linux-kernel - Re: Doubt on first access for PCIe device

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Message-ID: <20170421155900.GA19059@red-moon>
Date:   Fri, 21 Apr 2017 16:59:00 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     abhijit naik <abhijitnaik27@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: Doubt on first access for PCIe device

On Fri, Apr 21, 2017 at 03:33:45PM +0530, abhijit naik wrote:
>    Hi Lorenzo,
>    Thank you very much for reply.
>    As you mentioned I noted that, "If a Function must generate a Completion
>    prior to the initial device Configuration Write Request, 0's must be
>    entered into the Bus Number and Device Number fields"
>    By above statement this means that completion ID will be 0x0000. For the
>    first device on any bus,  device number will be 0 and function number will
>    be 0.
>    Isn't completer (for configuration read) packet will be addressing itself
>    instead of root complex with bus, device and function number 0?

I don't understand what you are asking. The completion (containing
the read data) is routed back to the initiator via ID (of the requester)
routing.

2.2.4.2
2.2.9

Lorenzo

>    On Wed, Apr 19, 2017 at 3:56 PM, Lorenzo Pieralisi
>    <lorenzo.pieralisi@....com> wrote:
> 
>      On Tue, Apr 11, 2017 at 07:45:40PM +0530, abhijit wrote:
>      >
>      > Hi All,
>      >
>      > I was referring Linux code for PCIe enumeration and I have one doubt
>      > w.r.t to very first operation that must be done on device.
>      >
>      > Currently while scanning for device, we directly read vendor ID from
>      > device. But PCIE base specification
>      > "CB-PCI_Express_Base_4.0r0.7_November-11-2016.pdf" section 2.2.6.2
>      > specifies that,
>      >
>      > "Functions must capture the Bus and Device Numbers supplied with all
>      > Type 0 Configuration Write Requests completed by the Function and
>      > supply these numbers in the Bus and Device Number fields of the
>      > Requester ID for all Requests initiated by the Device/Function."
> 
>      Please note:
>      "for all Requests initiated by the Device/Function".
>                        ^^^^^^^^^
>      > Here I am assuming, the completer ID will be device number and
>      > function number that will eventually programmed in to  device. In
>      > that case, my question is, without first write, how read
>      > request(VENDOR ID read) is serviced/routed?
> 
>      That's not initiated by the endpoint device, it is routed by
>      requester ID, which is the requester ID of the configuration
>      request - ie master (ie root complex) requester ID.
> 
>      Read 2.2.9 - Completion rules
> 
>      HTH,
>      Lorenzo
> 
>    --
>    Best wishes,
>    Abhijit Naik