[PATCH v3 0/4] Introduce hardware spinlock framework

Ohad Ben-Cohen
2010-12-03T23:50:43Z

OMAP4 introduces a Hardware Spinlock device, which provides hardware
assistance for synchronization and mutual exclusion between heterogeneous
processors and those not operating under a single, shared operating system
(e.g. OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP).

The intention of this hardware device is to allow remote processors,
that have no alternative mechanism to accomplish synchronization and mutual
exclusion operations, to share resources (such as memory and/or any other
hardware resource).

This patchset adds hwspinlock framework that makes it possible
for drivers to use those hwspinlock devices and stay platform-independent.

Currently there are two users for this hwspinlock interface:

1. Inter-processor communications: on OMAP4, cpu-intensive multimedia
tasks are offloaded by the host to the remote M3 and/or C64x+ slave
processors.

To achieve fast message-based communications, a minimal kernel support
is needed to deliver messages arriving from a remote processor to the
appropriate user process.

This communication is based on a simple data structure that is shared between
the remote processors, and access to it is synchronized using the hwspinlock
module (remote processor directly places new messages in this shared data
structure).

2. On some OMAP4 boards, the I2C bus is shared between the A9 and the M3,
and the hwspinlock is used to synchronize access to it.

While (2) can get away with an omap-specific hwspinlock implementation,
(1) is by no means omap-specific, and a common hwspinlock interface is
needed to keep it generic, in hopes that it will be useful for other
platforms as well.

Changes v2->v3:
- Remove the timeout-less _lock API variant (Tony)
- s/state->io_base/io_base/ (Ionut)
- Remove the "generic" wording (David)
- s/hwspinlock_/hwspin_lock_/ (Mugdha)
- Use MAX_SCHEDULE_TIMEOUT to indicate no timeout (Mugdha)
- Be more verbose on egregious API misuse (Olof)
- locking API misuse is BUG_ON material (Russell)

Note: Russell also suggested compiling out NULL checks on ARM.
I've posted an initial proposal for this (see
https://lkml.org/lkml/2010/11/29/96), which I'm going to resubmit
separately. If accepted, I'll adopt hwspinlocks to use it.

Patches are tested against linux-omap-2.6 master, which is 2.6.37-rc4 plus
omap material. All, but the third (the hwmod omap patch), apply on top of
mainline 2.6.37-rc4 as well

Changes v1->v2:
- Convert to a generic interface (Tony)
- API should silently succeed if framework isn't built (Greg)
- Don't use ERR_PTR pattern (Grant)
- Use tristate, fix and extend commentary (Kevin)
- Provide API flexibility regarding irq handling (Arnd, Grant)

Note: after reviewing OMAP's L4 access times, and comparing them with
external memory latencies, I can say that there is no notable difference.
Because of that, we can safely treat the hwspinlock like we do
with regular spinlocks: preemption should be disabled, but whether
to disable interrupts or not is up to the caller.

So despite the TRM's recommendation to always disable local interrupts
when taking an OMAP Hardware Spinlock, I have decided to allow callers
not to do that (by providing the full extent of hwspin_lock(),
hwspin_lock_irq() and hwspin_lock_irqsave() API).

Just like regular spinlocks, it's up to the callers to decide whether
interrupts should be disabled or not.

Sleeping, btw, is still prohibited of course.

Contributions:

Previous versions of an omap-specific hwspinlock driver circulated in
linux-omap several times, and received substantial attention and contribution
from many developers (see [1][2][3][4][5][6]):

Simon Que did the initial implementation and pushed several iterations
Benoit Cousson provided extensive review, help, improvements and hwmod support
Hari Kanigeri helped out when Simon was away
Sanjeev Premi, Santosh Shilimkar and Nishanth Menon did lots of review

I'd like to thank Benoit Cousson, Steve Krueger, Hari Kanigeri,
Nourredine Hamoudi and Richard Woodruff for useful discussions about
the OMAP Spinlock requirements and use-cases.

Relevant linux-omap threads:

[1] http://thread.gmane.org/gmane.linux.ports.arm.omap/38755
[2] http://thread.gmane.org/gmane.linux.ports.arm.omap/38917
[3] http://thread.gmane.org/gmane.linux.ports.arm.omap/39187
[4] http://thread.gmane.org/gmane.linux.ports.arm.omap/39365
[5] http://thread.gmane.org/gmane.linux.ports.arm.omap/39815
[6] http://thread.gmane.org/gmane.linux.ports.arm.omap/40901

Benoit Cousson (1):
OMAP4: hwmod data: Add hwspinlock

Ohad Ben-Cohen (1):
drivers: hwspinlock: add framework

Simon Que (2):
drivers: hwspinlock: add OMAP implementation
omap: add hwspinlock device

Documentation/hwspinlock.txt | 299 +++++++++++++++
arch/arm/mach-omap2/Makefile | 1 +
arch/arm/mach-omap2/hwspinlock.c | 63 ++++
arch/arm/mach-omap2/omap_hwmod_44xx_data.c | 64 ++++
drivers/Kconfig | 2 +
drivers/Makefile | 1 +
drivers/hwspinlock/Kconfig | 22 ++
drivers/hwspinlock/Makefile | 6 +
drivers/hwspinlock/hwspinlock.h | 61 +++
drivers/hwspinlock/hwspinlock_core.c | 557 ++++++++++++++++++++++++++++
drivers/hwspinlock/omap_hwspinlock.c | 231 ++++++++++++
include/linux/hwspinlock.h | 298 +++++++++++++++
12 files changed, 1605 insertions(+), 0 deletions(-)
create mode 100644 Documentation/hwspinlock.txt
create mode 100644 arch/arm/mach-omap2/hwspinlock.c
create mode 100644 drivers/hwspinlock/Kconfig
create mode 100644 drivers/hwspinlock/Makefile
create mode 100644 drivers/hwspinlock/hwspinlock.h
create mode 100644 drivers/hwspinlock/hwspinlock_core.c
create mode 100644 drivers/hwspinlock/omap_hwspinlock.c
create mode 100644 include/linux/hwspinlock.h