[Skiboot] [PATCH] witherspoon: Squash spurious I2C errors

[Oliver O'Halloran]

On Wed, Nov 27, 2019 at 3:48 PM Oliver O'Halloran <oohall at gmail.com> wrote:
>
> On witherspoon there's an I2C bus connecting the each chip to the GPUs
> connected to that chip via NVLink. That bus has a 750kOhm pullup on the
> system planar with prevents the bus from operating correctly.
>
> Each GPU has a smaller pullup which makes the bus usable when a GPU is
> plugged in, but on systems without GPUs we get a lot of spurious I2C
> master errors. Specificly, because of the oversized pullup the SDA and
> SCL for that bus cannot return to '1' fast enough, so the master assumes
> that another master is driving the bus and that it should stop.
>
> Work around this by disabling the affected port when there's no GPUs
> detected in the system.
>
> Cc: Andrew Donnellan <andrew.donnellan at au1.ibm.com>
> Signed-off-by: Oliver O'Halloran <oohall at gmail.com>

Merged as dc85bd46d20db4d878216283f818573cb0c8c05b