linux-kernel - Re: Query regarding the use of pcie-designware-plat.c file

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Date:   Wed, 9 Jun 2021 11:49:20 +0200
From:   Thierry Reding <treding@...dia.com>
To:     Vidya Sagar <vidyas@...dia.com>
CC:     Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "amurray@...goodpenguin.co.uk" <amurray@...goodpenguin.co.uk>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Krishna Thota <kthota@...dia.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Query regarding the use of pcie-designware-plat.c file

On Wed, Jun 09, 2021 at 10:56:48AM +0530, Vidya Sagar wrote:
> 
> 
> On 6/9/2021 2:47 AM, Gustavo Pimentel wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > Hi Vidya,
> > 
> > The pcie-designware-plat.c is the driver for the Synopsys PCIe RC IP
> > prototype.
> Thanks for the info Gustavo.
> But, I don't see any DT file having only "snps,dw-pcie" compatibility
> string. All the DT files that have "snps,dw-pci" compatibility string also
> have their platform specific compatibility string and their respective host
> controller drivers. Also, it is the platform specific compatibility string
> that is used for binding purpose with their respective drivers and not the
> "snps,dw-pcie". So, wondering when will pcie-designware-plat.c be used as
> there is not DT file which has only "snps,dw-pcie" as the compatibility
> string.

Sounds to me like we have two options:

  1. If there's indeed real hardware that's identified by the existing
     "snps,dw-pcie" compatible string, then it's wrong for other devices
     to list that in their compatible string because they are likely not
     compatible with that (i.e. they might be from a register point of
     view, but at least from an integration point of view they usually
     differ).

  2. If "snps,dw-pcie" is meant to describe the fact that these are all
     based off the same IP but may be differently integrated, then there
     should be no driver matching on that compatible string.

Option 2 is not very robust because somebody could easily add a matching
driver at some point in the future. Also, if we don't match on a
compatible string there's not a lot of use in listing it in DT in the
first place.

So I think option 1 would be preferred.

Thierry

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