linux-kernel - Re: [PATCH v3] PCI: Workaround wrong flags completions for IDT switch

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Message-ID: <20170613221015.GB7012@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 13 Jun 2017 17:10:15 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Yinghai Lu <yinghai@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        James Puthukattukaran <james.puthukattukaran@...cle.com>
Subject: Re: [PATCH v3] PCI: Workaround wrong flags completions for IDT switch

On Tue, Jun 13, 2017 at 10:00:33AM -0700, Yinghai Lu wrote:
> On Mon, Jun 12, 2017 at 2:48 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Fri, Jun 09, 2017 at 04:16:17PM -0700, Yinghai Lu wrote:
> >> From: James Puthukattukaran <james.puthukattukaran@...cle.com>
> >>
> >> The IDT switch incorrectly flags an ACS source violation on a read config
> >> request to an end point device on the completion (IDT 89H32H8G3-YC,
> >> errata #36) even though the PCI Express spec states that completions are
> >> never affected by ACS source violation (PCI Spec 3.1, Section 6.12.1.1).
> ...

> > Have you considered ways to make this patch apply only to the affected
> > IDT switches?  Currently it applies to *all* devices.
> 
> But we need to apply that workaround before we know vendorid/deviceid
> under that root port or downstream port.

If I understand correctly, the problem (the ACS source violation)
occurs when we enumerate a device below an IDT switch.  We enumerate
a switch before we enumerate any devices under the switch, so I don't
understand why this can't be made IDT-specific.

> > The purpose of the pci_bus_read_dev_vendor_id() path is to support the
> > Configuration Request Retry Status feature (see PCIe r3.1, sec 2.3.2),
> > which works by special handling of config reads of the Vendor ID after
> > a reset.  Normally, that Vendor ID read would be the first access to
> > a device when we enumerate it.
> >
> > This patch adds config reads and writes of the ACS capability *before*
> > the Vendor ID read.  At that point we don't even know whether the
> > device exists.  If it doesn't exist, pci_find_ext_capability() would
> > read 0xffffffff data, and it probably fails reasonably gracefully.
> >
> > But if the device *does* exist, I think this patch breaks the CRS
> > Software Visibility feature.  Without this patch, we try to read
> > Vendor ID, and the device may return a CRS Completion Status.  If CRS
> > visibility is enabled, the root complex may complete the request by
> > returning 0x0001 for the Vendor ID, in which case we sleep and try
> > again later.
> >
> > With this patch, we first try to read the ACS capability.  If the
> > device returns a CRS Completion Status, the root complex is required
> > to reissue the config request.  This is the required behavior
> > regardless of whether CRS Software Visibility is enabled, so I think
> > this effectively disables that feature.
> 
> The workaround (acs reading etc) is applied to root port or downstream port.
> and pci_bus_read_dev_vendor_id() is for reading vendorid of device
> under that root port or downstream port.

OK, I see what you're saying: pci_std_enable_acs_sv() twiddles the ACS
capability of the upstream bridge when we're enumerating a device
*below* the bridge.  Since the bridge has already been enumerated,
we've already read its Vendor ID, so looking up its ACS capability
should not cause any CRS completions.

Still, I think all this fiddling around with the ACS capability is too
intrusive for non-IDT devices.  Caching the ACS capability location
would help a little bit, but it's still too much in my opinion.

If ACS is broken on the IDT switch, one obvious possibility is a quirk
to disable use of ACS on those switches.

The current patch appears to penalize everybody for the sins of IDT,
which I'd like to avoid.

Bjorn