Date: Thu, 11 Jul 2013 13:13:30 +0300
From: Mika Westerberg <mika.westerberg@linux.intel.com>
To: Christian Ruppert <christian.ruppert@abilis.com>
Cc: linux-i2c@vger.kernel.org, Wolfram Sang <wsa@the-dreams.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] i2c-designware: make *CNT values configurable
Message-ID: <20130711101330.GP4898@intel.com>
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On Thu, Jul 11, 2013 at 10:36:00AM +0300, Mika Westerberg wrote:
> On Wed, Jul 10, 2013 at 06:56:35PM +0200, Christian Ruppert wrote:
> > On Wed, Jul 10, 2013 at 01:52:15PM +0300, Mika Westerberg wrote:
> > > On Tue, Jul 09, 2013 at 06:19:28PM +0200, Christian Ruppert wrote:
> > > > What I meant is the following: The clock cycle time Tc is composed of
> > > > the four components
> > > > 
> > > >   Tc = Th + Tf + Tl + Tr
> > > > 
> > > > where
> > > >   Th: Time during which the signal is high
> > > >   Tf: Falling edge transition time
> > > >   Tl: Time during which the signal is low
> > > >   Tr: Rising edge transition time
> > > > 
> > > > The I2C specification specifies a minimum for Tl and Th and a range (or
> > > > maximum) for Tr and Tf. A maximum frequency is specified as the
> > > > frequency obtained by adding the minima for Th and Tl to the maxima of
> > > > Tr ant Tf.
> > > > Since as you said, transition times are very much PCB dependent, one way
> > > > to guarantee the max. frequency constraint (and to achieve a constant
> > > > frequency at its max) is to define the constants
> > > > Th' = Th + Tf := Th_min + Tf_max
> > > > Tl' = Tl + Tr := Tl_min + Tr_max
> > > > 
> > > > and to calculate the variables
> > > > Th = Th' - Tf
> > > > Tl = Tl' - Tr
> > > > in function of Tf and Tr of the given PCB.
> > > 
> > > If I understand the above, it leaves Tf and Tr to be PCB specific and then
> > > these values are passed to the core driver from platform data, right?
> > 
> > That would be the idea: Calculate Th' and Tl' in function of the desired
> > clock frequency and duty cycle and then adapt these values using
> > measured transition times. What prevented me from implementing this
> > rather academic approach are the following comments in
> > i2c-designware-core.c:
> > 
> > /*
> >  * DesignWare I2C core doesn't seem to have solid strategy to meet
> >  * the tHD;STA timing spec.  Configuring _HCNT based on tHIGH spec
> >  * will result in violation of the tHD;STA spec.
> >  */
> > 
> > /* ...
> >  * This is just experimental rule; the tHD;STA period
> >  * turned out to be proportinal to (_HCNT + 3).  With this setting,
> >  * we could meet both tHIGH and tHD;STA timing specs.
> >  * ...
> >  */
> > 
> > If I interpret this right, the slow down of the clock is intentional to
> > meet tHD;STA timing constraints.
> 
> Yeah, looks like so. tHD;STA is the SDA hold time. I wonder if the above
> comments apply to some earlier version of the IP that didn't have the SDA
> hold register?

Scratch that.

I re-read the spec and tHD;STA is hold time for (repeated) start. There is
a constraint that says that the device must internally provide a hold time
of at least 300ns for the SDA signal. Maybe that's the constraint the
comment above is referring to?
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