linux-kernel - Re: [PATCH v3 3/3] PCI: imx6: Add support for i.MX8MQ

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Date:   Thu, 20 Dec 2018 09:00:01 -0600
From:   Rob Herring <robh@...nel.org>
To:     Trent Piepho <tpiepho@...inj.com>
Cc:     Lucas Stach <l.stach@...gutronix.de>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        Dong Aisheng <aisheng.dong@....com>,
        Richard Zhu <hongxing.zhu@....com>,
        Chris Healy <cphealy@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Fabio Estevam <fabio.estevam@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Leonard Crestez <leonard.crestez@....com>,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v3 3/3] PCI: imx6: Add support for i.MX8MQ

On Wed, Dec 19, 2018 at 7:22 PM Trent Piepho <tpiepho@...inj.com> wrote:
>
> On Wed, 2018-12-19 at 16:47 -0800, Andrey Smirnov wrote:
> >
> > > > This series initially added explicit offsets but I suggested a single
> > > > "controller-id" because:
> > > >   * There are multiple bit and byte offsets
> > > >   * Other imx8 SOCs also have 2x pcie with other bit/byte offsets
> > > >
> > > > Hiding this behind a compatible string and single "controller-id" seem
> > > > preferable to elaborating register maps in dt bindings. It also makes
> > > > upgrades simpler: if features are added which use other bits there is no
> > > > need to describe them in DT and deal with compatibility headaches.
> > >
> > > You already have an id for the controllers: the address. Use that if
> > > you don't want to put the register offsets in DT.
> > >
> >
> > Lucas, are you on board with this?
>
> Does address here mean the address from the controller's reg property?

Yes.

> How do you map that address to the controller's index?  A giant table
> of every soc so the soc type plus controller register address pair than
> can be looked up in the driver?

You only need the 2-Nth instance addresses. A non-matching address can
be instance 0.

> I.e., on iMX8MQ the controller at 0x33800000 is controller 0 and so on
> for every possible SoC address combination?
>
> Not really a fan of that.

Well, this was not my first suggestion.

> The situation here is that some registers for these controllers are
> interleaved, right?  I.e., there's one register somewhere where bit 0
> means enable controller 0 and bit 1 means enable controller 1 and so
> on.

So? The only difference here is an additional mapping step.

> Isn't cell-index already the standard device tree property for this
> kind of setup?
>
> I know cell-index was historically also (ab)used in an attempt to
> provide a fixed kernel device enumeration order, something now handled
> better by chosen node aliases.

Don't use cell-index. Consider it a deprecated relic from OF that is
not used on FDT (though there probably are some cases it did get
used).

Rob