linux-kernel - Re: fwnode_for_each_child_node() and OF backend discrepancy

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Date:   Fri, 1 Jul 2022 00:21:20 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Michael Walle <michael@...le.cc>
Cc:     Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: fwnode_for_each_child_node() and OF backend discrepancy

On Thu, Jun 30, 2022 at 11:00:37PM +0200, Michael Walle wrote:
> > > > It is not possible to have a defined for the MAX number of ports that
> > > > supported by lan966x. Which is 8. And assigned that define to
> > > > num_phys_ports instead of counting the entries in DT?
> > > 
> > > You mean also for the lan9662? I'm pretty sure that doesn't
> > > work. Have a look where num_phys_ports is used. One random
> > > example:
> > > https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/microchip/lan966x/lan966x_main.c#L874
> > > 
> > > So if your switch only has 4 ports, then I'd guess you'll
> > > access a non-existing register.
> > 
> > Underneath lan662 and lan668 is the same chip. The HW people disable
> > some ports/features on each platform but from what I know you will still
> > be able to access the registers.
> 
> I noticed that there are still 8 ports in the register description and
> assumed that it was wrong [1]. But ok, that makes sense in some way.
> OTOH that means, we cannot do the guesswork Vladimir proposed.
> 
> -michael
> 
> [1] https://microchip-ung.github.io/lan9662_reginfo/reginfo_LAN9662.html

Are you 100% positive that the default values for the flooding PGIDs are
GENMASK(8, 0) for a 4-port switch? And that the packet buffer has the
same size for a switch with half as many ports? Ok...

But in that case, what exactly is the problem if the port count of 8 is
a synthesis time constant for lan966x, and if the CPU port module is
always at index 8 in the analyzer (with a gap between indices 4 and 7)?
Just hardcode lan966x->num_phys_ports to 8 and work with that throughout.
Allocate lan966x->ports as an array of 8 pointers to struct lan966x_port
(which they are already), and the pointers themselves are populated as
being the netdev_priv of the interfaces that are actually present and
used. Literally the only thing you need to fix is that you need to
hardcode num_phys_ports to 8, problem solved. It means that lan9662 is
nothing but a lan9668 where the last 4 ports have 'status = "disabled"'
in the device tree.