linux-kernel - Re: [PATCH v6 1/1] PCI: Avoid putting some root ports into D3 on some Ryzen chips

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Date:   Mon, 10 Jul 2023 17:42:56 -0500
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        stable@...r.kernel.org, Iain Lane <iain@...ngesquash.org.uk>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH v6 1/1] PCI: Avoid putting some root ports into D3 on some
 Ryzen chips


On 7/10/2023 3:33 PM, Bjorn Helgaas wrote:
>
> It sounds like there's someplace the hardware designers specify how
> this should work?  Where is that?  "Modern Standby" doesn't mean
> anything to me, but maybe there's some spec that covers it?

https://learn.microsoft.com/en-us/windows-hardware/design/device-experiences/modern-standby

It quickly devolves into Microsoft specific stuff though and
I can't find anything interesting to our specific issue.
> Maybe this is the clue we need.  My eyes glaze over when reading that
> section, but if we can come up with a commit log that starts with a
> sentence from that section and connects the dots all the way to the
> platform_pci_power_manageable() implementation, that might be a good
> argument that 9d26d3a8f1b0 was a little too aggressive.
Yeah.
> I like the fact that v5 ([1] for anybody following along at home) is
> very generic:
>
>    @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
>    ...
>    +       if (pci_pcie_type(bridge) == PCI_EXP_TYPE_ROOT_PORT &&
>    +           !platform_pci_power_manageable(bridge))
>    +               return false;
>
> My objection was that we didn't have a clear connection to any specs,
> so even though the code is perfectly generic, the commit log mentioned
> specific cases (USB keyboard/mouse on xHCI device connected to USB-C
> on AMD USB4 router).
>
> But maybe we *could* make a convincing generic commit log.  I guess
> we'd also have to explain the PCI_EXP_TYPE_ROOT_PORT part of the
> patch.
OK, I'll take a stab at rewriting the v5 commit message to be more
generic as you suggested as a v7.

We might be able to drop the PCI_EXP_TYPE_ROOT_PORT part well but I
would be more worried about regressions from this.