linux-kernel - Re: [PATCH] PCI/ASPM: Should not report ASPM support to BIOS if FADT indicates ASPM is unsupported

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Date:   Fri, 15 Jul 2022 10:32:36 +0100
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     Manyi Li <limanyi@...ontech.com>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Bjorn Helgaas <helgaas@...nel.org>, bhelgaas@...gle.com,
        refactormyself@...il.com, kw@...ux.com, rajatja@...gle.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vidya Sagar <vidyas@...dia.com>, rafael@...nel.org
Subject: Re: [PATCH] PCI/ASPM: Should not report ASPM support to BIOS if FADT
 indicates ASPM is unsupported

On Fri, Jul 15, 2022 at 05:19:25PM +0800, Manyi Li wrote:
> 
> 
> On 2022/7/15 16:29, Matthew Garrett wrote:
> > On Fri, Jul 15, 2022 at 03:40:36PM +0800, Manyi Li wrote:
> > 
> > > Please see the details of this issus:
> > > https://bugzilla.kernel.org/show_bug.cgi?id=216245
> > 
> > Hmm. The only case where changing aspm_support_enabled to false should
> > matter is in pcie_aspm_init_link_state(), where it looks like we'll
> > potentially rewrite some registers even if aspm_disabled is true. I
> > think in theory we shouldn't actually modify anything as a result, and
> > the lspcis from the bug don't show any ASPM values having changed, but I
> > don't trust Realtek hardware in the general case so maybe it gets upset
> > here? If the proposed patch is to just set aspm_support_enabled to false
> > when we see the FADT bit set then I think this is fine.
> > 
> 
> "aspm_support_enabled" alse be used in calculate_support():
> if (pcie_aspm_support_enabled())
>     support |= OSC_PCI_ASPM_SUPPORT | OSC_PCI_CLOCK_PM_SUPPORT;
> When set OSC_PCI_ASPM_SUPPORT | OSC_PCI_CLOCK_PM_SUPPORT, cause this AER
> issue. I want don't set OSC_PCI_ASPM_SUPPORT | OSC_PCI_CLOCK_PM_SUPPORT when
> we see the FADT bit set.

Oh hm. Are you sure it's the OSC call that breaks it? I have some 
recollection that I verified the behaviour of Windows here, but it's 
been over 10 years since I touched this so I could well be wrong. I can 
try to set up a test env to verify the behaviour of Windows when it 
comes to _OSC if the FADT says ASPM is unsupported.