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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> |
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