linux-kernel - Re: [PATCH REBASE] i2c-designware: make SDA hold time configurable

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Date:	Mon, 17 Jun 2013 11:23:31 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Christian Ruppert <christian.ruppert@...lis.com>
Cc:	Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>
Subject: Re: [PATCH REBASE] i2c-designware: make SDA hold time configurable

On Wed, Jun 12, 2013 at 04:47:45PM +0200, Christian Ruppert wrote:
> On Mon, Jun 10, 2013 at 05:29:55PM +0200, Wolfram Sang wrote:
> > On Tue, May 14, 2013 at 03:04:02PM +0200, Christian Ruppert wrote:
> > > This patch makes the SDA hold time configurable through device tree.
> > > 
> > > [rebased to i2c-current/i2c-next/mainline-3.10-rc1]
> > > 
> > > Signed-off-by: Christian Ruppert <christian.ruppert@...lis.com>
> > > Signed-off-by: Pierrick Hascoet <pierrick.hascoet@...lis.com>
> > 
> > Hmm, I really have problems adding a generic property. I need to better
> > understand why this is needed? What is the usecase? Can't a safe value
> > be calculated depending on the bus-speed? Is there a public datasheet
> > available somewhere?
> 
> I checked with our PCB/Applications team and the data sheets for the
> peripherals in question (DVB demodulator front ends) are under NDA.
> Mika, you seem to be interested in this patch as well. Do you know of
> any publicly available data sheets for hardware requiring this
> adjustment?

Sorry for the delay, I was on vacation.

I'm not aware of any publicly available datasheets for this block,
unfortunately.

> In the case of the Designware block, the parameter both changes SDA and
> START hold times, however, and you'll find lots of data sheets for
> hardware with START hold time requirements on the net, e.g.
> http://ww1.microchip.com/downloads/en/DeviceDoc/21805B.pdf
> 
> The empirical solution in the function i2c_dw_scl_hcnt does not seem to
> work in all cases: Our lab guys confirmed that we have several PCB
> designs which do not work without adjusting the sda-hold-time parameter
> to an appropriate value. The value seems to be different for different
> PCBs.

Yeah, we see the same problem. Our platform guys have solved this so that
they made the SDA hold time available from ACPI to the device driver.

> I suspect that this kind of configurability is not the same for all i2c
> bus master hardware. If you don't want to add a generic property, would
> you accept the patch with the property renamed to dwi2c,sda-hold-time?
> 
> Greetings,
>   Christian
> 
> 
> -- 
>   Christian Ruppert              ,          <christian.ruppert@...lis.com>
>                                 /|
>   Tel: +41/(0)22 816 19-42     //|                 3, Chemin du Pré-Fleuri
>                              _// | bilis Systems   CH-1228 Plan-les-Ouates


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