netdev - Re: [v4] dsa: mv88e6xxx: make serdes SGMII/Fiber tx amplitude configurable

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Date:   Tue, 8 Feb 2022 17:12:19 +0100
From:   Marek Behún <kabel@...nel.org>
To:     Holger Brunck <holger.brunck@...achienergy.com>
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [v4] dsa: mv88e6xxx: make serdes SGMII/Fiber tx amplitude
 configurable

On Tue,  8 Feb 2022 10:44:55 +0100
Holger Brunck <holger.brunck@...achienergy.com> wrote:

> +static struct mv88e6352_serdes_p2p_to_val mv88e6352_serdes_p2p_to_val[] = {
> +	/* Mapping of configurable mikrovolt values to the register value */
> +	{ 14000, 0},
> +	{ 112000, 1},
> +	{ 210000, 2},
> +	{ 308000, 3},
> +	{ 406000, 4},
> +	{ 504000, 5},
> +	{ 602000, 6},
> +	{ 700000, 7},
> +};

...

> +	reg = (reg & MV88E6352_SERDES_OUT_AMP_MASK) | val;

This is weird: normally in mask we have those bits set that are to be changed.
So amplitude mask should be bits that specify the amplitue, and this
should be
  reg &= ~MV88E6352_SERDES_OUT_AMP_MASK;
  reg |= val & MV88E6352_SERDES_OUT_AMP_MASK;
and mask should be defined inversely.

...

> +#define MV88E6352_SERDES_OUT_AMP_MASK		0xfffc

And this is also weird. 0xfffc is all bits set except last 2, but in
the mapping above the maximum value is 7, so you use 3 bits for
amplitude...

Marek