MySQL 1422, explicit or implicit commit is not allowed in trigger - mysql

Question

I understand that I cannot do a transaction inside a trigger, but I CAN inside a procedure. And I should be able to call a procedure from a trigger. Which is exactly what I'm doing.
TABLE A has an "after update trigger".
I only want to call the procedure if one of two columns has been changed.
TABLE A TRIGGER:
IF (
NEW.colA != OLD.colA
OR
NEW.colB != OLD.colB
) THEN
call setBonuses( NEW.colA, NEW.colB );
END IF
The setBonuses SP effects TABLE B by doing some calculations using TABLE_A.colA,TABLE_A.colB (I don't think it's relevant, but TABLE B does have an "after update trigger", but it has a conditional to only do its update if fields NOT effected by the below procedure are changed, so it does 'nothing' when the bonus field is updated.)
THE STORED PROCEDURE:
begin
IF A > B THEN
SET n = A;
...
ELSEIF A < B THEN
SET n = B;
...
END IF;
start transaction;
-- Bonuses
UPDATE TABLE_B
SET bonus = 0
WHERE... ;
UPDATE TABLE_B
SET bonus = bonus + n
WHERE...;
UPDATE TABLE_B
SET bonus = bonus + n
WHERE...;
UPDATE TABLE_B
SET bonus = bonus + n
WHERE...;
UPDATE TABLE_B
SET bonus = bonus + n
WHERE...;
UPDATE TABLE_B
SET bonus = bonus + n
WHERE...;
commit;
end
The SP works fine if called directly by the application, however, I don't want to have to explicitly call it as a chain in some event, I really just need it to do its calcs and update TABLE B - whenever someone changed TABLE A (colA or colB).
Any reason why this isn't possible.