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References: <cover.1267225701.git.gardner.ben@gmail.com>
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Date: Sat, 27 Feb 2010 12:36:24 -0600
Message-ID: <808c8e9d1002271036s387b4638u9c754da1e88b6e1b@mail.gmail.com>
Subject: Re: [PATCH 1/3] gpiolib: add gpio_set_direction()
From: Ben Gardner <gardner.ben@gmail.com>
To: David Brownell <david-b@pacbell.net>
Cc: linux-kernel@vger.kernel.org, Andres Salomon <dilinger@collabora.co.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Jani Nikula <ext-jani.1.nikula@nokia.com>
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> NAK.
>
> When we discussed the programming interface originally, having the
> direction be part of the function name was explicitly requested as
> a way to reduce programming errors.
>
> I recall that ?a gpio_set_direction() was in the original proposal...
> removing that was one of the handful of fixes that went into the final
> set of calls in this programming interface.
>
> And in fact ?... it *was* very easy to make errors with a few GPIO
> interfaces which worked like that. ?With even fewer parameters to
> create confusion than in your proposal.
>
> Let's have no retrograde motion...
>
>
>> Add 'none' and 'inout' directions to the sysfs interface.
>
> Both of those seem nonsensical:
>
> ?"none" ... since it's not even a GPIO, why would it show
> ? ? ? ?up through the GPIO subsystem???
>
> ?"inout" ... is not a direction. ?But if you want to do this,
> ? ? ? ?you really ought to bite the bullet and come up with a
> ? ? ? ?way the implementation can pass up its capabilities.
>
> ? ? ? ?Did you read the definition of gpio_get_value()? ?It says
> ? ? ? ?"When reading the value of an output pin, the value returned
> ? ? ? ?should be what's seen on the pin ... that won't always match
> ? ? ? ?the specified output value, because of issues including
> ? ? ? ?open-drain signaling and output latencies." ?Also: ?"note that
> ? ? ? ?not all platforms can read the value of output pins; those that
> ? ? ? ?can't should always return zero."
>
> ? ? ? ?Another way to say that is that "output" means 'inout" except
> ? ? ? ?when the platform can't do that. ?So you'd need to address the
> ? ? ? ?case of hardware that's truly output-only ... instead of
> ? ? ? ?ignoring that, as done here. ?(That is: ?you'd need to have a
> ? ? ? ?way to reject "inout" mode ... or for that matter, "output-only".)
>
>
> ? ? ? ?Doing that well might be a Good Thing ... if for example it
> ? ? ? ?lets the initial mode of a GPIO show up properly ... there's
> ? ? ? ?currently an assumption in sysfs that they start up as "input",
> ? ? ? ?which of course makes sense for any power-sensitive system
> ? ? ? ?(you don't enable output drivers unless that power consumption
> ? ? ? ?is for some reason required).
>
> ? ? ? ?I've never, for example, seen GPIO hardware that would support
> ? ? ? ?the equivalent of that "open in <bitmask> mode". ? It's either
> ? ? ? ?unidirectional (rarely), or (normally) the only real option is
> ? ? ? ?whether the output drivers are disabled. ?So you always get the
> ? ? ? ?"inout" semantics described above, or input-only ones. ?Asking
> ? ? ? ?for output-only would need to fail. ?(Or in the less typical
> ? ? ? ?cases, it's asking for "inout" that would need to fail.)

OK. Lets dump this patch series and I'll send off a patch that fixes
the cs5535-gpio driver to behave as indicated.

Ben
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