Message-ID: <a92ff542-6534-4e2e-bef9-6f10c8362d86@kernel.org>
Date: Sat, 21 Dec 2024 21:04:09 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Florian Fainelli <florian.fainelli@...adcom.com>
Cc: linux-arm-kernel@...ts.infradead.org, Rob Herring <robh@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>,
 "open list:MEMORY CONTROLLER DRIVERS" <linux-kernel@...r.kernel.org>,
 "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
 <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/2] dt-bindings: memory-controller: Document rev c.1.5
 compatible

On 20/12/2024 00:43, Florian Fainelli wrote:
> On 12/19/24 00:58, Krzysztof Kozlowski wrote:
>> 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@...adcom.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.
> 
> Yes, but that is not how the binding has been defined until now, so all 
> of the DTBs out there have:
> 
> compatible = "brcm,brcmstb-memc-ddr-rev-b.2.x", "brcm,brcmstb-memc-ddr"

I don't understand the problem. We talk about new devices here, it does
not matter what existing/old devices have in binding in that matter.

> 
> (where X is in range [1..5])
> 
> and there is no fallback defined to "brcm,brcmstb-memc-ddr-rev-b.2.1", 
> so it is not like we can retrofit that easily by adding one now.
> 
>  > > 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.
> 
> Yes, there are advantages to using fallbacks and we (ab)use that 
> whenever practical.
> 
> The driver only uses a very limited subset of registers (for now), the 
> registers change between minor revisions as well in a way that using a 
> fallback like "brcm,brcmstb-memc-ddr-rev-b.2.1" is not accurate enough 
> not practical. In particular for some of the changes that I am thinking 
> of adding later on, we would need the precise minor version because the 
> behavior and/or register interface is subtly different that this matters.

Devices work fine now with the same driver data, so they are compatible.
Just because you have some differences or new features does not
invalidate that this is exactly the point for what compatibility was
created.


Best regards,
Krzysztof

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