linux-kernel - Re: [PATCH] PCI: Remove not needed check in disable aspm link

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Message-ID: <CAErSpo6vEOw10YV=Sou6wjjSc2GWr-oX=Dvbt7Xt=RJKi4RELw@mail.gmail.com>
Date:	Fri, 14 Jun 2013 11:44:42 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Jiang Liu <jiang.liu@...wei.com>,
	Roman Yepishev <roman.yepishev@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] PCI: Remove not needed check in disable aspm link

On Fri, Jun 14, 2013 at 10:57 AM, Yinghai Lu <yinghai@...nel.org> wrote:
> On Fri, Jun 14, 2013 at 9:33 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>> On Fri, Jun 14, 2013 at 10:17 AM, Yinghai Lu <yinghai@...nel.org> wrote:
>>
>> Can you please refer to specific function names?  I can't read your mind.
>>
>> You might be referring to quirk_disable_aspm_l0s().  This is a
>> pci_fixup_final quirk that calls pci_disable_link_state().  In the
>> current tree, we enumerate devices before requesting _OSC control.
>> However, pci_fixup_final quirks are not run until the
>> pci_apply_final_quirks() fs_initcall, which is after we request _OSC
>> control.
>>
>> As far as I can tell, we never call pci_disable_link_state() before
>> calling pcie_no_aspm().
>
> ok, you are right, that is not pci_disable_link_state.
>
> It is pcie_aspm_init_link_state ==> pcie_aspm_sanity_check in booting path
> that disable aspm.  It has  "if (aspm_disabled)" in it, and it cause
> the difference.

Yes, I agree, the pcie_aspm_init_link_state() path uses aspm_disabled
before we set it:

    acpi_pci_root_add
      pci_acpi_scan_root
        pci_scan_child_bus
          pci_scan_slot
            pcie_aspm_init_link_state
              pcie_aspm_sanity_check
                if (aspm_disabled)              # used before set
                  ...
      acpi_pci_osc_control_set
      pcie_no_aspm
        aspm_disabled = 1                       # set

That might mean we do some ASPM configuration during enumeration (in
pci_scan_slot()) even though the BIOS hasn't given us permission.  It
looks like we did that even in v3.7, since we did the enumeration
before the _OSC there as well.  That looks like a bug to me.

I don't think the fact that we have been doing ASPM config during
enumeration before _OSC is an argument for dropping the check in
pci_disable_link_state().

Bjorn
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