linux-kernel - Re: [PATCH v2, net-next, 2/2] net: stmmac: PCI driver for BCM8958X SoC

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Date: Mon, 13 May 2024 10:32:19 -0700
From: Jitendra Vegiraju <jitendra.vegiraju@...adcom.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: "Russell King (Oracle)" <linux@...linux.org.uk>, netdev@...r.kernel.org, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	bcm-kernel-feedback-list@...adcom.com, alexandre.torgue@...s.st.com, 
	joabreu@...opsys.com, mcoquelin.stm32@...il.com, richardcochran@...il.com, 
	linux-kernel@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2, net-next, 2/2] net: stmmac: PCI driver for BCM8958X SoC

On Sat, May 11, 2024 at 10:50 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Sat, May 11, 2024 at 06:12:38PM +0100, Russell King (Oracle) wrote:
> > On Sat, May 11, 2024 at 06:16:52PM +0200, Andrew Lunn wrote:
> > > > + /* This device interface is directly attached to the switch chip on
> > > > +  *  the SoC. Since no MDIO is present, register fixed_phy.
> > > > +  */
> > > > + brcm_priv->phy_dev =
> > > > +          fixed_phy_register(PHY_POLL,
> > > > +                             &dwxgmac_brcm_fixed_phy_status, NULL);
> > > > + if (IS_ERR(brcm_priv->phy_dev)) {
> > > > +         dev_err(&pdev->dev, "%s\tNo PHY/fixed_PHY found\n", __func__);
> > > > +         return -ENODEV;
> > > > + }
> > > > + phy_attached_info(brcm_priv->phy_dev);
> > >
> > > What switch is it? Will there be patches to extend SF2?
> >
> > ... and why is this legacy fixed_phy even necessary when stmmac uses
> > phylink which supports fixed links, including with custom fixed status?
>
> And now you mentions legacy Fixed link:
>
> +MODULE_DESCRIPTION("Broadcom 10G Automotive Ethernet PCIe driver");
>
> This claims it is a 10G device. You cannot represent 10G using legacy
> fixed link.
>
> Does this MAC directly connect to the switch within the SoC? There is
> no external MII interface? Realtek have been posting a MAC driver for
> something similar were the MAC is directly connected to the switch
> within the SoC. The MAC is fixed at 5G, there is no phylink/phylib
> support, set_link_ksetting return -EOPNOTSUPP and get_link_ksettings
> returns hard coded 5G.
>
> We need a better understanding of the architecture here, before we can
> advise the correct way to do this.
>
Yes, the MAC directly connects to switch within the SoC with no external MII.
The SoC is BCM89586M/BCM89587 automotive ethernet switch.
The SOC presents PCIE interfaces on BCM89586M/BCM89587 automotive
ethernet switch.
The switch supports many ethernet interfaces out of which one or two
interfaces are presented as PCIE endpoints to the host connected on
the PCIE bus.
The MAC connects to switch using XGMII interface internal to the SOC.
The high level diagram is shown below:

+==================================================+
   +--------+                     |                     BCM8958X
switch SoC               +----------------+         |
   | Host   |                      |  +----------------+
    +-------+                 |                     |         | ===
more ethernet IFs
   | CPU   | ===PCIE===| PCIE endpoint |==DMA==| MAC |==XGMII==|
switch fabric |         | === more ethernet IFs
   |Linux   |                      | +----------------+
   +-------+                 |                      |         |
   +-------+                       |
                                      +-----------------+        |

+==================================================+
Since the legacy fixed link cannot support 10G, we are initializing to
fixed speed 1G.
>       Andrew