linux-kernel - Re: [PATCH v10] i2c-designware: make SDA hold time configurable

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Message-Id: <201307031642.30380.arnd@arndb.de>
Date:	Wed, 3 Jul 2013 16:42:30 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Christian Ruppert <christian.ruppert@...lis.com>
Cc:	vincent.stehle@...escale.lan, Wolfram Sang <wsa@...-dreams.de>,
	Rob Herring <robherring2@...il.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	linux-i2c@...r.kernel.org,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	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 v10] i2c-designware: make SDA hold time configurable

On Wednesday 03 July 2013, Christian Ruppert wrote:
> On Wed, Jul 03, 2013 at 04:20:03PM +0200, Arnd Bergmann wrote:
> > On Wednesday 03 July 2013, Christian Ruppert wrote:
> > > On Wed, Jul 03, 2013 at 01:43:11PM +0200, Arnd Bergmann wrote:
> > > > On Wednesday 26 June 2013, Wolfram Sang wrote:
> > > > >   On Wed, Jun 26, 2013 at 10:55:06AM +0200, Christian Ruppert wrote:
> > > > > > This patch makes the SDA hold time configurable through device tree.
> > > > > > 
> > > > > > Signed-off-by: Christian Ruppert <christian.ruppert@...lis.com>
> > > > > > Signed-off-by: Pierrick Hascoet <pierrick.hascoet@...lis.com>
> > > > > 
> > > > > Applied to for-next, thanks for keeping at it and providing lots of
> > > > > useful information. Much appreciated!
> > > > 
> > > > Sorry, but I got a regression that I didn't find reported elsewhere
> > > > so far, even though it breaks a lot of the ARM defconfig builds:
> > > > 
> > > > drivers/built-in.o: In function `dw_i2c_probe':
> > > > /git/arm-soc/drivers/i2c/busses/i2c-designware-platdrv.c:125: undefined reference to `__udivdi3'
> > > > 
> > > > I suspect you want something like the change below.
> > > 
> > > This looks similar to a patch Vincent Stehle submitted yesterday, see
> > > https://lkml.org/lkml/2013/7/2/145
> > 
> > Thanks for the link. Actually his patch looks wrong to me, because
> > 
> >  dev->sda_hold_time = div_u64((u64)ic_clk * ht + 500000, 1000000); 
> > 
> > assigns the division remainder to sda_hold_time, not the quotient.
> 
> Hrmmm... At least when I tested it this morning on an ARC architecture
> it worked as intended and returned the quotient. Does that mean we have
> an issue with this function on ARC? Can anyone who knows these functions
> better than I comment?

ARC just uses the generic version of div_u64, which is defined in lib/div64.c.

I suspect that the division remainder just happens to work well enough for
you to not cause any run-time error. You could try adding a printk
in that function to show the values you get on ARC.

	Arnd
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