netdev - RE: [PATCH net-next 1/5] dt-bindings: net: r8a779f0-ether-switch: Add ACLK

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Message-ID:
 <TYBPR01MB53413C7E1E5AE74ABE84ABE8D84B9@TYBPR01MB5341.jpnprd01.prod.outlook.com>
Date: Tue, 30 May 2023 00:19:46 +0000
From: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To: Conor Dooley <conor@...nel.org>, Andrew Lunn <andrew@...n.ch>
CC: "s.shtylyov@....ru" <s.shtylyov@....ru>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"krzysztof.kozlowski+dt@...aro.org" <krzysztof.kozlowski+dt@...aro.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>, "geert+renesas@...der.be"
	<geert+renesas@...der.be>, "magnus.damm@...il.com" <magnus.damm@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH net-next 1/5] dt-bindings: net: r8a779f0-ether-switch: Add
 ACLK

Hello Conor, Andrew,

> From: Conor Dooley, Sent: Tuesday, May 30, 2023 5:44 AM
> On Mon, May 29, 2023 at 10:11:12PM +0200, Andrew Lunn wrote:
> > On Mon, May 29, 2023 at 07:36:03PM +0100, Conor Dooley wrote:
> > > On Mon, May 29, 2023 at 05:08:36PM +0900, Yoshihiro Shimoda wrote:
> > > > Add ACLK of GWCA which needs to calculate registers' values for
> > > > rate limiter feature.
> > > >
> > > > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> > > > ---
> > > >  .../bindings/net/renesas,r8a779f0-ether-switch.yaml    | 10 ++++++++--
> > > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/net/renesas,r8a779f0-ether-switch.yaml
> b/Documentation/devicetree/bindings/net/renesas,r8a779f0-ether-switch.yaml
> > > > index e933a1e48d67..cbe05fdcadaf 100644
> > > > --- a/Documentation/devicetree/bindings/net/renesas,r8a779f0-ether-switch.yaml
> > > > +++ b/Documentation/devicetree/bindings/net/renesas,r8a779f0-ether-switch.yaml
> > > > @@ -75,7 +75,12 @@ properties:
> > > >        - const: rmac2_phy
> > > >
> > > >    clocks:
> > > > -    maxItems: 1
> > > > +    maxItems: 2
> > > > +
> > > > +  clock-names:
> > > > +    items:
> > > > +      - const: fck
> > > > +      - const: aclk
> > >
> > > Since having both clocks is now required, please add some detail in the
> > > commit message about why that is the case. Reading it sounds like this
> > > is an optional new feature & not something that is required.
> >
> > This is something i wondered about, backwards compatibility with old
> > DT blobs. In the C code it is optional, and has a default clock rate
> > if the clock is not present.

I'm sorry for lacking explanation. You're correct. this is backwards
compatibility with old DT blobs.

> Yeah, I did the cursory check of the code to make sure that an old dtb
> would still function, which is part of why I am asking for the
> explanation of the enforcement here. I'm not clear on what the
> consequences of getting the default rate is. Perhaps if I read the whole
> series and understood the code I would be, but this commit should
> explain the why anyway & save me the trouble ;)

The following clock rates are the same (400MHz):
 - default rate (RSWITCH_ACLK_DEFAULT) in the C code
 - R8A779F0_CLK_S0D2_HSC from dtb

Only for backwards compatibility with old DT blobs, I added
the RSWITCH_ACLK_DEFAULT, and got the aclk as optional.

By the way, R8A779F0_CLK_S0D2_HSC is fixed rate, and the r8a779f0-ether-switch
only uses the rswitch driver. Therefore, the clock rate is always 400MHz.
So, I'm thinking that the following implementation is enough.
 - no dt-bindings change. (In other words, don't add aclk in the dt-bindings.)
 - hardcoded the clock rate in the C code as 400MHz.

> > So the yaml should not enforce an aclk member.
> 
> This however I could go either way on. If the thing isn't going to
> function properly with the fallback rate, but would just limp on on
> in whatever broken way it has always done, I would agree with making
> the second clock required so that no new devicetrees are written in a
> way that would put the hardware into that broken state.
> On the other hand, if it works perfectly fine for some use cases without
> the second clock & just using the default rathe then I don't think the
> presence of the second clock should be enforced.

Thank you very much for your comments! The it works perfectly fine for
all use cases without the second clock & just using the default rate.
That's why I'm now thinking that adding aclk into the dt-bindings is not
a good way...

Best regards,
Yoshihiro Shimoda

> Cheers,
> Conor.