[PATCH 0/2] net: phy: dp83867: add numeric impedance DT property

[PATCH 0/2] net: phy: dp83867: add numeric impedance DT property

[Teresa Remmet]

Introduce a new way to set the output impedance over device tree.
So far it is possible to pass the value either over an nvmem
cell or to set the min and max booleans over device tree.

In our use case we want to be able to set the impedance over
device tree but the boolean values are not the right fit to 
provide best signal integrity for the hardware.

To: Andrew Lunn <andrew+netdev@lunn.ch>
To: David S. Miller <davem@davemloft.net>
To: Eric Dumazet <edumazet@google.com>
To: Jakub Kicinski <kuba@kernel.org>
To: Paolo Abeni <pabeni@redhat.com>
To: Rob Herring <robh@kernel.org>
To: Krzysztof Kozlowski <krzk+dt@kernel.org>
To: Conor Dooley <conor+dt@kernel.org>
To: Andrew Davis <afd@ti.com>
To: Andrew Lunn <andrew@lunn.ch>
To: Heiner Kallweit <hkallweit1@gmail.com>
To: Russell King <linux@armlinux.org.uk>
Cc: netdev@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: upstream@lists.phytec.de

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
---
Teresa Remmet (2):
      dt-bindings: dp83867: add binding for ti,output-impedance property
      net: phy: dp83867: add numeric io impedance DT property

 Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19 ++++++++++++++-----
 drivers/net/phy/dp83867.c                             | 17 +++++++++++++----
 2 files changed, 27 insertions(+), 9 deletions(-)
---
base-commit: 1f97d9dcf53649c41c33227b345a36902cbb08ad
change-id: 20260128-wip-t-remmet-phytec-de-bspimx95-132_upstream-48188a567544

Best regards,
-- 
Teresa Remmet <t.remmet@phytec.de>

[PATCH 1/2] dt-bindings: dp83867: add binding for ti,output-impedance property

[Teresa Remmet]

Add an optional device tree property, "ti,output-impedance", which
specifies the output impedance using a raw register field value from
0x0 to 0x1f.

0x0 corresponds to the highest impedance (approximately 70 ohms),
while 0x1f represents the lowest (approximately 35 ohms).

This property allows the impedance to be configured through the
device-tree to any required value rather than being limited to fixed
minimum or maximum settings.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
---
 Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/Documentation/devicetree/bindings/net/ti,dp83867.yaml b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
index 4bc1f98fd9fe..a8d8bfb68bee 100644
--- a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
+++ b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
@@ -52,11 +52,20 @@ properties:
     description: |
       MAC Interface Impedance control to set the programmable output impedance
       to a maximum value (70 ohms).
-      Note: Specifying an io_impedance_ctrl nvmem cell or one of the
-        ti,min-output-impedance, ti,max-output-impedance properties
-        are mutually exclusive. If more than one is present, an nvmem
-        cell takes precedence over ti,max-output-impedance, which in
-        turn takes precedence over ti,min-output-impedance.
+      Note: Specifying an io_impedance_ctrl nvmem cell, ti,output-impedance
+        or one of the boolean ti,min-output-impedance and ti,max-output-impedance
+        properties is mutually exclusive.
+        If more than one is present the priority order is nvmem cell,
+        ti,output-impedance, ti,max-output-impedance and last
+        ti,min-output-impedance.
+
+  ti,output-impedance:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      MAC Interface Impedance control to set the raw register value from 0x0
+      (approx. 70 ohms) to 0x1f (approx. 35 ohms).
+    minimum: 0
+    maximum: 31
 
   tx-fifo-depth:
     $ref: /schemas/types.yaml#/definitions/uint32

-- 
2.43.0

[PATCH 2/2] net: phy: dp83867: add numeric io impedance DT property

[Teresa Remmet]

The DP83867 MAC-side IO impedance is currently configured in the driver
either via an NVMEM cell or via the two boolean device tree properties
ti,min-output-impedance and ti,max-output-impedance.

