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

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Date:   Tue, 29 Aug 2017 01:09:53 +0530
From:   Oza Oza <oza.oza@...adcom.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
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 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.

Regards,
Oza.