linux-kernel - Re: [PATCH] mfd: syscon: Decouple syscon interface from syscon devices

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Message-ID: <20140902083400.GF17117@lee--X1>
Date:	Tue, 2 Sep 2014 09:34:00 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, kgene.kim@...sung.com,
	linux@....linux.org.uk, naushad@...sung.com,
	Pankaj Dubey <pankaj.dubey@...sung.com>,
	Tomasz Figa <t.figa@...sung.com>, linux-kernel@...r.kernel.org,
	joshi@...sung.com, vikas.sajjan@...sung.com,
	linux-samsung-soc@...r.kernel.org, broonie@...nel.org,
	thomas.ab@...sung.com, chow.kim@...sung.com
Subject: Re: [PATCH] mfd: syscon: Decouple syscon interface from syscon
 devices

On Tue, 02 Sep 2014, Arnd Bergmann wrote:

> On Tuesday 02 September 2014 09:05:16 Lee Jones wrote:
> > On Mon, 01 Sep 2014, Arnd Bergmann wrote:
> > 
> > > On Monday 01 September 2014 17:04:26 Lee Jones wrote:
> > > > On Mon, 01 Sep 2014, Arnd Bergmann wrote:
> > > > > Maybe I'm misreading the patch, but I don't see how it creates a
> > > > > migration path. What I want to end up with is infrastructure that
> > > > > lets anybody call syscon_regmap_lookup_by_pdevname or
> > > > > syscon_regmap_lookup_by_compatible (if they really need to)
> > > > > without needing the platform_driver for syscon. That should not
> > > > > require any form of compatibility layer because to the driver
> > > > > using it there is no API change.
> > > > 
> > > > Somehow I think the likelyhood is that I am misreading the patch.
> > > > 
> > > > I thought that before this patch drivers we had to register a syscon
> > > > device to bind to this driver, which was fine for the first use-cases
> > > > of syscon as it wasn't required too early during boot.  However, now
> > > > there are use-cases where systems require access to syscon registers
> > > > eariler in boot we require a means to obtain access prior to device
> > > > probing.  I thought this patch not only provides that possibilty, but
> > > > also leaves in the ability to register direct from DT.
> > > 
> > > Right, it does provide the ability to have syscon before devices
> > > are registered, I missed that part.
> > > 
> > > > > In contrast, this patch introduces a new of_syscon_{un,}register()
> > > > > interface that would get removed after the the above has
> > > > > been implemented, causing extra churn for any driver that also
> > > > > wants to provide a regmap-like interface.
> > > > 
> > > > When will we ever not have to register syscon?
> > > 
> > > The idea is that we implicitly register the syscon block when someone
> > > calls syscon_regmap_lookup_by_compatible or syscon_regmap_lookup_by_phandle
> > > and then return a reference to that new syscon. When another driver
> > > looks up the same device node, we just pass a reference to the existing
> > > syscon.
> > 
> > Doesn't sound too unreasonable.  So how about instead of exporting
> > these new of_syscon_{un,}register() calls, we make them static and
> > call them from syscon_regmap_lookup_by_{phandle,compatible}?
> 
> Yes, that would be a good start. We should think about whether we want
> to remove the existing DT probing at the same time, since it becomes
> unused, and we might want to move the code to drivers/base/regmap_*.c
> at some point

I'd support that move - and can even draft the patch if required.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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