[PATCH v9 1/4] PCI: Add new PCIe Fabric End Node flag, PCI_DEV_FLAGS_NO_RELAXED_ORDERING

Bjorn Helgaas
2017-08-09T03:02:36Z

On Wed, Aug 09, 2017 at 01:40:01AM +0000, Casey Leedom wrote:
> | From: Bjorn Helgaas <helgaas@kernel.org>
> | Sent: Tuesday, August 8, 2017 4:22 PM
> |
> | This needs to include a link to the Intel spec
> | (https://software.intel.com/sites/default/files/managed/9e/bc/64-ia-32-architectures-optimization-manual.pdf,
> | sec 3.9.1).
>
> In the commit message or as a comment? Regardless, I agree. It's always
> nice to be able to go back and see what the official documentation says.
> However, that said, links on the internet are ... fragile as time goes by,
> so we might want to simply quote section 3.9.1 in the commit message since
> it's relatively short:
>
> 3.9.1 Optimizing PCIe Performance for Accesses Toward Coherent Memory
> and Toward MMIO Regions (P2P)
>
> In order to maximize performance for PCIe devices in the processors
> listed in Table 3-6 below, the soft- ware should determine whether the
> accesses are toward coherent memory (system memory) or toward MMIO
> regions (P2P access to other devices). If the access is toward MMIO
> region, then software can command HW to set the RO bit in the TLP
> header, as this would allow hardware to achieve maximum throughput for
> these types of accesses. For accesses toward coherent memory, software
> can command HW to clear the RO bit in the TLP header (no RO), as this
> would allow hardware to achieve maximum throughput for these types of
> accesses.
>
> Table 3-6. Intel Processor CPU RP Device IDs for Processors Optimizing
> PCIe Performance
>
> Processor CPU RP Device IDs
>
> Intel Xeon processors based on 6F01H-6F0EH
> Broadwell microarchitecture
>
> Intel Xeon processors based on 2F01H-2F0EH
> Haswell microarchitecture

Agreed, links are prone to being broken. I would include in the
changelog the complete title and order number, along with the link as
a footnote. Wouldn't hurt to quote the section too, since it's short.

> | It should also include a pointer to the AMD erratum, if available, or
> | at least some reference to how we know it doesn't obey the rules.
>
> Getting an ACK from AMD seems like a forlorn cause at this point. My
> contact was Bob Shaw <Bob.Shaw@amd.com> and he stopped responding to me
> messages almost a year ago saying that all of AMD's energies were being
> redirected towards upcoming x86 products (likely Ryzen as we now know). As
> far as I can tell AMD has walked away from their A1100 (AKA "Seattle") ARM
> SoC.
>
> On the specific issue, I can certainly write up somthing even more
> extensive than I wrote up for the comment in drivers/pci/quirks.c. Please
> review the comment I wrote up and tell me if you'd like something even more
> detailed -- I'm usually acused of writing comments which are too long, so
> this would be a new one on me ... :-)

If you have any bug reports with info about how you debugged it and
concluded that Seattle is broken, you could include a link (probably
in the changelog). But if there isn't anything, there isn't anything.

I might reorganize those patches as:

1) Add a PCI_DEV_FLAGS_RELAXED_ORDERING_BROKEN flag, the quirk that
sets it, and the current patch [2/4] that uses it.

2) Add the Intel DECLARE_PCI_FIXUP_CLASS_EARLY()s with the Intel
details.

3) Add the AMD DECLARE_PCI_FIXUP_CLASS_EARLY()s with the AMD
details.