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

[Bjorn Helgaas]

On Tue, Jun 13, 2017 at 02:30:55PM -0400, james puthukattukaran wrote:
> On 6/13/2017 1:00 PM, Yinghai Lu wrote:
> >On Mon, Jun 12, 2017 at 2:48 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> >>On Fri, Jun 09, 2017 at 04:16:17PM -0700, Yinghai Lu wrote:
> >>>From: James Puthukattukaran <james.puthukattukaran@xxxxxxxxxx>
> >>>
> >>>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).
> >>Can you include a URL where this erratum is published?  If not, can
> >>you include the actual erratum text here?
> >Ok.
> 
> Here's the errata text
> ------------------------------------
> Item #36 - Downstream port applies ACS Source Validation to Completions
> âSection 6.12.1.1" of the PCI Express Base Specification 3.1 states
> that completions are never affected
> by ACS Source Validation. However, completions received by a
> downstream port of the PCIe switch from a device that has not yet
> captured a PCIe bus number are incorrectly dropped by ACS source
> validation by the switch downstream port.
> 
> Workaround: Issue a CfgWr1 to the downstream device before issuing
> the first CfgRd1 to the device.
> This allows the downstream device to capture its bus number; ACS
> source validation no longer stops
> completions from being forwarded by the downstream port. It has been
> observed that Microsoft Windows implements this workaround already;
> however, some versions of Linux and other operating systems may not.

This doesn't mention anything about disabling ACS.  Issuing a config
write to devices downstream of an IDT bridge sounds simpler than what
this patch does.  Why don't you do that?

This patch does write to PCI_VENDOR_ID, which purports to be part of
the workaround, but that happens *after* the first config read (which
happens inside __pci_bus_read_dev_vendor_id()).

Bjorn