Re: [PATCH v1 1/2] driver core: fw_devlink: Add support for FWNODE_FLAG_BROKEN_PARENT

From: Saravana Kannan <saravanak@google.com>
To: Vladimir Oltean <olteanv@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Vivien Didelot <vivien.didelot@gmail.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Len Brown <lenb@kernel.org>,
	Alvin Sipraga <ALSI@bang-olufsen.dk>,
	kernel-team@android.com, linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org, linux-acpi@vger.kernel.org
Subject: Re: [PATCH v1 1/2] driver core: fw_devlink: Add support for FWNODE_FLAG_BROKEN_PARENT
Date: Tue, 31 Aug 2021 19:00:58 -0700	[thread overview]
Message-ID: <CAGETcx-ktuU1RqXwj_qV8tCOLAg3DXU-wCAm6+NukyxRencSjw@mail.gmail.com> (raw)
In-Reply-To: <20210831231804.zozyenear45ljemd@skbuf>

On Tue, Aug 31, 2021 at 4:18 PM Vladimir Oltean <olteanv@gmail.com> wrote:
>
> On Wed, Sep 01, 2021 at 01:02:09AM +0200, Andrew Lunn wrote:
> > Rev B is interesting because switch0 and switch1 got genphy, while
> > switch2 got the correct Marvell PHY driver. switch2 PHYs don't have
> > interrupt properties, so don't loop back to their parent device.
>
> This is interesting and not what I really expected to happen. It goes to
> show that we really need more time to understand all the subtleties of
> device dependencies before jumping on patching stuff.
>
> In case the DSA tree contains more than one switch, different things
> will happen in dsa_register_switch().
> The tree itself is only initialized when the last switch calls
> dsa_register_switch(). All the other switches just mark themselves as
> present and exit probing early. See this piece of code in dsa_tree_setup:
>
>         complete = dsa_tree_setup_routing_table(dst);
>         if (!complete)
>                 return 0;
>
> So it should be a general property of cross-chip DSA trees that all
> switches except the last one will have the specific PHY driver probed
> properly, and not the genphy.
>
> Because all (N - 1) switches of a tree exit early in dsa_register_switch,
> they have successfully probed by the time the last switch brings up the
> tree, and brings up the PHYs on behalf of every other switch.
>
> The last switch can connect to the PHY on behalf of the other switches
> past their probe ending, and those PHYs should not defer probing because
> their supplier is now probed. It is only that the last switch cannot
> connect to the PHYs of its own ports.

I'm not saying this with any intention of making things easier for me
(I'm not even sure it does). But your description about how multiple
switches are handled by DSA has me even more convinced than before
that DSA needs to use a component device model. This is like the
textbook example for component devices.

-Saravana