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To:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Phil Edworthy <phil.edworthy@renesas.com>,
        Russell King <linux@armlinux.org.uk>
From:   Stephen Boyd <sboyd@kernel.org>
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Cc:     Michael Turquette <mturquette@baylibre.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Simon Horman <horms@verge.net.au>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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        Linux ARM <linux-arm-kernel@lists.infradead.org>
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Subject: RE: [PATCH] clk: Add functions to get optional clocks
Date:   Wed, 25 Jul 2018 15:37:02 -0700
Sender: linux-kernel-owner@vger.kernel.org
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Quoting Phil Edworthy (2018-07-18 06:56:26)
> Hi Russell,
> =

> On 18 July 2018 14:19, Geert Uytterhoeven wrote:
> > On Wed, Jul 18, 2018 at 3:02 PM Russell King - ARM Linux  wrote:
> > > On Wed, Jul 18, 2018 at 01:57:38PM +0100, Phil Edworthy wrote:
> > > > Behaves the same as (devm_)clk_get except where there is no clock
> > > > producer. In this case, instead of returning -ENOENT, the function
> > > > returns NULL. This makes error checking simpler and allows
> > > > clk_prepare_enable, etc to be called on the returned reference
> > > > without additional checks.
> > >
> > > How does this work with non-DT systems, where looking a clock up which
> > > isn't yet registered with clkdev returns -ENOENT ?
> > >
> > > (clkdev doesn't know when all clocks are registered with it.)
> > =

> > Good question.
> > =

> > I guess all drivers trying to handle optional clocks this way are alrea=
dy broken
> > on non-DT systems where clocks may be registered late...
> =

> So how do non-DT systems that look a clock up which isn't yet
> registered with clkdev, determine that an optional clock is there
> or not?
> =


Short answer is they don't. I'd still prefer we have this API though.

Can you rework this patch to be a little more invasive into the
clk_get() path, perhaps by reworking __of_clk_get_by_name() a little to
take an 'optional' argument, so that it only returns NULL when the clk
is looked up from DT? The fallback path in clkdev where we have a DT
based system looking up a clk through clkdev lookups doesn't seem to be
a real scenario that we should worry about here. I think sometimes
people use clkdev lookups when they're migrating to DT systems and
things aren't wired up properly in DT, but that isn't the norm.