linux-kernel - Re: [PATCH V2] i2c-designware: Add Intel Baytrail PMIC I2C bus support

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Date:	Tue, 23 Sep 2014 12:58:54 -0700
From:	"David E. Box" <david.e.box@...ux.intel.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	wsa@...-dreams.de, jdelvare@...e.de, arnd@...db.de,
	dianders@...omium.org, u.kleine-koenig@...gutronix.de,
	laurent.pinchart+renesas@...asonboard.com,
	boris.brezillon@...e-electrons.com, maxime.coquelin@...com,
	andrew@...n.ch, sjg@...omium.org, markus.mayer@...aro.org,
	ch.naveen@...sung.com, jacob.jun.pan@...ux.intel.com,
	max.schwarz@...ine.de, mika.westerberg@...ux.intel.com,
	skuribay@...ox.com, Romain.Baeriswyl@...lis.com,
	wenkai.du@...el.com, chiau.ee.chew@...el.com,
	christian.ruppert@...lis.com, alan@...ux.intel.com,
	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH V2] i2c-designware: Add Intel Baytrail PMIC I2C bus
 support

Hi Maxime,

On Tue, Sep 23, 2014 at 09:00:57PM +0200, Maxime Ripard wrote:
> Hi David,
> 
> On Tue, Sep 23, 2014 at 11:40:26AM -0700, David E. Box wrote:
> > This patch implements an I2C bus sharing mechanism between the host and platform
> > hardware on select Intel BayTrail SoC platforms using the X-Powers AXP288 PMIC.
> > 
> > On these platforms access to the PMIC must be shared with platform hardware. The
> > hardware unit assumes full control of the I2C bus and the host must request
> > access through a special semaphore. Hardware control of the bus also makes it
> > necessary to disable runtime pm to avoid interfering with hardware transactions.
> > 
> > Signed-off-by: David E. Box <david.e.box@...ux.intel.com>
> 
> Sorry for stepping in like this without really knowing your platform,
> but wouldn't using the hwspinlock framework make more sense than
> hardcoding your own internal functions here?

I looked into this but didn't see a clear way on our platform to identify the
semaphore seperately from doing it in the designware platform driver. The way
we can find it now is through evaluating an ACPI _SEM object on every i2c device
that gets probed by the dw driver since at probe time we can get the acpi handle.
Without this handle however there isn't a clear way of evaluating the _SEM
object which would be needed to register a hwspinlock in separate code.

Plus it would still require changes to the designware i2c core, though admittedly
having a generic hwspinlock pointer added to the struct is cleaner.

Dave
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