Some boards require an impedance setting different from the default,
minimum, or maximum values, and may not have NVMEM storage available.
For such cases, a device tree configuration option is needed.

Add a optional device-tree property "ti,output-impedance", that passes
the raw register field value (0x0 - 0x1f) representing the range
between approximately 70 ohms and 35 ohms. Because the actual impedance
range and step size varies depending on the production process, no exact
mapping between register value and impedance can be made. The values
are determined empirically.

If the property is not set the existing behavior is maintained. NVMEM cell
has still priority but ti,output-impedance is checked before the
boolean min and max values.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
---
 drivers/net/phy/dp83867.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/net/phy/dp83867.c b/drivers/net/phy/dp83867.c
index 5f5de01c41e1..b06d851c3487 100644
--- a/drivers/net/phy/dp83867.c
+++ b/drivers/net/phy/dp83867.c
@@ -516,13 +516,22 @@ static int dp83867_of_init_io_impedance(struct phy_device *phydev)
 			return phydev_err_probe(phydev, ret,
 						"failed to get nvmem cell io_impedance_ctrl\n");
 
-		/* If no nvmem cell, check for the boolean properties. */
-		if (of_property_read_bool(of_node, "ti,max-output-impedance"))
+		/* If no nvmem cell, check for the device tree entries */
+		ret = of_property_read_u32(of_node, "ti,output-impedance",
+					   (u32 *)&dp83867->io_impedance);
+		if (!ret) {
+			if (dp83867->io_impedance > DP83867_IO_MUX_CFG_IO_IMPEDANCE_MASK) {
+				phydev_err(phydev, "'ti,output-impedance' value %u out of range\n",
+					   dp83867->io_impedance);
+				return -EINVAL;
+			}
+		} else if (of_property_read_bool(of_node, "ti,max-output-impedance")) {
 			dp83867->io_impedance = DP83867_IO_MUX_CFG_IO_IMPEDANCE_MAX;
-		else if (of_property_read_bool(of_node, "ti,min-output-impedance"))
+		} else if (of_property_read_bool(of_node, "ti,min-output-impedance")) {
 			dp83867->io_impedance = DP83867_IO_MUX_CFG_IO_IMPEDANCE_MIN;
-		else
+		} else {
 			dp83867->io_impedance = -1; /* leave at default */
+		};
 
 		return 0;
 	}

-- 
2.43.0

Re: [PATCH 2/2] net: phy: dp83867: add numeric io impedance DT property

[Jakub Kicinski]

On Thu, 29 Jan 2026 15:13:34 +0100 Teresa Remmet wrote:
> +		} else {
>  			dp83867->io_impedance = -1; /* leave at default */
> +		};

drivers/net/phy/dp83867.c:534:3-4: Unneeded semicolon

reminder: please wait 24h between reposts on netdev:
https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#tl-dr
-- 
pw-bot: cr

Re: [PATCH 0/2] net: phy: dp83867: add numeric impedance DT property

[Andrew Davis]

On 1/29/26 8:13 AM, Teresa Remmet wrote:
> Introduce a new way to set the output impedance over device tree.
> So far it is possible to pass the value either over an nvmem
> cell or to set the min and max booleans over device tree.
> 

We have a similar issue in our networking driver, we can either get
the MAC address though a NVMEM cell, or when that is unavailable use
fallback to using a hardcoded value in DT.

I wonder if it would make sense to add a new type of NVMEM cell that
encodes the content of the cell in DT itself..

That way we can use the same NVMEM interface in both cases, without
adding all these custom fallback properties to every NVMEM consumer.

