linux-kernel - Re: [PATCH v5 3/8] mfd: syscon: Add reference counting and device managed support

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Message-ID: <20240903180116.717a499b@bootlin.com>
Date: Tue, 3 Sep 2024 18:01:16 +0200
From: Herve Codina <herve.codina@...tlin.com>
To: Lee Jones <lee@...nel.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Andy Shevchenko
 <andy.shevchenko@...il.com>, Simon Horman <horms@...nel.org>, Arnd Bergmann
 <arnd@...db.de>, Derek Kiernan <derek.kiernan@....com>, Dragan Cvetic
 <dragan.cvetic@....com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 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>, UNGLinuxDriver@...rochip.com, Rob Herring
 <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Horatiu Vultur
 <horatiu.vultur@...rochip.com>, Andrew Lunn <andrew@...n.ch>,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 devicetree@...r.kernel.org, Allan Nielsen <allan.nielsen@...rochip.com>,
 Luca Ceresoli <luca.ceresoli@...tlin.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, Clément Léger
 <clement.leger@...tlin.com>
Subject: Re: [PATCH v5 3/8] mfd: syscon: Add reference counting and device
 managed support

Hi Lee,

On Tue, 3 Sep 2024 16:38:39 +0100
Lee Jones <lee@...nel.org> wrote:

> On Thu, 08 Aug 2024, 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 retrieves 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>
> > ---
> >  drivers/mfd/syscon.c       | 138 ++++++++++++++++++++++++++++++++++---
> >  include/linux/mfd/syscon.h |  16 +++++
> >  2 files changed, 144 insertions(+), 10 deletions(-)  
> 
> This doesn't look very popular.
> 
> What are the potential ramifications for existing users?
> 

Existing user don't use devm_syscon_regmap_lookup_by_phandle() nor
syscon_put_regmap().

So refcount is incremented but never decremented. syscon is never
released. Exactly the same as current implementation.
Nothing change for existing users.

Best regards,
Hervé