Openblocks AX3-4 i2c bus lockup

[Jason Cooper]

On Tue, Dec 31, 2013 at 01:50:34PM +0100, Sebastian Hesselbarth wrote:
> On 12/31/2013 01:28 PM, Gregory CLEMENT wrote:
> >>First I wanted to be sure that there the issue was not introduce by a
> >>commit so reverted one by one the commits on the file
> >>drivers/i2c/busses/i2c-mv64xxx.c. I tested it on both version of the
> >>OpenBlock AX-4 (with CPU A0 and B0). After each commit the kernel
> >>continue to work on the B0 version as expected, but it was when I
> >>reverted the commit "i2c: mv64xxx: Add I2C Transaction Generator
> >>support" that it worked also on the A0 version.
> >>
> >>Then I had a look on the errata datasheet and I found issues that I
> >>missed when I worked on it. This issues were fixed in B0 version.
> >>
> >>The fix should be pretty simple: disabling the offload_enabled flag when
> >>an A0 version of the CPU is used. For this there are 2 solutions:
> >>introducing a new compatible string or trying to detect the CPU
> >>stepping at runtime. I would prefer the second solution and I am looking
> >>for a way to get this information.
> >
> >
> >We can have this information in the same way that it is currently done
> >by the other mvebu SoC: accessing the PCIE_DEV_REV_OFF register. In
> >arch/arm/plat-orion/pcie.c there were functions named
> >orion_pcie_dev_id() and orion_pcie_dev_id() to retrieve information
> >about the CPU variant and its version.
> 
> Depending on running pcie when calling is tricky, as it can be clock
> gated. Maybe we should have some mach code to get the SoC revision
> early for all SoCs. That should look for a pcie controller node,
> enable the clock, store the revision once, and disable the clock
> again. The callback can then return the stored value.

Agreed.

> >We could the same in drivers/pci/host/pci-mvebu.c, however it would
> >add a dependency between PCIe and I2C for the mvebu SoCs. I can think
> >of several options:
> >
> >1. Using only a new compatible strings: mv78230-A0-i2c. The benefits
> >of it are that it is very easy to implement and it don't touch anything
> >else than the driver itself. The drawback is that we need to add an
> >new dts file for the A0 variant of the AX3-4.

If we decide to do this, it should be mv78230-i2c runs assuming it is
on the A0 variant.  mv78230-B0-i2c would permit offloading.

> I know that DT should describe HW, but at this point I tend to not
> fork off another dts. If it is probable, we should probe it. SoC
> revisions are really hard to see even from looking at the pcb, there
> is no way for users to determine the correct dts.

In theory, this is something that could be tweaked at runtime by the
bootloader.  But the bootloaders aren't there yet, and requiring a
bootloader upgrade isn't an option.  However, this is something that
should definitely be expressed in the DT.

I have no problem with doing both.  eg check for -B0-i2c, if that's
missing, retrieve the CPU variant and then enable/disable offloading.

thx,

Jason.