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When the switcher is active, there is no straightforward way to
figure out which logical CPU a given physical CPU maps to.
This patch provides a function
bL_switcher_get_logical_index(mpidr), which is analogous to
get_logical_index().
This function returns the logical CPU on which the specified
physical CPU is grouped (or -EINVAL if unknown).
If the switcher is inactive or not present, -EUNATCH is returned instead.
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
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This patch exports a bL_switcher_trace_trigger() function to
provide a means for drivers using the trace events to get the
current status when starting a trace session.
Calling this function is equivalent to pinging the trace_trigger
file in sysfs.
Signed-off-by: Dave Martin <dave.martin@linaro.org>
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There is no explicit way to know when a switch started via
bL_switch_request() is complete. This can lead to unpredictable
behaviour when the switcher is controlled by a subsystem which
makes dynamic decisions (such as cpufreq).
The CPU PM notifier is not really suitable for signalling
completion, because the CPU could get suspended and resumed for
other, independent reasons while a switch request is in flight.
Adding a whole new notifier for this seems excessive, and may tempt
people to put heavyweight code on this path.
This patch implements a new bL_switch_request_cb() function that
allows for a per-request lightweight callback, private between the
switcher and the caller of bL_switch_request_cb().
Overlapping switches on a single CPU are considered incorrect if
they are requested via bL_switch_request_cb() with a callback (they
will lead to an unpredictable final state without explicit external
synchronisation to force the requests into a particular order).
Queuing requests robustly would be overkill because only one
subsystem should be attempting to control the switcher at any time.
Overlapping requests of this kind will be failed with -EBUSY to
indicate that the second request won't take effect and the
completer will never be called for it.
bL_switch_request() is retained as a wrapper round the new function,
with the old, fire-and-forget semantics. In this case the last request
will always win. The request may still be denied if a previous request
with a completer is still pending.
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
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Some subsystems will need to respond synchronously to runtime
enabling and disabling of the switcher.
This patch adds a dedicated notifier interface to support such
subsystems. Pre- and post- enable/disable notifications are sent
to registered callbacks, allowing safe transition of non-b.L-
transparent subsystems across these control transitions.
Notifier callbacks may veto switcher (de)activation on pre notifications
only. Post notifications won't revert the action.
If enabling or disabling of the switcher fails after the pre-change
notification has been sent, subsystems which have registered
notifiers can be left in an inappropriate state.
This patch sends a suitable post-change notification on failure,
indicating that the old state has been reestablished.
For example, a failed initialisation will result in the following
sequence:
BL_NOTIFY_PRE_ENABLE
/* switcher initialisation fails */
BL_NOTIFY_POST_DISABLE
It is the responsibility of notified subsystems to respond in an
appropriate way.
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
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Some subsystems will need to know for sure whether the switcher is
enabled or disabled during certain critical regions.
This patch provides a simple mutex-based mechanism to discover
whether the switcher is enabled and temporarily lock out further
enable/disable:
* bL_switcher_get_enabled() returns true iff the switcher is
enabled and temporarily inhibits enable/disable.
* bL_switcher_put_enabled() permits enable/disable of the switcher
again after a previous call to bL_switcher_get_enabled().
Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
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The workqueues are problematic as they may be contended.
They can't be scheduled with top priority either. Also the optimization
in bL_switch_request() to skip the workqueue entirely when the target CPU
and the calling CPU were the same didn't allow for bL_switch_request() to
be called from atomic context, as might be the case for some cpufreq
drivers.
Let's move to dedicated kthreads instead.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
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This is the core code implementing big.LITTLE switcher functionality.
Rationale for this code is available here:
http://lwn.net/Articles/481055/
The main entry point for a switch request is:
void bL_switch_request(unsigned int cpu, unsigned int new_cluster_id)
If the calling CPU is not the wanted one, this wrapper takes care of
sending the request to the appropriate CPU with schedule_work_on().
At the moment the core switch operation is handled by bL_switch_to()
which must be called on the CPU for which a switch is requested.
What this code does:
* Return early if the current cluster is the wanted one.
* Close the gate in the kernel entry vector for both the inbound
and outbound CPUs.
* Wake up the inbound CPU so it can perform its reset sequence in
parallel up to the kernel entry vector gate.
* Migrate all interrupts in the GIC targeting the outbound CPU
interface to the inbound CPU interface, including SGIs. This is
performed by gic_migrate_target() in drivers/irqchip/irq-gic.c.
* Call cpu_pm_enter() which takes care of flushing the VFP state to
RAM and save the CPU interface config from the GIC to RAM.
* Modify the cpu_logical_map to refer to the inbound physical CPU.
* Call cpu_suspend() which saves the CPU state (general purpose
registers, page table address) onto the stack and store the
resulting stack pointer in an array indexed by the updated
cpu_logical_map, then call the provided shutdown function.
This happens in arch/arm/kernel/sleep.S.
At this point, the provided shutdown function executed by the outbound
CPU ungates the inbound CPU. Therefore the inbound CPU:
* Picks up the saved stack pointer in the array indexed by its MPIDR
in arch/arm/kernel/sleep.S.
* The MMU and caches are re-enabled using the saved state on the
provided stack, just like if this was a resume operation from a
suspended state.
* Then cpu_suspend() returns, although this is on the inbound CPU
rather than the outbound CPU which called it initially.
* The function cpu_pm_exit() is called which effect is to restore the
CPU interface state in the GIC using the state previously saved by
the outbound CPU.
* Exit of bL_switch_to() to resume normal kernel execution on the
new CPU.
However, the outbound CPU is potentially still running in parallel while
the inbound CPU is resuming normal kernel execution, hence we need
per CPU stack isolation to execute bL_do_switch(). After the outbound
CPU has ungated the inbound CPU, it calls mcpm_cpu_power_down() to:
* Clean its L1 cache.
* If it is the last CPU still alive in its cluster (last man standing),
it also cleans its L2 cache and disables cache snooping from the other
cluster.
* Power down the CPU (or whole cluster).
Code called from bL_do_switch() might end up referencing 'current' for
some reasons. However, 'current' is derived from the stack pointer.
With any arbitrary stack, the returned value for 'current' and any
dereferenced values through it are just random garbage which may lead to
segmentation faults.
The active page table during the execution of bL_do_switch() is also a
problem. There is no guarantee that the inbound CPU won't destroy the
corresponding task which would free the attached page table while the
outbound CPU is still running and relying on it.
To solve both issues, we borrow some of the task space belonging to
the init/idle task which, by its nature, is lightly used and therefore
is unlikely to clash with our usage. The init task is also never going
away.
Right now the logical CPU number is assumed to be equivalent to the
physical CPU number within each cluster. The kernel should also be
booted with only one cluster active. These limitations will be lifted
eventually.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
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