Re: Erratic MPC8248 CPM2 I2C behaviour

[Laurent Pinchart <laurentp@cse-semaphore.com>]

Hi Joakim,

On Tuesday 02 December 2008 09:39:59 Joakim Tjernlund wrote:
> On Mon, 2008-12-01 at 15:28 -0800, Mike Ditto wrote:
> > Laurent Pinchart <laurentp@cse-semaphore.com> wrote:
> > > Transmission timeout after one second. The first TX buffer descriptor
> > > status hasn't been modified by the CPM. The CPM state dump shows that
> > > processing of
> >
> > ...
> >
> > This sounds very similar to a problem I have seen on MPC8247 and
> > MPC8248.
> >
> > It could be a variation of the CPM bug documented by Freescale as
> > erratum CPM98.  But the key difference is that we see a persistent
> > failure, while the erratum only mentions a problem with "the next
> > transaction".  What we see is that once the I2C engine gets confused
> > by some anomaly on the SCL signal, it stops processing buffer
> > descriptors entirely until it is turned off and back on.  You can
> > observe this bug by momentarily grounding the SCL line (it's an
> > open-collector bus) with a jumper while you attempt an I/O.
> >
> > Our production equipment is using Linux 2.6 with the out-of-tree
> > i2c-algo-8260.c by Dan Malek and Brad Parker.  I modified this
> > driver to shut off the I2C controller and turn it back on when it
> > hits this problem, and now it can recover from this condition.
> >
> > However, this does not explain how the controller gets into the
> > frozen state in the first place.  We see it very rarely in production
> > units and have not been able to identify the cause.  If it is
> > similar to erratum CPM98 then it could be noise on the SCL signal or
> > perhaps an I2C device that doesn't conform to the protocol perfectly.
> > Also beware, if you are using some kind of multiplexer, that you don't
> > direct the multiplexer to switch to a different bus (segment) while a
> > transaction is in progress.
> >
> > In testing with the current (2.6.27) i2c-cpm.c driver, I found that
> > it is sufficient to #define I2C_CHIP_ERRATA to allow it to recover
> > from the CPM I2C freeze.  However, I don't know if I like this
> > approach because it turns off the I2C engine after every transfer,
> > even successful ones, and I don't know if this will affect reliability
> > or performance.  And I don't know if this will actually prevent the
> > freeze from happening, although I presume that it would protect the
> > I2C engine from getting confused by a glitch that happens while no
> > transfer is in progress.
> >
> > I am not aware of any documentation for what exactly the I2C_CHIP_ERRATA
> > conditional code in i2c-cpm.c is meant to work around.
>
> The I2C_CHIP_ERRATA is mine and refers to mpc8xx, mpc860 in my case. The
> driver was adapted by me some years ago in 2.4 and now someone has
> ported it to 2.6 it seems. From memory, you should check that mrblr is
> one byte bigger than your max transfer size.

I've checked that, there's no problem there. The transfer that fails doesn't 
involve any rx buffer anyway, so mrblr is irrelevant here.

> I have include my old driver that we still use in 2.4, as I recall, not
> all bits made into the kernel.

Thanks. The out-of-tree i2c-mpc8260.c driver that I was using with the ppc 
architecture seems to have been a port of your code. It has been replaced by 
the i2c-cpm.c driver in the powerpc architecture. Both drivers use a similar 
approach, but the i2c-cpm.c driver allocates rx/tx buffers differently.

I spent some more time last Friday investigating the problem, and it seems the 
baud rate generator configuration plays an important role. The default 
configuration (60kHz nominal => 65.104kHz using a 25MHz brg clock and a /32 
predivider) leads to timeouts, while I haven't been able to reproduce the 
problem with the i2c-mpc8260.c configuration (100kHz nominal => 104.167kHz 
using a 25MHz brg clock and a /8 predivider).

This might be a coincidence, and I can't verify any of the results right now 
as my system decide to work correctly even at 60kHz :-/ I hope I won't have to 
through clock jitter and system temperature into the mix, otherwise this is 
going to be very complex to debug.

Best regards,

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
B-1410 Waterloo
Belgium

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