Andrew

> In our use case we want to be able to set the impedance over
> device tree but the boolean values are not the right fit to
> provide best signal integrity for the hardware.
> 
> To: Andrew Lunn <andrew+netdev@lunn.ch>
> To: David S. Miller <davem@davemloft.net>
> To: Eric Dumazet <edumazet@google.com>
> To: Jakub Kicinski <kuba@kernel.org>
> To: Paolo Abeni <pabeni@redhat.com>
> To: Rob Herring <robh@kernel.org>
> To: Krzysztof Kozlowski <krzk+dt@kernel.org>
> To: Conor Dooley <conor+dt@kernel.org>
> To: Andrew Davis <afd@ti.com>
> To: Andrew Lunn <andrew@lunn.ch>
> To: Heiner Kallweit <hkallweit1@gmail.com>
> To: Russell King <linux@armlinux.org.uk>
> Cc: netdev@vger.kernel.org
> Cc: devicetree@vger.kernel.org
> Cc: upstream@lists.phytec.de
> 
> Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
> ---
> Teresa Remmet (2):
>        dt-bindings: dp83867: add binding for ti,output-impedance property
>        net: phy: dp83867: add numeric io impedance DT property
> 
>   Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19 ++++++++++++++-----
>   drivers/net/phy/dp83867.c                             | 17 +++++++++++++----
>   2 files changed, 27 insertions(+), 9 deletions(-)
> ---
> base-commit: 1f97d9dcf53649c41c33227b345a36902cbb08ad
> change-id: 20260128-wip-t-remmet-phytec-de-bspimx95-132_upstream-48188a567544
> 
> Best regards,

Re: [PATCH 0/2] net: phy: dp83867: add numeric impedance DT property

[Rob Herring]

On Thu, Jan 29, 2026 at 11:31:29AM -0600, Andrew Davis wrote:
> On 1/29/26 8:13 AM, Teresa Remmet wrote:
> > Introduce a new way to set the output impedance over device tree.
> > So far it is possible to pass the value either over an nvmem
> > cell or to set the min and max booleans over device tree.
> > 
> 
> We have a similar issue in our networking driver, we can either get
> the MAC address though a NVMEM cell, or when that is unavailable use
> fallback to using a hardcoded value in DT.
> 
> I wonder if it would make sense to add a new type of NVMEM cell that
> encodes the content of the cell in DT itself..

I don't think DT should be providing fake nvmem...

The firmware/bootloader could read nvmem and always populate the DT MAC 
address property.

Or Linux can provide a single API that reads from different sources.

Rob

Re: [PATCH 1/2] dt-bindings: dp83867: add binding for ti,output-impedance property

[Rob Herring]

On Thu, Jan 29, 2026 at 03:13:33PM +0100, Teresa Remmet wrote:
> Add an optional device tree property, "ti,output-impedance", which
> specifies the output impedance using a raw register field value from
> 0x0 to 0x1f.
> 
> 0x0 corresponds to the highest impedance (approximately 70 ohms),
> while 0x1f represents the lowest (approximately 35 ohms).
> 
> This property allows the impedance to be configured through the
> device-tree to any required value rather than being limited to fixed
> minimum or maximum settings.
> 
> Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
> ---
>  Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/net/ti,dp83867.yaml b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> index 4bc1f98fd9fe..a8d8bfb68bee 100644
> --- a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> +++ b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> @@ -52,11 +52,20 @@ properties:
>      description: |
>        MAC Interface Impedance control to set the programmable output impedance
>        to a maximum value (70 ohms).
> -      Note: Specifying an io_impedance_ctrl nvmem cell or one of the
> -        ti,min-output-impedance, ti,max-output-impedance properties
> -        are mutually exclusive. If more than one is present, an nvmem
> -        cell takes precedence over ti,max-output-impedance, which in
> -        turn takes precedence over ti,min-output-impedance.
> +      Note: Specifying an io_impedance_ctrl nvmem cell, ti,output-impedance
> +        or one of the boolean ti,min-output-impedance and ti,max-output-impedance
> +        properties is mutually exclusive.
> +        If more than one is present the priority order is nvmem cell,
> +        ti,output-impedance, ti,max-output-impedance and last
> +        ti,min-output-impedance.
> +
> +  ti,output-impedance:
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      MAC Interface Impedance control to set the raw register value from 0x0
> +      (approx. 70 ohms) to 0x1f (approx. 35 ohms).
> +    minimum: 0
> +    maximum: 31

How does this compare to 'mac-termination-ohms'? Seems like this should 
be a common property whether it's the same or for a different location.

