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

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Date:   Tue, 13 Jun 2017 19:19:57 -0400
From:   James Puthukattukaran <james.puthukattukaran@...cle.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Yinghai Lu <yinghai@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] PCI: Workaround wrong flags completions for IDT switch



> On Jun 13, 2017, at 6:14 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> 
>> 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@...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).
>>>> 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
>> “Section 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?

The issue is how will we know is the config write succeeds if the device is not ready? I thought it was simpler to disable acs for the sake of the read and when we know that the device is ready ( returns vendor id from read), it's ready for subsequent config write. 

> 
> 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