linux-kernel - Re: [PATCH v9 1/4] PCI: Add new PCIe Fabric End Node flag, PCI_DEV_FLAGS_NO_RELAXED

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Date:   Tue, 8 Aug 2017 22:02:36 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Casey Leedom <leedom@...lsio.com>
Cc:     Ding Tianhong <dingtianhong@...wei.com>,
        "ashok.raj@...el.com" <ashok.raj@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        Michael Werner <werner@...lsio.com>,
        Ganesh GR <ganeshgr@...lsio.com>,
        "asit.k.mallick@...el.com" <asit.k.mallick@...el.com>,
        "patrick.j.cramer@...el.com" <patrick.j.cramer@...el.com>,
        "Suravee.Suthikulpanit@....com" <Suravee.Suthikulpanit@....com>,
        "Bob.Shaw@....com" <Bob.Shaw@....com>,
        "l.stach@...gutronix.de" <l.stach@...gutronix.de>,
        "amira@...lanox.com" <amira@...lanox.com>,
        "gabriele.paoloni@...wei.com" <gabriele.paoloni@...wei.com>,
        "David.Laight@...lab.com" <David.Laight@...lab.com>,
        "jeffrey.t.kirsher@...el.com" <jeffrey.t.kirsher@...el.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "alexander.duyck@...il.com" <alexander.duyck@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>
Subject: Re: [PATCH v9 1/4] PCI: Add new PCIe Fabric End Node flag,
 PCI_DEV_FLAGS_NO_RELAXED_ORDERING

On Wed, Aug 09, 2017 at 01:40:01AM +0000, Casey Leedom wrote:
> | From: Bjorn Helgaas <helgaas@...nel.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@....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.