linux-kernel - Re: [PATCH] OMAP: I2C driver for TI OMAP boards #3

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Date:	Tue, 8 Aug 2006 05:57:30 -0700 (PDT)
From:	Komal Shah <komal_shah802003@...oo.com>
To:	Tony Lindgren <tony@...mide.com>, Jean Delvare <khali@...ux-fr.org>
Cc:	Komal Shah <komal_shah802003@...oo.com>,
	David Brownell <david-b@...bell.net>, r-woodruff2@...com,
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Greg KH <gregkh@...e.de>, i2c@...sensors.org
Subject: Re: [PATCH] OMAP: I2C driver for TI OMAP boards #3

--- Tony Lindgren <tony@...mide.com> wrote:

> Hi,
> 
> * Jean Delvare <khali@...ux-fr.org> [060805 11:31]:
> > 
> > > >> +		if (armxor_rate > 16000000)
> > > >> +			psc = (armxor_rate + 8000000) / 12000000;
> > > >> +		else
> > > >> +			psc = 0;
> > > 
> > > >Can you please explain this formula?
> > > 
> > > The OMAP core uses 8-bit value to divide the system clock (SCLK)
> and
> > > generates its own sampling clock (ICLK), and the core logic is
> sampled
> > > at clock rate of the system clock for the module, divided by
> (prescaler value + 1)
> > 
> > I should have been more precise, I guess. What surprises me are the
> > numbers themselves. It's frequent to see forumlae of the form
> > "a = (b + c/2) / c" to divide with proper rounding, but here you
> have
> > 2c/3 instread of c/2. My question was more like: is it intentional,
> or a
> > typo? Also, with the code above, psc will never have value 1. The
> "if"
> > part will always compute to at least 2, and the "else" part to 0.
> Is
> > this OK?
> > 
> 
> Hmmm, this sounds like a bug somewhere. TRM for 5912 says the I2C
> clock
> must be prescaled to be between 7 - 12 MHz [1]. The XOR input clock
> is
> typically 12, 13 or 19.2 MHz. So we should have code that produces:
> 
> XOR Mhz	Divider	Prescaler
> 12	1	0
> 13	2	1
> 19.2	2	1
> 
> Then again the original old code produces something different too
> [2]...
> 
> I suspect the original code had some hw workarounds and and later
> code
> may have a conversion bug somewhere :)
> 
> I suggest we keep the code as is for now since it's known to work on
> all omaps, and then submit a follow-up patch later once we have
> verified that that code based on the TRM works on all omaps.

I have updated the driver with all other review comments except comment
on formula above by Jean. I can some part of description from Tony's
explanation along with removing the following code line

/* Set Own Address */
omap_i2c_write_reg(dev, OMAP_I2C_OA_REG, dev->own_address);
 
As it is not needed. Please confirm so that I can send the revised
patch  soon.

---Komal Shah
http://komalshah.blogspot.com/

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