linux-kernel - Re: [PATCH 1/2] dt-bindings: mtd: spi-nor: Add output-driver-strength property

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Date:   Tue, 12 Oct 2021 07:52:17 -0500
From:   Rob Herring <robh@...nel.org>
To:     Alexander Stein <Alexander.Stein@...systems.com>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Pratyush Yadav <p.yadav@...com>,
        Michael Walle <michael@...le.cc>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Alexander Stein <alexander.stein@...tq-group.com>,
        linux-mtd@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: mtd: spi-nor: Add
 output-driver-strength property

On Mon, Oct 04, 2021 at 01:15:28PM +0200, Alexander Stein wrote:
> From: Alexander Stein <alexander.stein@...tq-group.com>
> 
> This property is for optimizing output voltage impedance and is
> specific to each board.
> 
> Signed-off-by: Alexander Stein <alexander.stein@...tq-group.com>
> ---
> I checked Micron and Macronix datasheets. Both have similar but not
> identical supported values. Also the register locations are different.
> For those reasons I decided to specify the Ohms value directly and let
> the device specfic driver figure out if it is supported where to write
> it to.
> BTW: Are the Ohm values and the corresponding register bits standardized
> somewhere?
> 
>  Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> index ed590d7c6e37..7d7f20a741b5 100644
> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> @@ -72,6 +72,12 @@ properties:
>        be used on such systems, to denote the absence of a reliable reset
>        mechanism.
>  
> +  output-driver-strength:
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      Output driver strength in Ohms which optimizes the impedance at Vcc/2
> +      output voltage.

Use a standard unit suffix.

Though, specifying 'drive strength' in ohms rather than amps is a bit 
strange.