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

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Message-ID: <20170419102652.GA23487@red-moon>
Date:   Wed, 19 Apr 2017 11:26:52 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     abhijit <abhijitnaik27@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: Doubt on first access for PCIe device

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