From mboxrd@z Thu Jan  1 00:00:00 1970
From: marex@denx.de (Marek Vasut)
Date: Fri, 24 May 2013 03:00:59 +0200
Subject: [PATCH 2/2] ARM: dts: imx28: Adjust i2c interrupt bindings
In-Reply-To: <1369322904.4142.36.camel@weser.hi.pengutronix.de>
References: <1368713639-21485-1-git-send-email-fabio.estevam@freescale.com>
 <201305231648.17356.marex@denx.de>
 <1369322904.4142.36.camel@weser.hi.pengutronix.de>
Message-ID: <201305240301.00137.marex@denx.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Dear Lucas Stach,

> Am Donnerstag, den 23.05.2013, 16:48 +0200 schrieb Marek Vasut:
> > Dear Juergen Beisert,
> > 
> > > Hi Marek,
> > > 
> > > Marek Vasut wrote:
> > > > > > i2c slowness is a different issue.
> > > > > 
> > > > > Same happens here for my i.M23 based platform. It seems the PIO
> > > > > mode does not work, or at least not like it works on a i.MX28.
> > > > > Each short transfer needs about one second (without an error
> > > > > message) but does not send anything on the I2C lines.
> > > > > 
> > > > > I need the following patches to make I2C master work within a
> > > > > 3.10-rc2 kernel:
> > > > > 
> > > > > Subject: [PATCH] I2C/MXS: distinguish i.MX23 and i.MX28 based I2C
> > > > 
> > > > I'm all for it, but then ... won't it be better if you actually fixed
> > > > the PIO and mixed-mode on MX23 instead of implementing such hack?
> > > 
> > > If the PIO mode or my patch is a hack depends on the point of view:
> > > Lucas told me the PIO mode is *mentioned* but *not specified* in the
> > > i.MX23/i.MX28 datasheets.
> > 
> > The PIO works the same way DMA does -- set up bits and then pump data
> > into the DATA register.
> > 
> > > So, the PIO mode seems to depend on some undocumented status bits in
> > > the i.MX28 I2C controller implementation.
> > 
> > How would DMA work then if it used undocumented registers ? It's in the
> > documentation, just read it or ask FSL ;-)
> 
> While the PIO mode might use the same controller mechanisms as the DMA
> mode, PIO mode is _not_ a documented mode of operation for the i.MX23.
> 
> To quote the i.MX28 RM: "The I2C block on the i.MX28 supports a new PIO
> mode or soft-DMA mode.", which implies the PIO mode to be a new mode of
> operation not found on earlier i.MX SoCs.
> The doc is slightly fuzzy here as the i.MX23 RM in contrary states:
> "Short transmission (up to three bytes plus address) can be easily
> triggered using only PIO operations, i.e., no DMA setup required." But
> again it's not a documented mode of operation, i.MX23 doc only describes
> the DMA mode.

I'm CCing a Amaury Pouly so he can comment on this stuff. He did some hardware 
digging.

> So while we _might_ be able to get the PIO mode to work on the i.MX23
> there is nothing in the doc stating that it's even meant to work. Even
> while PIO and DMA mode use the same internal mechanisms, there's still
> plenty of opportunities of fail in there. After all PIO mode relies on
> reading a debug register in the course of normal operation.

What's the problem with that ? The debug register contains status bits.

> Only more extensive experimentation could show if we are in fact able to
> make it work, a first shot of using PIO mode on MX23 failed, so it might
> as well be that Juergens quick fix is correct and we have to disable PIO
> mode on MX23 altogether. That said please stop slapping the word "hack"
> over this patch until proven otherwise.
> 
> Regards,
> Lucas

Best regards,
Marek Vasut