i2c: Support command version numbers

Currently, I2C commands look like this:

Input:
  cmd8 [params bytes] checksum
Output:
  response8 [response_ptr bytes] checksum

Use a prefix byte of (0xDC + cmd_version) to indicate the command
version.  This is compatible with the existing protocol, since there
are no host commands in the range 0xDC-0xFB.  If the first byte of
the from-host data is 0x00-0xDB, it's a version 0 command.

There is no change to the output format, since the EC needs to hand
back a response which matches the version requested by the host.

New input:
  (0xDC+ver8) cmd8 paramlen8 [params bytes] checksum
New output:
  response8 responselen8 [response_ptr bytes] checksum

If the host gets a response of EC_RES_INVALID_COMMAND, it knows it's
talking to an old EC, and at most version 0 of the command is supported.

BUG=chrome-os-partner:11317
TEST=manual and a bit ad-hoc:
(note, this testing is not completed yet, so far only snow is tested)
Check that snow and link still process commands correctly over I2C
from U-Boot.
SMDK5250 # mkbp test
Old interface:

New interface:
Test passed

Change-Id: I1c21f2b036091e9122b4f980ca5f5af34f7fc070
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/27470

0 files changed, 0 insertions, 0 deletions