linux-kernel - Re: [PATCH v7 1/3] dt-bindings: media: stm32-dcmi: Add DCMI min frequency property

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Message-ID: <CAL_Jsq+VgXTJy1SQr6B63kLZ3wcRMe4YfYiRNCT6s=gUO_tmTw@mail.gmail.com>
Date:   Fri, 10 Jul 2020 14:01:02 -0600
From:   Rob Herring <robh@...nel.org>
To:     Benjamin GAIGNARD <benjamin.gaignard@...com>
Cc:     Hugues FRUCHET <hugues.fruchet@...com>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "mcoquelin.stm32@...il.com" <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v7 1/3] dt-bindings: media: stm32-dcmi: Add DCMI min
 frequency property

On Fri, Jul 10, 2020 at 1:33 AM Benjamin GAIGNARD
<benjamin.gaignard@...com> wrote:
>
>
>
> On 7/9/20 10:37 PM, Rob Herring wrote:
> > On Wed, Jul 01, 2020 at 03:01:27PM +0200, Benjamin Gaignard wrote:
> >> Document st,stm32-dcmi-min-frequency property which is used to
> >> request CPUs minimum frequency when streaming frames.
> >>
> >> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...com>
> >> ---
> >>   Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml | 8 ++++++++
> >>   1 file changed, 8 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml b/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
> >> index 3fe778cb5cc3..05ca85a2411a 100644
> >> --- a/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
> >> +++ b/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
> >> @@ -44,6 +44,13 @@ properties:
> >>         bindings defined in
> >>         Documentation/devicetree/bindings/media/video-interfaces.txt.
> >>
> >> +  st,stm32-dcmi-min-frequency:
> >> +    description: DCMI minimum CPUs frequency requirement (in KHz).
> >> +    allOf:
> >> +      - $ref: /schemas/types.yaml#/definitions/uint32
> >> +      - minimum: 0
> >> +      - default: 0
> > I think this is questionable to be in DT and if it is, it's something
> > that's hardly specific to ST or this block. IIRC, we already have a way
> > to specify minimum OPPs.
> This binding is only needed on some STM32 SoC when DVFS is activated
> with low frequency setting in opp. The value also depends of the targeted
> video format and framerate.

As those 2 things are not in the DT, then neither should this value be.

> It is not an opp because it doesn't define a voltage-current-frequency
> combination
> but only set a minimum target for the CPUs frequency to guaranty a good
> reaction
> time when handling IRQs for the sensor.

OPPs can be frequency only. This is pretty clearly defining the CPU
must Operate at a certain minimum Performance Point.

Rob