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

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: "Vladimir Oltean" <olteanv@gmail.com>,
	"Andrew Lunn" <andrew@lunn.ch>,
	"Heiner Kallweit" <hkallweit1@gmail.com>,
	"Alexandre Belloni" <alexandre.belloni@bootlin.com>,
	"Alvin __ipraga" <alsi@bang-olufsen.dk>,
	"Claudiu Manoil" <claudiu.manoil@nxp.com>,
	"Daniel Scally" <djrscally@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	"DENG Qingfang" <dqfext@gmail.com>,
	"Eric Dumazet" <edumazet@google.com>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	"George McCollister" <george.mccollister@gmail.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Hauke Mehrtens" <hauke@hauke-m.de>,
	"Heikki Krogerus" <heikki.krogerus@linux.intel.com>,
	"Jakub Kicinski" <kuba@kernel.org>,
	"Kurt Kanzenbach" <kurt@linutronix.de>,
	"Landen Chao" <Landen.Chao@mediatek.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org,
	"Matthias Brugger" <matthias.bgg@gmail.com>,
	netdev@vger.kernel.org, "Paolo Abeni" <pabeni@redhat.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	"Sakari Ailus" <sakari.ailus@linux.intel.com>,
	"Sean Wang" <sean.wang@mediatek.com>,
	UNGLinuxDriver@microchip.com,
	"Vivien Didelot" <vivien.didelot@gmail.com>,
	"Woojung Huh" <woojung.huh@microchip.com>,
	"Marek Behún" <kabel@kernel.org>
Subject: Re: [PATCH net-next 2/6] software node: allow named software node to be created
Date: Mon, 18 Jul 2022 22:24:09 +0300	[thread overview]
Message-ID: <YtWzWdkFVMg0Hyvf@smile.fi.intel.com> (raw)
In-Reply-To: <YtWxMrz3LcVQa43I@shell.armlinux.org.uk>

On Mon, Jul 18, 2022 at 08:14:58PM +0100, Russell King (Oracle) wrote:
> On Mon, Jul 18, 2022 at 09:53:39PM +0300, Andy Shevchenko wrote:
> > On Mon, Jul 18, 2022 at 09:43:42PM +0300, Andy Shevchenko wrote:
> > > 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.
> > 
> > Forgot to say (yes, it maybe obvious) that this API will be exported,
> > anyone can use it and trap into the similar issue, because, for example,
> > of testing in environment with a single instance of the caller.
> 
> I think we're coming to the conclusion that using swnodes is not the
> correct approach for this problem, correct?

If I understand the possibilities of the usage in _this_ case, then it's
would be problematic (it does not mean it's incorrect). It might be due to
swnode design restrictions which shouldn't be made, I dunno. That' why
it's better to ask the others for their opinions.

By design swnode's name makes not much sense, because the payload there
is a property set, where _name_ is a must.

Now, telling you this, I'm questioning myself why the heck I added names
to swnodes in the intel_quark_i2c_gpio driver...

-- 
With Best Regards,
Andy Shevchenko

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