linux-kernel - AW: (EXT) Re: [PATCH v2 1/2] dt-bindings: mtd: spi-nor: Add output-driver-strength property

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Date:   Wed, 13 Oct 2021 08:47:04 +0000
From:   "Alexander Stein" <Alexander.Stein@...tq-group.com>
To:     "Michael Walle" <michael@...le.cc>
Cc:     "Miquel Raynal" <miquel.raynal@...tlin.com>,
        "Richard Weinberger" <richard@....at>,
        "Vignesh Raghavendra" <vigneshr@...com>,
        "Rob Herring" <robh@...nel.org>, "Pratyush Yadav" <p.yadav@...com>,
        "Tudor Ambarus" <tudor.ambarus@...rochip.com>,
        linux-mtd@...ts.infradead.org 
        <linux-mtd@...ts.infradead.org>,
        devicetree@...r.kernel.org 
        <devicetree@...r.kernel.org>,
        linux-kernel@...r.kernel.org 
        <linux-kernel@...r.kernel.org>
Subject: AW: (EXT) Re: [PATCH v2 1/2] dt-bindings: mtd: spi-nor: Add
 output-driver-strength property

Hello Michael,


Am 2021-10-12 09:48, schrieb Michael Walle:
> 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@...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?

"Vcc/2" is taken from the datasheet description.

> 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.

Well, 100% must map to some actual value. Which then can be used to create
a discrete value table, which are then supported by the flash driver.
E.g. for Micron not every flash supports the same set of settings for
driver strength.
Macronix uses similar settings (values and bitmask), but in a different
register. But if some vendors have pretty much incompatible settings, it
might be feasible to provide vendor specific settings, e.g.
"micron,drive-strength = <45>" (for 45 Ohm) or "winbond,drive-strength = <100>"
(for 100%).

Best regards,
Alexander