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Date:   Tue, 12 Dec 2023 15:27:07 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc:     Johan Hovold <johan+linaro@kernel.org>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Andy Gross <agross@kernel.org>,
        Bjorn Andersson <andersson@kernel.org>,
        Konrad Dybcio <konrad.dybcio@linaro.org>,
        Manivannan Sadhasivam <mani@kernel.org>,
        Rob Herring <robh@kernel.org>,
        Nirmal Patel <nirmal.patel@linux.intel.com>,
        Jonathan Derrick <jonathan.derrick@linux.dev>,
        linux-arm-msm@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Michael Bottini <michael.a.bottini@linux.intel.com>,
        "David E . Box" <david.e.box@linux.intel.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Subject: Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link
 state
Message-ID: <20231212212707.GA1021099@bhelgaas>
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On Tue, Dec 12, 2023 at 11:48:27AM +0800, Kai-Heng Feng wrote:
> On Fri, Dec 8, 2023 at 4:47 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> ...

> > I hope we can obsolete this whole idea someday.  Using pci_walk_bus()
> > in qcom and vmd to enable ASPM is an ugly hack to work around this
> > weird idea that "the OS isn't allowed to enable more ASPM states than
> > the BIOS did because the BIOS might have left ASPM disabled because it
> > knows about hardware issues."  More history at
> > https://lore.kernel.org/linux-pci/20230615070421.1704133-1-kai.heng.feng@canonical.com/T/#u
> >
> > I think we need to get to a point where Linux enables all supported
> > ASPM features by default.  If we really think x86 BIOS assumes an
> > implicit contract that the OS will never enable ASPM more
> > aggressively, we might need some kind of arch quirk for that.
> 
> The reality is that PC ODM toggles ASPM to workaround hardware
> defects, assuming that OS will honor what's set by the BIOS.
> If ASPM gets enabled for all devices, many devices will break.

That's why I mentioned some kind of arch quirk.  Maybe we're forced to
do that for x86, for instance.  But even that is a stop-gap.

The idea that the BIOS ASPM config is some kind of handoff protocol is
really unsupportable.

Do we have concrete examples of where enabling ASPM for a device that
advertises ASPM support will break something?

Bjorn