linux-kernel - Re: [PATCH v6 13/52] dt-bindings: memory: tegra124: emc: Document new interconnect property

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Date:   Tue, 27 Oct 2020 23:16:29 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Georgi Djakov <georgi.djakov@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Mikko Perttunen <cyndis@...si.fi>,
        Viresh Kumar <vireshk@...nel.org>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v6 13/52] dt-bindings: memory: tegra124: emc: Document new
 interconnect property

27.10.2020 22:48, Krzysztof Kozlowski пишет:
> On Tue, Oct 27, 2020 at 10:19:28PM +0300, Dmitry Osipenko wrote:
>> 27.10.2020 13:25, Krzysztof Kozlowski пишет:
>>> On Mon, Oct 26, 2020 at 01:16:56AM +0300, Dmitry Osipenko wrote:
>>>> External memory controller is interconnected with memory controller and
>>>> with external memory. Document new interconnect property which turns
>>>> External Memory Controller into interconnect provider.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>> ---
>>>>  .../bindings/memory-controllers/nvidia,tegra124-emc.yaml   | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra124-emc.yaml b/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra124-emc.yaml
>>>> index 278549f9e051..ac00832ceac1 100644
>>>> --- a/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra124-emc.yaml
>>>> +++ b/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra124-emc.yaml
>>>> @@ -29,6 +29,9 @@ properties:
>>>>      items:
>>>>        - const: emc
>>>>  
>>>> +  "#interconnect-cells":
>>>> +    const: 0
>>>> +
>>>>    nvidia,memory-controller:
>>>>      $ref: /schemas/types.yaml#/definitions/phandle
>>>>      description:
>>>> @@ -327,6 +330,7 @@ required:
>>>>    - clocks
>>>>    - clock-names
>>>>    - nvidia,memory-controller
>>>> +  - "#interconnect-cells"
>>>
>>> Another required property, what about all existing users of this binding?
>>
>> EMC/devfreq drivers check presence of the new properties and ask users
>> to upgrade the DT. The kernel will continue to work fine using older
>> DTBs, but devfreq driver won't load.
> 
> If the devfreq was working fine before (with these older DTBs and older
> kernel) then you break the feature.
> 
> If devfreq was not working or was not stable enough, then nothing is
> broken so such change is accepted.
> 
> Which one is then?

Definitely the latter. The current devfreq works okay'ish, but we rely
on hardware to recover from temporal FIFO underflows and it's a
user-visible problem which this series addresses.