On Thu, May 13, 2010 at 11:27 PM, Paul Walmsley <paul@xxxxxxxxx> wrote: > > Hello, > > Another comment on the kernel suspend-blocker API. > > On Thu, 13 May 2010, Arve Hjønnevåg wrote: > >> Adds /sys/power/policy that selects the behaviour of /sys/power/state. >> After setting the policy to opportunistic, writes to /sys/power/state >> become non-blocking requests that specify which suspend state to enter >> when no suspend blockers are active. A special state, "on", stops the >> process by activating the "main" suspend blocker. >> >> Signed-off-by: Arve Hjønnevåg <arve@xxxxxxxxxxx> > > Looking at the way that suspend-blocks are used in the current Android > 'msm' kernel tree[1], they seem likely to cause layering violations, > fragile kernel code with userspace dependencies, and code that consumes > power unnecessarily. > > For example, in drivers/mmc/core/core.c:721, in mmc_rescan() [2], we find > the following code: > > /* give userspace some time to react */ > wake_lock_timeout(&mmc_delayed_work_wake_lock, HZ / 2); > > This is a layering violation. The MMC subsystem should have nothing to do > with "giving userspace time to react." That is the responsibility of the > Linux scheduler. > > This code is also intrinsically fragile and use-case dependent. What if > userspace occasionally needs more than (HZ / 2) to react? If the > distributor is lucky enough to catch this before the product ships, then > the distributor can patch in a new magic number. But if the device makes > it to the consumer, the result is an unstable device that unpredictably > suspends. > > The above code will also waste power. Even if userspace takes less than > (HZ / 2) to react, the system will still be prevented from entering a > system-wide low power state for the duration of the remaining time. This > is in contrast to an approach that uses the idle loop to enter a > system-wide low power state. The moment that the system has no further > work to do, it can start saving power. > Yes, preventing suspend for 1/2 second wastes some power, but are you seriously suggesting that it wastes more power then never suspending? Opportunitic suspend does not prevent entering low power modes from idle. -- Arve Hjønnevåg _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
