linux-kernel - Re: [PATCH 27/32] gpio: Add support for the FXL6408 GPIO expander.

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Date:	Mon, 06 Jun 2016 16:50:31 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Gerd Hoffmann <kraxel@...hat.com>,
	linux-rpi-kernel@...ts.infradead.org
Cc:	Florian Fainelli <f.fainelli@...il.com>,
	Ray Jui <rjui@...adcom.com>,
	Scott Branden <sbranden@...adcom.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	"open list\:BROADCOM BCM281XX\/BCM11XXX\/BCM216XX ARM ARCHITE..." 
	<bcm-kernel-feedback-list@...adcom.com>,
	"open list\:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@...r.kernel.org>,
	"moderated list\:ARM64 PORT \(AARCH64 ARCHITECTURE\)" 
	<linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list\:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH 27/32] gpio: Add support for the FXL6408 GPIO expander.

Gerd Hoffmann <kraxel@...hat.com> writes:

> From: Eric Anholt <eric@...olt.net>
>
> This commit needs to be split up and cleaned up.  Also we should add
> interrupt support before pushing upstream.
>
> Signed-off-by: Eric Anholt <eric@...olt.net>

OK, I've confirmed with the firmware maintainers: We really can't drive
this natively from Linux.  The firmware is polling one of the fxl6408's
pins 10 times a second for detecting under-voltage, so if we also try to
drive it we'll end up smashing each others' I2C transactions.  We'll
need to do a GPIO driver using the GET/SET_GPIO_STATE firmware calls,
instead.  Unfortunately, we don't have a direction-setting API for that
at the moment, but it'll at least let us read the HDMI HPD signal.

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