Date:   Thu, 2 Nov 2017 16:05:59 +0000
From:   "Chocron, Jonathan" <jonnyc@...zon.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Antoine Tenart <antoine.tenart@...e-electrons.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "thomas.petazzoni@...e-electrons.com" 
        <thomas.petazzoni@...e-electrons.com>,
        "arnd@...db.de" <arnd@...db.de>, "BSHARA, Said" <saeedb@...zon.com>
Subject: RE: [PATCH net-next 4/8] net: ethernet: add the Alpine Ethernet
 driver

 -----Original Message-----
> From: Andrew Lunn [mailto:andrew@...n.ch]
> Sent: Monday, August 28, 2017 9:10 PM
> To: Chocron, Jonathan <jonnyc@...zon.com>
> Cc: Antoine Tenart <antoine.tenart@...e-electrons.com>;
> netdev@...r.kernel.org; davem@...emloft.net; linux-arm-
> kernel@...ts.infradead.org; thomas.petazzoni@...e-electrons.com;
> arnd@...db.de
> Subject: Re: [PATCH net-next 4/8] net: ethernet: add the Alpine Ethernet
> driver
> 
> On Sun, Aug 27, 2017 at 01:47:19PM +0000, Chocron, Jonathan wrote:
> > This is a fixed version of my previous response (using proper indentation
> and leaving only the specific questions responded to).
> 
> Wow, this is old.  3 Feb 2017. I had to go dig into the archive to refresh my
> memory.
> 
> > > > +/* MDIO */
> > > > +#define AL_ETH_MDIO_C45_DEV_MASK     0x1f0000
> > > > +#define AL_ETH_MDIO_C45_DEV_SHIFT    16
> > > > +#define AL_ETH_MDIO_C45_REG_MASK     0xffff
> > > > +
> > > > +static int al_mdio_read(struct mii_bus *bp, int mii_id, int reg)
> > > > +{
> > > > +     struct al_eth_adapter *adapter = bp->priv;
> > > > +     u16 value = 0;
> > > > +     int rc;
> > > > +     int timeout = MDIO_TIMEOUT_MSEC;
> > > > +
> > > > +     while (timeout > 0) {
> > > > +             if (reg & MII_ADDR_C45) {
> > > > +                     netdev_dbg(adapter->netdev, "[c45]: dev %x reg %x val
> %x\n",
> > > > +                                ((reg & AL_ETH_MDIO_C45_DEV_MASK) >>
> AL_ETH_MDIO_C45_DEV_SHIFT),
> > > > +                                (reg & AL_ETH_MDIO_C45_REG_MASK), value);
> > > > +                     rc = al_eth_mdio_read(&adapter->hw_adapter, adapter-
> >phy_addr,
> > > > +                             ((reg & AL_ETH_MDIO_C45_DEV_MASK) >>
> AL_ETH_MDIO_C45_DEV_SHIFT),
> > > > +                             (reg & AL_ETH_MDIO_C45_REG_MASK), &value);
> > > > +             } else {
> > > > +                     rc = al_eth_mdio_read(&adapter->hw_adapter, adapter-
> >phy_addr,
> > > > +                                           MDIO_DEVAD_NONE, reg, &value);
> > > > +             }
> > > > +
> > > > +             if (rc == 0)
> > > > +                     return value;
> > > > +
> > > > +             netdev_dbg(adapter->netdev,
> > > > +                        "mdio read failed. try again in 10
> > > > + msec\n");
> > > > +
> > > > +             timeout -= 10;
> > > > +             msleep(10);
> > > > +     }
> > >
> > > This is rather unusual, retrying MDIO operations. Are you working
> > > around a hardware bug? I suspect this also opens up race conditions,
> > > in particular with PHY interrupts, which can be clear on read.
> >
> > The MDIO bus is shared between the ethernet units. There is a HW lock
> > used to arbitrate between different interfaces trying to access the
> > bus, therefore there is a retry loop. The reg isn't accessed before
> > obtaining the lock, so there shouldn't be any clear on read issues.
> >
> > > > +/* al_eth_mdiobus_setup - initialize mdiobus and register to
> > > > +kernel */ static int al_eth_mdiobus_setup(struct al_eth_adapter
> > > > +*adapter) {
> > > > +     struct phy_device *phydev;
> > > > +     int i;
> > > > +     int ret = 0;
> > > > +
> > > > +     adapter->mdio_bus = mdiobus_alloc();
> > > > +     if (!adapter->mdio_bus)
> > > > +             return -ENOMEM;
> > > > +
> > > > +     adapter->mdio_bus->name     = "al mdio bus";
> > > > +     snprintf(adapter->mdio_bus->id, MII_BUS_ID_SIZE, "%x",
> > > > +              (adapter->pdev->bus->number << 8) | adapter->pdev-
> >devfn);
> > > > +     adapter->mdio_bus->priv     = adapter;
> > > > +     adapter->mdio_bus->parent   = &adapter->pdev->dev;
> > > > +     adapter->mdio_bus->read     = &al_mdio_read;
> > > > +     adapter->mdio_bus->write    = &al_mdio_write;
> > > > +     adapter->mdio_bus->phy_mask = ~BIT(adapter->phy_addr);
> > >
> > > Why do this?
> >
> > Since the MDIO bus is shared, we want each interface to probe only for the
> PHY associated with it.
> 
> So i think this is the core of the problem. You have one physical MDIO bus,
> yet you register it twice with the MDIO framework.
> 
> How about you only register it once? A lot of the complexity then goes away.
> The mutex in the mdio core per bus means you don't need your hardware
> locking. All that code goes away. All the retry code goes away. Life is simple.
> 
> 	Andrew

We indeed have one physical MDIO bus, but have multiple masters on it,
each "behind" a different internal PCIe device. Since the accesses to the bus
are done "indirectly" through each master, we can't register the bus only once.
Think of the scenario that we register it in the driver context of PCIe device A,
and then the driver is unbound from just this device. Device B won't be able
to access the bus since it was registered with callbacks that use a PCIe BAR of
device A, which is no longer valid.

Is it possible to register the mdio_bus struct as a global instance at driver load,
and someway pass the offset to the specific device's MDIO master, as part of
each read/write transaction towards the MDIO bus?
Or perhaps you have another suggestion which takes into account the issues I've described?

Jonathan

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