linux-kernel - Re: [PATCH v2 01/19] mfd: syscon: Add reference counting and device managed support

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Date: Tue, 18 Jun 2024 16:53:30 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Herve Codina" <herve.codina@...tlin.com>,
 "Simon Horman" <horms@...nel.org>,
 "Sai Krishna Gajula" <saikrishnag@...vell.com>,
 "Thomas Gleixner" <tglx@...utronix.de>, "Rob Herring" <robh@...nel.org>,
 "Krzysztof Kozlowski" <krzk+dt@...nel.org>,
 "Conor Dooley" <conor+dt@...nel.org>,
 "David S . Miller" <davem@...emloft.net>,
 "Eric Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>,
 "Paolo Abeni" <pabeni@...hat.com>, "Lee Jones" <lee@...nel.org>,
 "Horatiu Vultur" <horatiu.vultur@...rochip.com>,
 UNGLinuxDriver@...rochip.com, "Andrew Lunn" <andrew@...n.ch>,
 "Heiner Kallweit" <hkallweit1@...il.com>,
 "Russell King" <linux@...linux.org.uk>,
 "Saravana Kannan" <saravanak@...gle.com>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>,
 "Philipp Zabel" <p.zabel@...gutronix.de>,
 "Lars Povlsen" <lars.povlsen@...rochip.com>,
 "Steen Hegelund" <Steen.Hegelund@...rochip.com>,
 "Daniel Machon" <daniel.machon@...rochip.com>,
 "Alexandre Belloni" <alexandre.belloni@...tlin.com>
Cc: linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 Netdev <netdev@...r.kernel.org>, linux-pci@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org,
 "Allan Nielsen" <allan.nielsen@...rochip.com>,
 "Luca Ceresoli" <luca.ceresoli@...tlin.com>,
 "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
 "Clement Leger" <clement.leger@...tlin.com>
Subject: Re: [PATCH v2 01/19] mfd: syscon: Add reference counting and device managed
 support

On Mon, May 27, 2024, at 18:14, Herve Codina wrote:
> From: Clément Léger <clement.leger@...tlin.com>
>
> Syscon releasing is not supported.
> Without release function, unbinding a driver that uses syscon whether
> explicitly or due to a module removal left the used syscon in a in-use
> state.
>
> For instance a syscon_node_to_regmap() call from a consumer retrieve a
> syscon regmap instance. Internally, syscon_node_to_regmap() can create
> syscon instance and add it to the existing syscon list. No API is
> available to release this syscon instance, remove it from the list and
> free it when it is not used anymore.
>
> Introduce reference counting in syscon in order to keep track of syscon
> usage using syscon_{get,put}() and add a device managed version of
> syscon_regmap_lookup_by_phandle(), to automatically release the syscon
> instance on the consumer removal.
>
> Signed-off-by: Clément Léger <clement.leger@...tlin.com>
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>

This all looks correct from an implementation perspective,
but it does add a lot of complexity if now every syscon user
feels compelled to actually free up their resources again,
while nothing else should actually depend on this.

The only reference I found in your series here is the
reset controller, and it only does a single update to
the regmap in the probe function.

Would it be possible to just make the syscon support in
the reset driver optional and instead poke the register
in the mfd driver itself when this is used as a pci device?
Or do you expect to see the syscon get used in other
places in the future for the PCI case?

      Arnd