linux-kernel - Re: [PATCH v8 2/3] PCI: iproc: retry request when CRS returned from EP

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Message-ID: <20170828195402.GK8154@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 28 Aug 2017 14:54:02 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Oza Oza <oza.oza@...adcom.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Jon Mason <jonmason@...adcom.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Andy Gospodarek <gospo@...adcom.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel@...r.kernel.org,
        Oza Pawandeep <oza.pawandeep@...il.com>
Subject: Re: [PATCH v8 2/3] PCI: iproc: retry request when CRS returned from
 EP

On Tue, Aug 29, 2017 at 01:09:53AM +0530, Oza Oza wrote:
> On Tue, Aug 29, 2017 at 12:47 AM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Thu, Aug 24, 2017 at 10:34:25AM +0530, Oza Pawandeep wrote:
> >> PCIe spec r3.1, sec 2.3.2
> >> If CRS software visibility is not enabled, the RC must reissue the
> >> config request as a new request.
> >>
> >> - If CRS software visibility is enabled,
> >> - for a config read of Vendor ID, the RC must return 0x0001 data
> >> - for all other config reads/writes, the RC must reissue the
> >>   request
> >>
> >> iproc PCIe Controller spec:
> >> 4.7.3.3. Retry Status On Configuration Cycle
> >> Endpoints are allowed to generate retry status on configuration
> >> cycles. In this case, the RC needs to re-issue the request. The IP
> >> does not handle this because the number of configuration cycles needed
> >> will probably be less than the total number of non-posted operations
> >> needed.
> >>
> >> When a retry status is received on the User RX interface for a
> >> configuration request that was sent on the User TX interface,
> >> it will be indicated with a completion with the CMPL_STATUS field set
> >> to 2=CRS, and the user will have to find the address and data values
> >> and send a new transaction on the User TX interface.
> >> When the internal configuration space returns a retry status during a
> >> configuration cycle (user_cscfg = 1) on the Command/Status interface,
> >> the pcie_cscrs will assert with the pcie_csack signal to indicate the
> >> CRS status.
> >> When the CRS Software Visibility Enable register in the Root Control
> >> register is enabled, the IP will return the data value to 0x0001 for
> >> the Vendor ID value and 0xffff  (all 1’s) for the rest of the data in
> >> the request for reads of offset 0 that return with CRS status.  This
> >> is true for both the User RX Interface and for the Command/Status
> >> interface.  When CRS Software Visibility is enabled, the CMPL_STATUS
> >> field of the completion on the User RX Interface will not be 2=CRS and
> >> the pcie_cscrs signal will not assert on the Command/Status interface.
> >>
> >> Per PCIe r3.1, sec 2.3.2, config requests that receive completions
> >> with Configuration Request Retry Status (CRS) should be reissued by
> >> the hardware except reads of the Vendor ID when CRS Software
> >> Visibility is enabled.
> >>
> >> This hardware never reissues configuration requests when it receives
> >> CRS completions.
> >> Note that, neither PCIe host bridge nor PCIe core re-issues the
> >> request for any configuration offset.
> >>
> >> For config reads, this hardware returns CFG_RETRY_STATUS data when
> >> it receives a CRS completion for a config read, regardless of the
> >> address of the read or the CRS Software Visibility Enable bit.
> >
> > I can't remember how Stingray handles the CRS Software Visibility Enable
> > bit.  Is it a read-only zero?  Is it writable?  Does the hardware look at
> > it all (I think not)?
> 
> HW doesnt look it at all, it is "dont care" bit.

Sigh, I made the classic mistake of asking more than one question, and
I guess I'm about to do it again :)  It'll save time if you can answer
all the questions at once.

Linux enables PCI_EXP_RTCTL_CRSSVE if PCI_EXP_RTCAP says it is
supported (see pci_enable_crs()).

  - What does PCI_EXP_RTCAP say?

  - Is PCI_EXP_RTCTL_CRSSVE writable?

With all the CRS-related work going on, I suspect we may someday want to
read PCI_EXP_RTCTL_CRSSVE to see if CRS SV is enabled.

Bjorn