net: mscc: ocelot: initialize watermarks to sane defaults

net: mscc: ocelot: initialize watermarks to sane defaults

This is meant to be a gentle introduction into the world of watermarks
on ocelot. The code is placed in ocelot_devlink.c because it will be
integrated with devlink, even if it isn't right now.

My first step was intended to be to replicate the default configuration
of the congestion watermarks programatically, since they are now going
to be tuned by the user.

But after studying and understanding through trial and error how they
work, I now believe that the configuration used out of reset does not do
justice to the word "reservation", since the sum of all reservations
exceeds the total amount of resources (otherwise said, all reservations
cannot be fulfilled at the same time, which means that, contrary to the
reference manual, they don't guarantee anything).

As an example, here's a dump of the reservation watermarks for frame
buffers, for port 0 (for brevity, the ports 1-6 were omitted, but they
have the same configuration):

BUF_Q_RSRV_I(port 0, prio 0) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 1) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 2) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 3) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 4) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 5) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 6) = max 3000 bytes
BUF_Q_RSRV_I(port 0, prio 7) = max 3000 bytes

Otherwise said, every port-tc has an ingress reservation of 3000 bytes,
and there are 7 ports in VSC9959 Felix (6 user ports and 1 CPU port).
Concentrating only on the ingress reservations, there are, in total,
8 [traffic classes] x 7 [ports] x 3000 [bytes] = 168,000 bytes of memory
reserved on ingress.
But, surprise, Felix only has 128 KB of packet buffer in total...
A similar thing happens with Seville, which has a larger packet buffer,
but also more ports, and the default configuration is also overcommitted.

This patch disables the (apparently) bogus reservations and moves all
resources to the shared area. This way, real reservations can be set up
by the user, using devlink-sb.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>