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    Subject[PATCH v3 0/4] Introduce hardware spinlock framework
    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

    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

    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, 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.


    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:


    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

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