firmware: qcom: scm: Fix missing read barrier in qcom_scm_is_available()

firmware: qcom: scm: Fix missing read barrier in qcom_scm_is_available()

Commit 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq
completion variable initialization") introduced a write barrier in probe
function to store global '__scm' variable.  It also claimed that it
added a read barrier, because as we all known barriers are paired (see
memory-barriers.txt: "Note that write barriers should normally be paired
with read or address-dependency barriers"), however it did not really
add it.

The offending commit used READ_ONCE() to access '__scm' global which is
not a barrier.

The barrier is needed so the store to '__scm' will be properly visible.
This is most likely not fatal in current driver design, because missing
read barrier would mean qcom_scm_is_available() callers will access old
value, NULL.  Driver does not support unbinding and does not correctly
handle probe failures, thus there is no risk of stale or old pointer in
'__scm' variable.

However for code correctness, readability and to be sure that we did not
mess up something in this tricky topic of SMP barriers, add a read
barrier for accessing '__scm'.  Change also comment from useless/obvious
what does barrier do, to what is expected: which other parts of the code
are involved here.

Fixes: 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq completion variable initialization")
Cc: stable@vger.kernel.org
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241209-qcom-scm-missing-barriers-and-all-sort-of-srap-v2-1-9061013c8d92@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>