Rob

Re: [PATCH 1/2] dt-bindings: dp83867: add binding for ti,output-impedance property

[Teresa Remmet]

Hello Rob,

Am Dienstag, dem 10.02.2026 um 08:48 -0600 schrieb Rob Herring:
> On Thu, Jan 29, 2026 at 03:13:33PM +0100, Teresa Remmet wrote:
> > Add an optional device tree property, "ti,output-impedance", which
> > specifies the output impedance using a raw register field value
> > from
> > 0x0 to 0x1f.
> > 
> > 0x0 corresponds to the highest impedance (approximately 70 ohms),
> > while 0x1f represents the lowest (approximately 35 ohms).
> > 
> > This property allows the impedance to be configured through the
> > device-tree to any required value rather than being limited to
> > fixed
> > minimum or maximum settings.
> > 
> > Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
> > ---
> >  Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19
> > ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > index 4bc1f98fd9fe..a8d8bfb68bee 100644
> > --- a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > +++ b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > @@ -52,11 +52,20 @@ properties:
> >      description: |
> >        MAC Interface Impedance control to set the programmable
> > output impedance
> >        to a maximum value (70 ohms).
> > -      Note: Specifying an io_impedance_ctrl nvmem cell or one of
> > the
> > -        ti,min-output-impedance, ti,max-output-impedance
> > properties
> > -        are mutually exclusive. If more than one is present, an
> > nvmem
> > -        cell takes precedence over ti,max-output-impedance, which
> > in
> > -        turn takes precedence over ti,min-output-impedance.
> > +      Note: Specifying an io_impedance_ctrl nvmem cell, ti,output-
> > impedance
> > +        or one of the boolean ti,min-output-impedance and ti,max-
> > output-impedance
> > +        properties is mutually exclusive.
> > +        If more than one is present the priority order is nvmem
> > cell,
> > +        ti,output-impedance, ti,max-output-impedance and last
> > +        ti,min-output-impedance.
> > +
> > +  ti,output-impedance:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      MAC Interface Impedance control to set the raw register
> > value from 0x0
> > +      (approx. 70 ohms) to 0x1f (approx. 35 ohms).
> > +    minimum: 0
> > +    maximum: 31
> 
> How does this compare to 'mac-termination-ohms'? Seems like this
> should 
> be a common property whether it's the same or for a different
> location.

the difference is that the termination is not passed in ohms but a raw
register value that can not directly be matched to a resistance value.

The datasheet says here "Output impedance approximate range from 35-
70ohms in 32 steps.
Lowest being 11111 and highest being 00000. Range and Step size
will vary with process."

So using 'mac-termination-ohms' seemed not the right fit here as the
documentation talks about selecting a resistance value which is not
possible in this case.

Teresa

> 
> Rob

-- 
PHYTEC Messtechnik GmbH | Barcelona-Allee 1 | 55129 Mainz, Germany

Geschäftsführer: Dipl.-Ing. Michael Mitezki, Dipl.-Ing. Bodo Huber,
Dipl.-Ing. (FH) Markus Lickes | Handelsregister Mainz HRB 4656 |
Finanzamt Mainz | St.Nr. 26/665/00608, DE 149059855

Re: [PATCH 1/2] dt-bindings: dp83867: add binding for ti,output-impedance property

[Andrew Lunn]

