netdev - Re: Potential issue with PHYs connected to the same MDIO bus and different MACs

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Date:   Sat, 5 Nov 2022 15:34:43 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     piergiorgio.beruto@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: Potential issue with PHYs connected to the same MDIO bus and
 different MACs

Hi Piergiorgio,

On Sat, Nov 05, 2022 at 01:23:36PM +0100, piergiorgio.beruto@...il.com wrote:
> Now, to use the third PHY I added a MAC IP in the FPGA (Altera Triple Speed
> MAC) for which a Linux driver already exists (altera_tse). However, unless I
> connect the PHY to the dedicated TSE mdio bus (which requires HW
> modifications), I get an error saying that the driver could not connect to
> the PHY. I assumed this could be a conflict between the phylink interface
> (used by STMMAC) and the "plain" PHY of APIs used by the TSE driver (which
> seems a bit old).

Can you share exactly what error message you get?
Also, btw, the tse driver was also converted to phylink in net-next, see
commit fef2998203e1 ("net: altera: tse: convert to phylink"). Maybe it
would be a good idea to retry with that.

> I then decided to use a different MAC IP for which I'm writing a driver
> using the phylink interface.
> I got stuck at a point where the function "phy_attach_direct" in
> phy_device.c gives an Oops (see log below).
>
> Doing some debug, it seems that the NULL pointer comes from
> dev->dev.parent->driver.
> The "parent" pointer seems to reference the SoC BUS which the MAC IP belongs
> to.
>
> Any clue of what I might be doing wrong? I also think it is possible that
> the problem I saw with the altera_tse driver could have something in common
> with this?

Not clear what problem you were seeing with the altera_tse driver...

> The MAC would be located inside the bus as well.
>
> My DTS looks like this (just listing the relevant parts):
>
> {
>    SoC {
>         gmac1 {
>            // This is the stmmac which has all the PHYs attached.
>            phy-handle=<&phy1>
>             ...
>            mdio {
>               phy1 {
>                  ...
>               }
>
>               phy2 {
>                  ...
>               }
>
>               phy3 {
>                  .
>               }
>          } // mdio
>       } // gmac1
>
>       gmac0 {
>          phy-handle=<&phy2>
>          ...
>       }
>
>       bridge {
>           ...
>          myMAC {
>             phy-handle=<&phy3>
>             ...
>          }
>       }
>    } // Soc
> }

Device tree looks more or less okay (not sure what's the "bridge"
though, this is potentially irrelevant).

>
> One more information: I can clearly see from the log my PHYs being scanned
> and enumerated. So again, I don't think the problem relates to the PHYs.
>
> This is the Oops log.
>
> 8<--- cut here ---
> [ 36.823689] Unable to handle kernel NULL pointer dereference at virtual address 00000008
> [ 36.835783] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
> [ 36.841090] Modules linked in: onsemi_tmac(O)
> [ 36.845452] CPU: 0 PID: 240 Comm: ifconfig Tainted: G O 5.19.12-centurion3-1.0.3.0 #10
> [ 36.854646] Hardware name: Altera SOCFPGA
> [ 36.858644] PC is at phy_attach_direct+0x98/0x328
> [ 36.863357] LR is at phy_attach_direct+0x8c/0x328
> [ 36.868057] pc : [<c051f218>] lr : [<c051f20c>] psr: 600d0013
> [ 36.874304] sp : d0eedd58 ip : 00000000 fp : d0eede78
> [ 36.992376] Process ifconfig (pid: 240, stack limit = 0xa94bafcf)
> [ 36.998455] Stack: (0xd0eedd58 to 0xd0eee000)
> [ 37.182221] phy_attach_direct from phylink_fwnode_phy_connect+0xa4/0xdc
> [ 37.188932] phylink_fwnode_phy_connect from tmac_open+0x44/0x9c [onsemi_tmac]
> [ 37.196160] tmac_open [onsemi_tmac] from __dev_open+0x110/0x128
> [ 37.202180] __dev_open from __dev_change_flags+0x168/0x17c
> [ 37.207758] __dev_change_flags from dev_change_flags+0x14/0x44
> [ 37.213680] dev_change_flags from devinet_ioctl+0x2ac/0x5fc
> [ 37.219349] devinet_ioctl from inet_ioctl+0x1ec/0x214

In phy_attach_direct() we currently have this in net-next (didn't check 5.19):

int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
		      u32 flags, phy_interface_t interface)
{
	/* For Ethernet device drivers that register their own MDIO bus, we
	 * will have bus->owner match ndev_mod, so we do not want to increment
	 * our own module->refcnt here, otherwise we would not be able to
	 * unload later on.
	 */
	if (dev)
		ndev_owner = dev->dev.parent->driver->owner;
	if (ndev_owner != bus->owner && !try_module_get(bus->owner)) {
		phydev_err(phydev, "failed to get the bus module\n");
		return -EIO;
	}

I think the (out-of-tree?!) driver you're writing needs to make a call
to SET_NETDEV_DEV() in order for the dev->dev.parent to be set. Also, a
bunch of other stuff relies on this. Is your driver making that call?