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

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Message-ID: <CAErSpo5yZdr4dgqjO6O9PSi3XZXfSOHmjjJRemaqAfmpv5WREA@mail.gmail.com>
Date:	Mon, 11 Jun 2012 19:52:03 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Myron Stowe <myron.stowe@...hat.com>
Cc:	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 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.

Bjorn
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