On Wed, Feb 11, 2026 at 12:50:13PM +0000, Teresa Remmet wrote:
> Hello Rob,
> 
> Am Dienstag, dem 10.02.2026 um 08:48 -0600 schrieb Rob Herring:
> > On Thu, Jan 29, 2026 at 03:13:33PM +0100, Teresa Remmet wrote:
> > > Add an optional device tree property, "ti,output-impedance", which
> > > specifies the output impedance using a raw register field value
> > > from
> > > 0x0 to 0x1f.
> > > 
> > > 0x0 corresponds to the highest impedance (approximately 70 ohms),
> > > while 0x1f represents the lowest (approximately 35 ohms).
> > > 
> > > This property allows the impedance to be configured through the
> > > device-tree to any required value rather than being limited to
> > > fixed
> > > minimum or maximum settings.
> > > 
> > > Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
> > > ---
> > >  Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19
> > > ++++++++++++++-----
> > >  1 file changed, 14 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > index 4bc1f98fd9fe..a8d8bfb68bee 100644
> > > --- a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > +++ b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > @@ -52,11 +52,20 @@ properties:
> > >      description: |
> > >        MAC Interface Impedance control to set the programmable
> > > output impedance
> > >        to a maximum value (70 ohms).
> > > -      Note: Specifying an io_impedance_ctrl nvmem cell or one of
> > > the
> > > -        ti,min-output-impedance, ti,max-output-impedance
> > > properties
> > > -        are mutually exclusive. If more than one is present, an
> > > nvmem
> > > -        cell takes precedence over ti,max-output-impedance, which
> > > in
> > > -        turn takes precedence over ti,min-output-impedance.
> > > +      Note: Specifying an io_impedance_ctrl nvmem cell, ti,output-
> > > impedance
> > > +        or one of the boolean ti,min-output-impedance and ti,max-
> > > output-impedance
> > > +        properties is mutually exclusive.
> > > +        If more than one is present the priority order is nvmem
> > > cell,
> > > +        ti,output-impedance, ti,max-output-impedance and last
> > > +        ti,min-output-impedance.
> > > +
> > > +  ti,output-impedance:
> > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > +    description:
> > > +      MAC Interface Impedance control to set the raw register
> > > value from 0x0
> > > +      (approx. 70 ohms) to 0x1f (approx. 35 ohms).
> > > +    minimum: 0
> > > +    maximum: 31
> > 
> > How does this compare to 'mac-termination-ohms'? Seems like this
> > should 
> > be a common property whether it's the same or for a different
> > location.
> 
> the difference is that the termination is not passed in ohms but a raw
> register value that can not directly be matched to a resistance value.
> 
> The datasheet says here "Output impedance approximate range from 35-
> 70ohms in 32 steps.
> Lowest being 11111 and highest being 00000. Range and Step size
> will vary with process."
> 
> So using 'mac-termination-ohms' seemed not the right fit here as the
> documentation talks about selecting a resistance value which is not
> possible in this case.

You need a really good argument to use raw register values in DT.

Given real life use case, how important is the "approximate"? Do you
need to tune the value for every single device? Or is one value likely
to work for all instances of a board?

So long as you state the values are approximate, i don't see why you
cannot use ohms, with a range of 34 - 70, and let the driver convert
to register value.

   Andrew

Re: [PATCH 1/2] dt-bindings: dp83867: add binding for ti,output-impedance property

[Teresa Remmet]

