Re: [PATCH 1/2] dt-bindings: memory-controller: Document rev c.1.5 compatible

[Krzysztof Kozlowski <krzk@kernel.org>]

On Wed, Dec 18, 2024 at 09:15:08AM -0800, Florian Fainelli wrote:
> On 12/18/24 03:37, Krzysztof Kozlowski wrote:
> > On Tue, Dec 17, 2024 at 11:44:38AM -0800, Florian Fainelli wrote:
> > > Document the revision c.1.5 compatible string that is present on newer
> > > Broadcom STB memory controllers (74165 and onwards).
> > > 
> > > Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
> > > ---
> > >   .../bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml       | 1 +
> > >   1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml b/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
> > > index 4b072c879b02..99d79ccd1036 100644
> > > --- a/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
> > > +++ b/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
> > > @@ -29,6 +29,7 @@ properties:
> > >             - brcm,brcmstb-memc-ddr-rev-c.1.2
> > >             - brcm,brcmstb-memc-ddr-rev-c.1.3
> > >             - brcm,brcmstb-memc-ddr-rev-c.1.4
> > > +          - brcm,brcmstb-memc-ddr-rev-c.1.5
> > 
> > You should use v2.1 fallback and drop driver patch. Or explain in
> > commit briefly why different approach is suitable.
> 
> Are you suggesting that we should have fallback compatible strings, such
> that we have something like this:
> 
> compatible = "brcm,brcmstb-memc-ddr-rev-c.1.5",
> "brcm,brcmstb-memc-ddr-rev-c", "brcm,brcmstb-memc-ddr"
> 
> and the driver only needs to match on "brcm,brcmstb-memc-ddr-rev-c" and
> apply the adequate register offset table?

Almost, fallback should be brcm,brcmstb-memc-ddr-rev-b.2.1 or whatever
was in the driver first or whatever is the oldest known common
interface.

brcm,brcmstb-memc-ddr-rev-c is not a specific compatible.

> If so, that is not how the current binding, and therefore DTBs are being
> deployed, so that will introduce a breakage until we update all DTBs in the
> field...

No. First, I thought about new comaptible so the one you add here. No
breakage, it's new compatible. This saves you these pointless updates of
driver everytime you add new compatible.

Second, you can introduce fallbacks to older compatibles as well - there
will be no breakage, because you add one more compatible. The old
compatibles (covered by fallback) of course stays in the driver, so
there is no breakage at all. We did it multiple times for several
different bindings in Qualcomm. People were doing exactly the same:
adding compatible for new device to binding and driver, without
considering the compatibility at all.

Except being logically correct choice - using fallbacks - this really
has huge benefits when later upstreaming complete, big SoCs, like we do
for latest Qualcomm SoCs: several changes will be only bindings updates.

Best regards,
Krzysztof