linux-kernel - Re: [PATCH v1 2/8] MT9M114: Add pad-slew-rate DT-binding

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Message-ID: <20250304-expend-isotope-cea613f4e9d5@spud>
Date: Tue, 4 Mar 2025 16:39:34 +0000
From: Conor Dooley <conor@...nel.org>
To: Mathis Foerst <mathis.foerst@...com>
Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	MauroCarvalhoChehab@...com, mchehab@...nel.org,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, linux-media@...r.kernel.org,
	devicetree@...r.kernel.org, manuel.traut@...com
Subject: Re: [PATCH v1 2/8] MT9M114: Add pad-slew-rate DT-binding

On Tue, Mar 04, 2025 at 12:48:58PM +0100, Mathis Foerst wrote:
> Hi Conor, Hi Sakari,
> 
> On Fri, Feb 28, 2025 at 07:11:31PM +0000, Conor Dooley wrote:
> > On Thu, Feb 27, 2025 at 10:14:02AM +0000, Sakari Ailus wrote:
> > > Hi Mathis,
> > > 
> > > On Wed, Feb 26, 2025 at 04:39:23PM +0100, Mathis Foerst wrote:
> > > > The MT9M114 supports the different slew rates (0 to 7) on the output pads.
> > > 
> > > "the output pads" probably means pixel data interface (DVP or CSI-2)
> > > signals in this cases? I suppose this is about clock modulation?
> > > It'd be good to say that.
> 
> The slew rate defines the slope of the voltage flanks on the output pads (how fast
> the pads go from LOW to HIGH or vice versa). I tried to clarify the description.
> 
> > > 
> > > > At the moment, this is hardcoded to 7 (the fastest rate).
> > > > The user might want to change this values due to EMC requirements.
> > > > 
> > > > Add the 'pad-slew-rate' property to the MT9M114 DT-bindings for selecting
> > > > the desired slew rate.
> > > > 
> > > > Signed-off-by: Mathis Foerst <mathis.foerst@...com>
> > > > ---
> > > >  .../devicetree/bindings/media/i2c/onnn,mt9m114.yaml         | 6 ++++++
> > > >  1 file changed, 6 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/media/i2c/onnn,mt9m114.yaml b/Documentation/devicetree/bindings/media/i2c/onnn,mt9m114.yaml
> > > > index 72e258d57186..666afe10c538 100644
> > > > --- a/Documentation/devicetree/bindings/media/i2c/onnn,mt9m114.yaml
> > > > +++ b/Documentation/devicetree/bindings/media/i2c/onnn,mt9m114.yaml
> > > > @@ -74,6 +74,12 @@ properties:
> > > >      description: Bypass the internal PLL of the sensor to use EXTCLK directly as SYSCLK.
> > > >      type: boolean
> > > >  
> > > > +  onnn,slew-rate:
> > > > +    $ref: /schemas/types.yaml#/definitions/uint8
> > > 
> > > No need to make this 8-bit (i.e. just use 32 bits).
> 
> Okay, I thought 8-bit would fit the small value range [0,7]. Changed it to 32 bits.
> 
> > > 
> > > > +    description: Slew rate ot the output pads DOUT[7:0], LINE_VALID, FRAME_VALID and PIXCLK
> > > 
> > > Please wrap at 80 characters.
> > > 
> > > Is there more information on the effect than 7 is the fastest?
> 
> There is no more information about the exact meaning of the values.
> As described above, the higher the value, the steeper the voltage flanks.
> 
> > > 
> > > > +    minimum: 0
> > > > +    maximum: 7
> > > 
> > > Please also add a default.
> 
> Sure, I added the default value 7 that matches the currently hardcoded 
> value in the driver.
> 
> > 
> > It'd also be great (IMO) if it were given in terms of actual units, not
> > nebulous values that I assume to be the register values.
> 
> I agree, but the documentation does not provide further details about the
> unit of the value. So using the register value is my best-effort approach.

If they don't provide em, how is anyone meant to calculate what's
correct? Trial and error?

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