soc: qcom: rpmh-rsc: Simplify locking by eliminating the per-TCS lock

soc: qcom: rpmh-rsc: Simplify locking by eliminating the per-TCS lock

The rpmh-rsc code had both a driver-level lock (sometimes referred to
in comments as drv->lock) and a lock per-TCS.  The idea was supposed
to be that there would be times where you could get by with just
locking a TCS lock and therefor other RPMH users wouldn't be blocked.

The above didn't work out so well.

Looking at tcs_write() the bigger drv->lock was held for most of the
function anyway.  Only the __tcs_buffer_write() and
__tcs_set_trigger() calls were called without holding the drv->lock.
It actually turns out that in tcs_write() we don't need to hold the
drv->lock for those function calls anyway even if the per-TCS lock
isn't there anymore.  From the newly added comments in the code, this
is because:
- We marked "tcs_in_use" under lock.
- Once "tcs_in_use" has been marked nobody else could be writing
  to these registers until the interrupt goes off.
- The interrupt can't go off until we trigger w/ the last line
  of __tcs_set_trigger().
Thus, from a tcs_write() point of view, the per-TCS lock was useless.

Looking at rpmh_rsc_write_ctrl_data(), only the per-TCS lock was held.
It turns out, though, that this function already needs to be called
with the equivalent of the drv->lock held anyway (we either need to
hold drv->lock as we will in a future patch or we need to know no
other CPUs could be running as happens today).  Specifically
rpmh_rsc_write_ctrl_data() might be writing to a TCS that has been
borrowed for writing an active transation but it never checks this.

Let's eliminate this extra overhead and avoid possible AB BA locking
headaches.

Suggested-by: Maulik Shah <mkshah@codeaurora.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200504104917.v6.4.Ib8dccfdb10bf6b1fb1d600ca1c21d9c0db1ef746@changeid
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>