clk: divider: Fix clk_divider_bestdiv() returning min rate for large rate requests

clk_divider_bestdiv() clamps maxdiv using:

    maxdiv = min(ULONG_MAX / rate, maxdiv);

to avoid overflow in rate * i. However, requests like
clk_round_rate(clk, ULONG_MAX), which are used to determine the maximum
supported rate of a clock, result in maxdiv being clamped to 1. If no
valid divider of 1 exists in the table the loop is never entered and
bestdiv falls back to the maximum divider with the minimum parent rate,
causing clk_round_rate(clk, ULONG_MAX) to incorrectly return the minimum
supported rate instead of the maximum.

Fix this by removing the pre-loop maxdiv clamping and replacing the
unprotected rate * i multiplication with check_mul_overflow(). Guard
the exact-match short-circuit with !overflow to prevent a clamped
target_parent_rate of ULONG_MAX from falsely matching parent_rate_saved
and causing premature loop exit. Break out of the loop after evaluating
the first overflowing divider since clk_hw_round_rate(parent, ULONG_MAX)
returns a constant for all subsequent iterations, meaning no better
candidate can be found, and continuing would cause exponential recursive
calls in chained divider clocks.

Update the KUnit test expected values to reflect the corrected behaviour:
  - clk_divider_bestdiv_ulong_max_returns_max_rate: PARENT_RATE_1GHZ / 8
    (minimum rate, pre-fix) -> PARENT_RATE_1GHZ / 2 (maximum rate)
  - clk_divider_bestdiv_mux_ulong_max_returns_max_rate: 0 (invalid,
    pre-fix) -> PARENT_RATE_4GHZ / 2 (maximum rate with mux selecting
    the 4 GHz parent and applying the smallest table divider of 2)

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Brian Masney <bmasney@redhat.com>

0 files changed, 0 insertions, 0 deletions