linux-kernel - Re: [PATCH 0/4] PCI: PCIe capability structure related cleanup/fixes

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Message-ID: <CAErSpo42C0Uo5x6dzvPWRkfjvZhBzi5g1yCs+pNfpFnfvCtqKA@mail.gmail.com>
Date:	Tue, 12 Jun 2012 10:44:43 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Myron Stowe <mstowe@...hat.com>
Cc:	Myron Stowe <myron.stowe@...hat.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, xudong.hao@...ux.intel.com,
	ddutile@...hat.com, yu.zhao@...el.com
Subject: Re: [PATCH 0/4] PCI: PCIe capability structure related cleanup/fixes

On Tue, Jun 12, 2012 at 10:34 AM, Myron Stowe <mstowe@...hat.com> wrote:
> On Mon, 2012-06-11 at 19:52 -0700, Bjorn Helgaas wrote:
>> On Fri, Jun 1, 2012 at 2:16 PM, Myron Stowe <myron.stowe@...hat.com> wrote:
>> > The following series introduces PCI Express 'capability structure'
>> > related cleanup, fixes, and optimizations.
>> >
>> > Patch 1/4 changes pci_ltr_supported() to a static routine.
>> >
>> > Patch 2/4 removes redundant checking in various PCI Express features as
>> > suggested by Bjorn Helgaas in
>> > http://marc.info/?l=linux-pci&m=130463494319762&w=2
>> >
>> > There is a similar idiom in use that could be similarly be re-factored:
>> >    if (!pci_is_pcie(dev))
>> >        return;
>> >
>> >    pos = pci_find_ext_capability(dev, ...);
>> >    if (!pos)
>> >        return;
>> >
>> > At first it seemed incorrect to remove the redundant call of
>> > pci_is_pcie() in these cases as a PCI or PCI-X (< 2.0) device may be
>> > involved.  In such cases an "extended capability" list would not exist,
>> > as it was not introduced until PCI-X 2.0, and accesses outside of the
>> > device's configuration space would be attempted.  However, upon further
>> > review of pci_find_ext_capability() it looks as if such accesses would
>> > be handled correctly due to the short-circuiting logic involved -
>> >
>> >    if (pci_read_config_dword(dev, pos, &header) != PCIBIOS_SUCCESSFUL)
>> >        return 0;
>> >
>> > As such, I'll entertain comments as to whether or not we should also
>> > make similar removals of pci_is_pcie() in these cases.
>> >
>> > Patch 3/4 introduces pci_pcie_cap2() for use in v2 capability related
>> > feature code.  The makeup of Express' capability structure varies
>> > substantially between v1 and v2.
>> >
>> > There is still some redundancy in PCIe v2 capabilities checking related
>> > to the Latency Tolerance Reporting (LTR) feature routines that likely
>> > could be re-factored further; please feel free to respond with ideas.
>> >
>> > Patch 4/4 makes a minor optimization to the saving and restoring of
>> > PCI Express capability structures.
>> >
>> > Seems like the same type of optimization could be done to remove the
>> > 'if (pcie_cap_has_lnkctl(dev->pcie_type, flags))' check.  According to
>> > section 7.8 "PCI Express Capability Structure" of the PCI Express 1.0a
>> > specification:
>> >
>> >    "Figure 7-10 details allocation of register fields in the PCI
>> >     Express Capability structure. The PCI Express Capabilities,
>> >     Device Capabilities, Device Status/Control, Link Capabilities,
>> >     and Link Status/Control registers are required for all PCI
>> >     Express devices. Endpoints are not required to implement
>> >     registers other than those listed above and terminate the
>> >     capability structure."
>> >
>> > There may have been some early Express devices that did not properly
>> > follow the specification which required the introduction of
>> > 'pcie_cap_has_lnkctl()' so I did not make the additional optimization.
>> > ---
>> >
>> > Myron Stowe (4):
>> >      PCI: Remove redundant capabilities checking in pci_{save,restore}_pcie_state
>> >      PCI: Add pci_pcie_cap2() check for PCIe feature capabilities >= v2
>> >      PCI: Remove redundant checking in PCI Express capability routines
>> >      PCI: make pci_ltr_supported static.
>>
>> I added Don's acks, made a couple minor changes he suggested, removed
>> the static pci_ltr_supported() function declaration (unnecessary,
>> AFAICS), and pushed these to:
>>
>>   http://git.kernel.org/?p=linux/kernel/git/helgaas/pci.git;a=shortlog;h=refs/heads/topic/stowe-cap-cleanup
>>
>> If everything looks right to you, I'll merge it into "next" tomorrow.
>> Thanks for doing this; I think it's some nice cleanup and will make
>> things safer and easier to understand.
>
> Looks good - thanks to both Don and yourself for the suggestions and
> changes to make the patch headers more comprehensible with respect to
> the capabilities structure versions.

Great, I merged that topic branch to "next" and pushed it.

Bjorn
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