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If we were to have two users of the same clock, doing something like:

clk_set_rate_range(user1, 1000, 2000);
clk_set_rate_range(user2, 3000, 4000);

The second call would fail with -EINVAL, preventing from getting in a
situation where we end up with impossible limits.

However, this is never explicitly checked against and enforced, and
works by relying on an undocumented behaviour of clk_set_rate().

Indeed, on the first clk_set_rate_range will make sure the current clock
rate is within the new range, so it will be between 1000 and 2000Hz. On
the second clk_set_rate_range(), it will consider (rightfully), that our
current clock is outside of the 3000-4000Hz range, and will call
clk_core_set_rate_nolock() to set it to 3000Hz.

clk_core_set_rate_nolock() will then call clk_calc_new_rates() that will
eventually check that our rate 3000Hz rate is outside the min 3000Hz max
2000Hz range, will bail out, the error will propagate and we'll
eventually return -EINVAL.

This solely relies on the fact that clk_calc_new_rates(), and in
particular clk_core_determine_round_nolock(), won't modify the new rate
allowing the error to be reported. That assumption won't be true for all
drivers, and most importantly we'll break that assumption in a later
patch.

It can also be argued that we shouldn't even reach the point where we're
calling clk_core_set_rate_nolock().

Let's make an explicit check for disjoints range before we're doing
anything.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220225143534.405820-4-maxime@cerno.tech

Signed-off-by: Stephen Boyd <sboyd@kernel.org>