Hello Andrew,

Am Mittwoch, dem 11.02.2026 um 15:02 +0100 schrieb Andrew Lunn:
> On Wed, Feb 11, 2026 at 12:50:13PM +0000, Teresa Remmet wrote:
> > Hello Rob,
> > 
> > Am Dienstag, dem 10.02.2026 um 08:48 -0600 schrieb Rob Herring:
> > > On Thu, Jan 29, 2026 at 03:13:33PM +0100, Teresa Remmet wrote:
> > > > Add an optional device tree property, "ti,output-impedance",
> > > > which
> > > > specifies the output impedance using a raw register field value
> > > > from
> > > > 0x0 to 0x1f.
> > > > 
> > > > 0x0 corresponds to the highest impedance (approximately 70
> > > > ohms),
> > > > while 0x1f represents the lowest (approximately 35 ohms).
> > > > 
> > > > This property allows the impedance to be configured through the
> > > > device-tree to any required value rather than being limited to
> > > > fixed
> > > > minimum or maximum settings.
> > > > 
> > > > Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
> > > > ---
> > > >  Documentation/devicetree/bindings/net/ti,dp83867.yaml | 19
> > > > ++++++++++++++-----
> > > >  1 file changed, 14 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git
> > > > a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > > b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > > index 4bc1f98fd9fe..a8d8bfb68bee 100644
> > > > --- a/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > > +++ b/Documentation/devicetree/bindings/net/ti,dp83867.yaml
> > > > @@ -52,11 +52,20 @@ properties:
> > > >      description: |
> > > >        MAC Interface Impedance control to set the programmable
> > > > output impedance
> > > >        to a maximum value (70 ohms).
> > > > -      Note: Specifying an io_impedance_ctrl nvmem cell or one
> > > > of
> > > > the
> > > > -        ti,min-output-impedance, ti,max-output-impedance
> > > > properties
> > > > -        are mutually exclusive. If more than one is present,
> > > > an
> > > > nvmem
> > > > -        cell takes precedence over ti,max-output-impedance,
> > > > which
> > > > in
> > > > -        turn takes precedence over ti,min-output-impedance.
> > > > +      Note: Specifying an io_impedance_ctrl nvmem cell,
> > > > ti,output-
> > > > impedance
> > > > +        or one of the boolean ti,min-output-impedance and
> > > > ti,max-
> > > > output-impedance
> > > > +        properties is mutually exclusive.
> > > > +        If more than one is present the priority order is
> > > > nvmem
> > > > cell,
> > > > +        ti,output-impedance, ti,max-output-impedance and last
> > > > +        ti,min-output-impedance.
> > > > +
> > > > +  ti,output-impedance:
> > > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > > +    description:
> > > > +      MAC Interface Impedance control to set the raw register
> > > > value from 0x0
> > > > +      (approx. 70 ohms) to 0x1f (approx. 35 ohms).
> > > > +    minimum: 0
> > > > +    maximum: 31
> > > 
> > > How does this compare to 'mac-termination-ohms'? Seems like this
> > > should 
> > > be a common property whether it's the same or for a different
> > > location.
> > 
> > the difference is that the termination is not passed in ohms but a
> > raw
> > register value that can not directly be matched to a resistance
> > value.
> > 
> > The datasheet says here "Output impedance approximate range from
> > 35-
> > 70ohms in 32 steps.
> > Lowest being 11111 and highest being 00000. Range and Step size
> > will vary with process."
> > 
> > So using 'mac-termination-ohms' seemed not the right fit here as
> > the
> > documentation talks about selecting a resistance value which is not
> > possible in this case.
> 
> You need a really good argument to use raw register values in DT.
> 
> Given real life use case, how important is the "approximate"? Do you
> need to tune the value for every single device? Or is one value
> likely
> to work for all instances of a board?

There is already an nvmem binding which makes it possible to tune the
raw value per device, so I would say is clearly useful in some setups.
In our case I discussed this with our HW engineer. On the current board
design a single value is expected to give sufficiently good signal
integrity for all instances of that board. 

We use this PHY also in other designs where selecting either the
minimum or maximum output impedance value has been good enough and we
could rely on the existing min/max bindings. The difference now is that
for this new design we need a value in between those extremes, so we
can no longer use just the min/max values.
> 
> So long as you state the values are approximate, i don't see why you
> cannot use ohms, with a range of 34 - 70, and let the driver convert
> to register value.

According to the data sheet, the default impedance control register is
trimmed to 50 ohms during production, which means different devices
power up with different reset values. I checked the impedance reset
values on around 20 PHYs and observed that the default raw register
values ranged from 0xc to 0x10. 
A TI forum post mentions a tolerance of around +/-8% [1]. 
Given this spread, mapping 31 register steps directly to 35-70 ohms
would imply a level of accuracy that is not actually present.

A very similar concerns and arguments came up during the initial
discussion of the output impedance bindings [2]. The solution was to
create just the min/max settings.

Teresa

[1]https://e2e.ti.com/support/interface-group/interface/f/interface-forum/723512/dp83867ergz-r-evm-dp83867-tolerance-drift-of-output-impedance-control-bit-4-0-of-io_mux_cfg-0x0170


[2]
https://lore.kernel.org/all/959bac39-ccd6-8338-a9f4-7f56f63b0478@ti.com/

>    Andrew

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