linux-kernel - Re: [PATCH 1/2] i2c-designware: make *CNT values configurable

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Message-ID: <52120E16.8010503@pobox.com>
Date:	Mon, 19 Aug 2013 21:22:46 +0900
From:	Shinya Kuribayashi <skuribay@...ox.com>
To:	mika.westerberg@...ux.intel.com
CC:	christian.ruppert@...lis.com, linux-i2c@...r.kernel.org,
	wsa@...-dreams.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c-designware: make *CNT values configurable

Hi,

On 8/19/13 8:36 PM, Mika Westerberg wrote:
> On Fri, Aug 16, 2013 at 11:15:12AM +0900, Shinya Kuribayashi wrote:
>>> Actually, the I2C specification clearly defines f_SCL;max (and thus
>>> implies t_SCL;min), both in the tables and the timing diagrams. Why can
>>> we ignore this constraint while having to meet all the others?
>>
>> If we meet t_r, t_f, t_HIGH, t_LOW (and t_HIGH in this DW case),
>> f_SCL;max will be met by itself.  And again, all I2C master and
>> slave devices in the bus don't care about f_SCL; what they do care
>> are t_f, t_r, t_HIGH, t_LOW, and so on.  That's why I'm saying
>> f_SCL is pointless and has no value for HCNT/LCNT calculations.
>
> One thing that comes to mind regarding the bus speed is that even if we
> have all the minimal timing requirements met we still prefer resulting bus
> speeds closer to 400kHz than 315.41kHz for the reasons that we get more
> data transferred that way, no?

That depends I2C slave devices in the bus in your target systems.
As long as your slave devices can detect START/STOP conditions and
recognize SDA/SCL transitions properly, that should be Ok (you can
use HCNT/LCNT settings for 400 kHz without having all the minimal
timing requirements met).

My comments above was a reply to Christian's snippet code and how to
treat f_SCL;mas constraints, and unrelated to your case in question.
I'm for having a way to override HCNT/LCNT values as said before, and
that should nicely work for you.

   Shinya
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