The function was designed with multi-threaded concurrency in mind,
where threads can take over while the lock is held and simply block
trying to acquire it, which allows the lock holder to proceed.
However interrupt handlers are different; they have priority, so once
they start they must complete immediately. The cannot afford to block
on the lock as the program would simply freeze. In exchange, they clean
up before they leave, so there are some guarantees on the execution
state even when interrupted.
The correct protection is therefore not a lock but a temporary block on
interrupts. There is no data race on the value of the saved IMASK
because it is preserved during interrupt handling.
This change introduces new sleep_block() and sleep_unblock() functions
that control whether the sleep() function actually sleeps. This type of
behavior was already implemented in the DMA driver, since DMA access to
on-chip memory is paused when sleeping (on-chip memory being paused
itself), which would make waiting for a DMA transfer a freeze.
Because DMA transfers are now asynchronous, and USB transfers that may
involve on-chip memory are coming, this API change allows the DMA and
USB drivers to block the sleep() function so that user code can sleep()
for interrupts without having to worry about asynchronous tasks
requiring on-chip memory to complete.
Changes in the driver and world system:
* Rewrite driver logic to include more advanced concepts. The notion of
binding a driver to a device is introduced to formalize wait(); power
management is now built-in instead of being handled by the drivers
(for instance DMA). The new driver model is described in great detail
in <gint/drivers.h>
* Formalized the concept of "world switch" where the hardware state is
saved and later restored. As a tool, the world switch turns out to be
very stable, and allows a lot of hardware manipulation that would be
edgy at best when running in the OS world.
* Added a GINT_DRV_SHARED flag for drivers to specify that their state
is shared between worlds and not saved/restored. This has a couple of
uses.
* Exposed a lot more of the internal driver/world system as their is no
particular downside to it. This includes stuff in <gint/drivers.h>
and the driver's state structures in <gint/drivers/states.h>. This is
useful for debugging and for cracked concepts, but there is no
API stability guarantee.
* Added a more flexible driver level system that allows any 2-digit
level to be used.
Feature changes:
* Added a CPU driver that provides the VBR change as its state save.
Because the whole context switch relied on interrupts being disabled
anyway, there is no longer an inversion of control when setting the
VBR; this is just part of the CPU driver's configuration. The CPU
driver may also support other features such as XYRAM block transfer
in the future.
* Moved gint_inthandler() to the INTC driver under the name
intc_handler(), pairing up again with intc_priority().
* Added a reentrant atomic lock based on the test-and-set primitive.
Interrupts are disabled with IMASK=15 for the duration of atomic
operations.
* Enabled the DMA driver on SH7305-based fx-9860G. The DMA provides
little benefit on this platform because the RAM is generally faster
and buffers are ultimately small. The DMA is still not available on
SH3-based fx-9860G models.
* Solved an extremely obnoxious bug in timer_spin_wait() where the
timer is not freed, causing the callback to be called when interrupts
are re-enabled. This increments a random value on the stack. As a
consequence of the change, removed the long delays in the USB driver
since they are not actually needed.
Minor changes:
* Deprecated some of the elements in <gint/hardware.h>. There really is
no good way to "enumerate" devices yet.
* Deprecated gint_switch() in favor of a new function
gint_world_switch() which uses the GINT_CALL abstraction.
* Made the fx-9860G VRAM 32-aligned so that it can be used for tests
with the DMA.
Some features of the driver and world systems have not been implemented
yet, but may be in the future:
* Some driver flags should be per-world in order to create multiple
gint worlds. This would be useful in Yatis' hypervisor.
* A GINT_DRV_LAZY flag would be useful for drivers that don't want to
be started up automatically during a world switch. This is relevant
for drivers that have a slow start/stop sequence. However, this is
tricky to do correctly as it requires dynamic start/stop and also
tracking which world the current hardware state belongs to.
