linux-kernel - Re: drivers binding to device node with multiple compatible strings

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Date:   Thu, 4 Oct 2018 10:32:56 +0100
From:   Grant Likely <grant.likely@...retlab.ca>
To:     leoyang.li@....com
Cc:     Rob Herring <robh+dt@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Frank Rowand <frowand.list@...il.com>,
        Grant Likely <grant.likely@....com>
Subject: Re: drivers binding to device node with multiple compatible strings

On Fri, Sep 28, 2018 at 10:01 PM Li Yang <leoyang.li@....com> wrote:
>
> On Fri, Sep 28, 2018 at 3:07 PM Rob Herring <robh+dt@...nel.org> wrote:
> >
> > On Thu, Sep 27, 2018 at 5:25 PM Li Yang <leoyang.li@....com> wrote:
> > >
> > > Hi Rob and Grant,
> > >
> > > Various device tree specs are recommending to include all the
> > > potential compatible strings in the device node, with the order from
> > > most specific to most general.  But it looks like Linux kernel doesn't
> > > provide a way to bind the device to the most specific driver, however,
> > > the first registered compatible driver will be bound.
> > >
> > > As more and more generic drivers are added to the Linux kernel, they
> > > are competing with the more specific vendor drivers and causes problem
> > > when both are built into the kernel.  I'm wondering if there is a
> > > generic solution (or in plan) to make the most specific driver bound
> > > to the device.   Or we have to disable the more general driver or
> > > remove the more general compatible string from the device tree?
> >
> > It's been a known limitation for a long time. However, in practice it
> > doesn't seem to be a common problem. Perhaps folks just remove the
> > less specific compatible from their DT (though that's not ideal). For
> > most modern bindings, there's so many other resources beyond
> > compatible (clocks, resets, pinctrl, etc.) that there are few generic
> > drivers that can work.
> >
> > I guess if we want to fix this, we'd need to have weighted matching in
> > the driver core and unbind drivers when we get a better match. Though
> > it could get messy if the better driver probe fails. Then we've got to
> > rebind to the original driver.
>
> Probably we can populate the platform devices from device tree after
> the device_init phase?  So that all built-in drivers are already
> registered when the devices are created and we can try find the best
> match in one go?  For more specific loadable modules we probably need
> to unbind from the old driver and bind to the new one.  But I agree
> with you that it could be messy.

It's a tradeoff.
>
> >
> > Do you have a specific case where you hit this?
>
> Maybe not a new issue but "snps,dw-pcie" is competing with various
> "fsl,<chip>-pcie" compatibles.  Also a specific PHY
> "ethernet-phy-idAAAA.BBBB" with generic "ethernet-phy-ieee802.3-c45".
>
> Regards,
> Leo