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Sender: Corey Minyard <tcminyard@gmail.com>
Date:   Sun, 7 Aug 2022 11:05:45 -0500
From:   Corey Minyard <minyard@acm.org>
To:     Tomer Maimon <tmaimon77@gmail.com>
Cc:     Avi Fishman <avifishman70@gmail.com>,
        Tali Perry <tali.perry1@gmail.com>,
        Joel Stanley <joel@jms.id.au>,
        Patrick Venture <venture@google.com>,
        Nancy Yuen <yuenn@google.com>,
        Benjamin Fair <benjaminfair@google.com>,
        Jonathan Cameron <jic23@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>,
        openipmi-developer@lists.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        devicetree <devicetree@vger.kernel.org>
Subject: Re: [PATCH v2] dt-binding: ipmi: add fallback to npcm845 compatible
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On Sun, Aug 07, 2022 at 05:54:28PM +0300, Tomer Maimon wrote:
> On Sun, 7 Aug 2022 at 15:11, Corey Minyard <minyard@acm.org> wrote:
> >
> > On Sun, Aug 07, 2022 at 11:03:56AM +0300, Tomer Maimon wrote:
> > > Hi Corey,
> > >
> > > Thanks for your comment.
> > >
> > > On Fri, 5 Aug 2022 at 14:58, Corey Minyard <minyard@acm.org> wrote:
> > > >
> > > > On Thu, Aug 04, 2022 at 09:18:00PM +0300, Tomer Maimon wrote:
> > > > > Add to npcm845 KCS compatible string a fallback to npcm750 KCS compatible
> > > > > string becuase NPCM845 and NPCM750 BMCs are using identical KCS modules.
> > > > >
> > > > > Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
> > > > > ---
> > > > >  Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt b/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
> > > > > index cbc10a68ddef..4fda76e63396 100644
> > > > > --- a/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
> > > > > +++ b/Documentation/devicetree/bindings/ipmi/npcm7xx-kcs-bmc.txt
> > > > > @@ -7,7 +7,7 @@ used to perform in-band IPMI communication with their host.
> > > > >  Required properties:
> > > > >  - compatible : should be one of
> > > > >      "nuvoton,npcm750-kcs-bmc"
> > > > > -    "nuvoton,npcm845-kcs-bmc"
> > > > > +    "nuvoton,npcm845-kcs-bmc", "nuvoton,npcm750-kcs-bmc"
> > > >
> > > > This is just wrong.  The compatible is supposed to identify the device,
> > > > not the board the device is on.  I think compatible here should be
> > > > "npcm7xx-kcs-bmc", and just use that everywhere.  It's fine if that is
> > > > used on a board named npcm845.
> > > The NPCM8XX is not a board, The Nuvoton NPCM8XX is a fourth-generation
> > > BMC SoC device family.
> >
> > Ok, but same principle applies.
> >
> > If the device is exactly the same, then you would only use one of the
> > "npcm7xx-kcs-bmc" and put that in both device trees.  You can use
> > "nuvoton,npcm750-kcs-bmc", it's really not that important.  Or even
> > "nuvoton,npcm-kcs-bmc"
> If we use "nuvoton, npcm-kcs-bmc" we should take care of backward dts
> compatibility, and I am not sure we like to change NPCM KCS driver.
> >
> > If the device has a minor difference that can be expressed in a
> > parameter, then create a parameter for it.
> >
> > If the device has enough differences that a parameter or two doesn't
> > cover it, then you put either nuvoton,npcm750-kcs-bmc or
> > nuvoton,npcm750-kcs-bmc in the device tree.  Not both.  Then you need
> > two entries in the of_device_id array and you use the data field or
> > something to express the difference.
> >
> > Since there appears to be no difference, just put
> > "nuvoton,npcm750-kcs-bmc" in the npcm845 and I will drop the patch
> > adding all this.  Then a patch can be added saying it applies to both
> > the 7xx and 8xx series of BMC SOCs.  If you want to change the name,
> > then a patch will be needed for that, but then you will need multiple
> > entries in your device tree, but you would not document it as such, as
> > there would only be one that applies for this kernel.
> 
> It little bit confusing to use nuvoton,npcm750-kcs-bmc that are
> related to NPCM7XX for NPCM8XX KCS.

A little, but it's not unusual.

> We can use the generic name "nuvoton, npcm-kcs-bmc" as you suggested
> above but we should take care of backward dts compatibility, and I am
> not sure we like to change NPCM KCS driver.
> 
> We had a disscation with Arnd, Arnd asked us to use a fallback as we
> did here if NPCM8XX device module is similar to NPCM7XX module:
> https://lore.kernel.org/lkml/20220522155046.260146-5-tmaimon77@gmail.com/
> 
> I think we should use a fallback to describe the NPCM8XX KCS in the
> dt-binding document.

I'm ok with that option.  I guess I should have mentioned it.  Add
nuvoton,npcm-kcs-bmc to the driver's of_device_id table.  Then use that
in that compatible string in the device tree.  Leave the 750 compatible
string in the table for backwards compatibility.

There's no point in having a bunch of those strings if they are all the
same.  If a new one comes out that is different, we can handle that when
the time comes.

-corey

> >
> > I'm pretty sure the only reason to have muliple compatible entries in a
> > device tree is to cover multiple kernels where the name changed.
> >
> > -corey
> >
> > > >
> > > > -corey
> > > >
> > > > >  - interrupts : interrupt generated by the controller
> > > > >  - kcs_chan : The KCS channel number in the controller
> > > > >
> > > > > --
> > > > > 2.33.0
> > > > >
> > >
> > > Best regards,
> > >
> > > Tomer
> 
> Best regards,
> 
> Tomer