ...
>   thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)
>
> This is even worse. None of the mandatory delays are performed. If this
> would be S3 instead of s2idle then according to PCI FW spec 3.2 section
> 4.6.8.  there is a specific _DSM that allows the OS to skip the delays
> but this platform does not provide the _DSM and does not go to S3 anyway
> so no firmware is involved that could already handle these delays.
>
> In this particular Intel Coffee Lake platform these delays are not
> actually needed because there is an additional delay as part of the ACPI
> power resource that is used to turn on power to the hierarchy but since
> that additional delay is not required by any of standards (PCIe, ACPI)
> it is not present in the Intel Ice Lake, for example where missing the
> mandatory delays causes pciehp to start tearing down the stack too early
> (links are not yet trained).
>
> For this reason, change the PCIe portdrv PM resume hooks so that they
> perform the mandatory delays before the downstream component gets
> resumed. We perform the delays before port services are resumed because
> otherwise pciehp might find that the link is not up (even if it is just
> training) and tears-down the hierarchy.
>

We have gotten multiple reports in Fedora that this patch has broken
suspend for users of 5.1.20 and 5.2 stable kernels.

Justin