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Date:   Tue, 12 Oct 2021 09:48:46 +0200
From:   Michael Walle <michael@walle.cc>
To:     Alexander Stein <alexander.stein@ew.tq-group.com>
Cc:     Miquel Raynal <miquel.raynal@bootlin.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Rob Herring <robh+dt@kernel.org>,
        Pratyush Yadav <p.yadav@ti.com>,
        Tudor Ambarus <tudor.ambarus@microchip.com>,
        linux-mtd@lists.infradead.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: mtd: spi-nor: Add
 output-driver-strength property
In-Reply-To: <20211012061704.284214-1-alexander.stein@ew.tq-group.com>
References: <20211012061704.284214-1-alexander.stein@ew.tq-group.com>
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Am 2021-10-12 08:17, schrieb Alexander Stein:
> This property is for optimizing output voltage impedance and is
> specific to each board. It overwrites the default set by the flash
> device. Various flash devices support different impedances.
> 
> Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
> ---
> Changes in v2:
> * Updated the property description and the commit message accordingly
> 
>  Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> index ed590d7c6e37..4c3c506a8853 100644
> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> @@ -72,6 +72,14 @@ 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. This property overwrites the default set by the 
> flash
> +      device. This is board specific and should be determined by the
> +      manufacturer. Various flash devices support different 
> impedances.

Mh, this seems to be very tailored to this flash chip. Eg. the "Vcc/2", 
is
this something specific to this flash or is this some kind of common 
usage?
For example, Winbond flashes specifies the output driver strength in 
percent.
Settings are 25%, 50%, 75%, 100% there.

I'd have to ask a hardware guy, if one could convert between these two
representations of the driver strength.

Rob, do you know how is this case ususally handled? Can there be 
different
properties, like:

output-driver-strength-ohms
output-driver-strength-percent

or similar.

-michael