linux-kernel - Re: [PATCH net-next 2/3] net: phy: add Broadcom BCM54140 support

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Date:   Fri, 17 Apr 2020 23:00:38 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Michael Walle <michael@...le.cc>
Cc:     Andrew Lunn <andrew@...n.ch>, linux-hwmon@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next 2/3] net: phy: add Broadcom BCM54140 support

Hi Michael,

On Fri, 17 Apr 2020 at 22:52, Michael Walle <michael@...le.cc> wrote:
>
> Hi Andrew,
>
> Am 2020-04-17 21:39, schrieb Andrew Lunn:
> > On Fri, Apr 17, 2020 at 09:28:57PM +0200, Michael Walle wrote:
> >
> >> +static int bcm54140_get_base_addr_and_port(struct phy_device *phydev)
> >> +{
> >> +    struct bcm54140_phy_priv *priv = phydev->priv;
> >> +    struct mii_bus *bus = phydev->mdio.bus;
> >> +    int addr, min_addr, max_addr;
> >> +    int step = 1;
> >> +    u32 phy_id;
> >> +    int tmp;
> >> +
> >> +    min_addr = phydev->mdio.addr;
> >> +    max_addr = phydev->mdio.addr;
> >> +    addr = phydev->mdio.addr;
> >> +
> >> +    /* We scan forward and backwards and look for PHYs which have the
> >> +     * same phy_id like we do. Step 1 will scan forward, step 2
> >> +     * backwards. Once we are finished, we have a min_addr and
> >> +     * max_addr which resembles the range of PHY addresses of the same
> >> +     * type of PHY. There is one caveat; there may be many PHYs of
> >> +     * the same type, but we know that each PHY takes exactly 4
> >> +     * consecutive addresses. Therefore we can deduce our offset
> >> +     * to the base address of this quad PHY.
> >> +     */
> >
> > Hi Michael
> >
> > How much flexibility is there in setting the base address using
> > strapping etc? Is it limited to a multiple of 4?
>
> You can just set the base address to any address. Then the following
> addresses are used:
>    base, base + 1, base + 2, base + 3, (base + 4)*
>
> It is not specified what happens if you set the base so that it would
> overflow. I guess that is a invalid strapping.
>
> * (base + 4) is some kind of special PHY address which maps some kind
> of moving window to a QSGMII address space. It is enabled by default,
> could be disabled in software, but it doesn't share the same PHY id
> for which this scans.
>
> So yes, if you look at the addresses and the phy ids, there are
> always 4 of this.
>
> -michael

What does the reading of the global register give you, when accessed
through the master PHY ID vs any other PHY ID? Could you use that as
an indication of this being the correct PHY ID, and scan only to the
left?

Regards,
-Vladimir