netdev - Re: [PATCH net-next 2/6] software node: allow named software node to be created

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Date:   Mon, 18 Jul 2022 21:43:41 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Vladimir Oltean <olteanv@...il.com>, Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Alvin __ipraga <alsi@...g-olufsen.dk>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Daniel Scally <djrscally@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        DENG Qingfang <dqfext@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        George McCollister <george.mccollister@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Sean Wang <sean.wang@...iatek.com>,
        UNGLinuxDriver@...rochip.com,
        Vivien Didelot <vivien.didelot@...il.com>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH net-next 2/6] software node: allow named software node to
 be created

On Mon, Jul 18, 2022 at 02:27:02PM +0100, Russell King (Oracle) wrote:
> On Mon, Jul 18, 2022 at 03:29:52PM +0300, Andy Shevchenko wrote:
> > On Fri, Jul 15, 2022 at 11:48:41PM +0300, Vladimir Oltean wrote:
> > > So won't kobject_init_and_add() fail on namespace collision? Is it the
> > > problem that it's going to fail, or that it's not trivial to statically
> > > determine whether it'll fail?
> > > 
> > > Sorry, but I don't see something actionable about this.
> > 
> > I'm talking about validation before a runtime. But if you think that is fine,
> > let's fail it at runtime, okay, and consume more backtraces in the future.
> 
> Is there any sane way to do validation of this namespace before
> runtime?

For statically compiled, I think we can do it (to some extent).
Currently only three drivers, if I'm not mistaken, define software nodes with
names. It's easy to check that their node names are unique.

When you allow such an API then we might have tracebacks (from sysfs) bout name
collisions. Not that is something new to kernel (we have seen many of a kind),
but I prefer, if possible, to validate this before sysfs issues a traceback.

> The problem in this instance is we need a node named "fixed-link" that
> is attached to the parent node as that is defined in the binding doc,
> and we're creating swnodes to provide software generated nodes for
> this binding.

And how you guarantee that it will be only a single one with unique pathname?

For example, you have two DSA cards (or whatever it's called) in the SMP system,
it mean that there is non-zero probability of coexisting swnodes for them.

> There could be several such nodes scattered around, but in this
> instance they are very short-lived before they are destroyed, they
> don't even need to be published to userspace (and its probably a waste
> of CPU cycles for them to be published there.)
> 
> So, for this specific case, is this the best approach, or is there
> some better way to achieve what we need here?

Honestly, I don't know.

The "workaround" (but it looks to me rather a hack) is to create unique swnode
and make fixed-link as a child of it.

Or entire concept of the root swnodes (when name is provided) should be
reconsidered, so somehow we will have a uniqueness so that the entire
path(s) behind it will be caller-dependent. But this I also don't like.

Maybe Heikki, Sakari, Rafael can share their thoughts...

Just for my learning, why PHY uses "fixed-link" instead of relying on a
(firmware) graph? It might be the actual solution to your problem.

How graphs are used with swnodes, you may look into IPU3 (Intel Camera)
glue driver to support devices before MIPI standardisation of the
respective properties.

-- 
With Best Regards,
Andy Shevchenko