linux-kernel - Re: [PATCH v8 12/12] mux: support simplified bindings for single-user gpio mux

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Date:   Mon, 23 Jan 2017 11:24:18 +0100
From:   Peter Rosin <peda@...ntia.se>
To:     Jonathan Cameron <jic23@...nel.org>, linux-kernel@...r.kernel.org
Cc:     Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-iio@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v8 12/12] mux: support simplified bindings for single-user
 gpio mux

On 2017-01-22 14:30, Jonathan Cameron wrote:
> On 18/01/17 15:57, Peter Rosin wrote:
>> Allow bindings for a GPIO controlled mux to be specified in the
>> mux consumer node.
>>
>> Signed-off-by: Peter Rosin <peda@...ntia.se>
> Code is good as far as I am concerned. Only question is whether this

Hmmm, now that I think some more about it, the code supporting the
simplified binding (patch 12/12) is a bit fishy in one respect.

A driver that calls mux_control_get and gets a mux_control that happens
to be backed by an implicit mux chip (i.e. using the simplified binding)
will not be able to reverse the resource allocation with less than a
complete destruction of itself. Now, this is likely not a problem in
most cases, but I bet it will creep up at the most inopportune time. And
your remark that I'm the one that has to maintain this makes me dislike
this concept...

I.e. mux_control_put *should* reverse mux_control_get, but this simply
does not happen for the implicit mux chips, as implicit mux chips are
not put away until the owning device is put away.

Every time I have tried to come up with a way to implement the simplified
bindings I seem to hit one of these subtleties.

> is worth the hassle given the normal bindings don't give that high
> a burden in complexity!

I am missing an ack from Rob though.

> I don't really care either way:)

But Rob seems to care, this series just has to find a way to get out of
his too-much-churn-will-look-at-it-later list. I sadly don't know how to
pull that trick...

Cheers,
peda