Date:	Thu, 17 Nov 2011 10:43:46 +0530
From:	Varun Wadekar <vwadekar@...dia.com>
To:	Varun Wadekar <vwadekar@...dia.com>
CC:	"ohad@...ery.com" <ohad@...ery.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hwspinlock: core: support for TEGRA hardware spinlocks

On Tuesday 15 November 2011 11:54 AM, Varun Wadekar wrote:
> +LKML ml
>
> This is an idea that I wanted some review comments on (to check if I am
> on the right path), before the TEGRA driver gets written.

Any feedback on this patch?

> On Tuesday 15 November 2011 10:49 AM, Varun Wadekar wrote:
>> TEGRA has a hardware spinlock block which is used
>> to get exclusive access to the hardware blocks either
>> from the CPU or the COP. The TEGRA hardware spinlock
>> block has the capability to interrupt the requester
>> on success. For this the core need not disable
>> preemption or interrupts before calling into the
>> actual driver. Add lock_timeout to the ops structure
>> to facilitate this working and to maintain backward
>> compatibility.
>>
>> Signed-off-by: Varun Wadekar <vwadekar@...dia.com>
>> ---
>>  Documentation/hwspinlock.txt             |   39 ++++++++++++++++++++++++++---
>>  drivers/hwspinlock/hwspinlock_core.c     |   16 ++++++++++--
>>  drivers/hwspinlock/hwspinlock_internal.h |    3 ++
>>  3 files changed, 51 insertions(+), 7 deletions(-)
>>
>> diff --git a/Documentation/hwspinlock.txt b/Documentation/hwspinlock.txt
>> index a903ee5..35730b8 100644
>> --- a/Documentation/hwspinlock.txt
>> +++ b/Documentation/hwspinlock.txt
>> @@ -29,6 +29,18 @@ the remote processors, and access to it is synchronized using the hwspinlock
>>  module (remote processor directly places new messages in this shared data
>>  structure).
>>  
>> +The Tegra line of processors has a CPU and a COP similar to the OMAP processors.
>> +The Tegra SoC has a hardware block which arbitrates access to different hardware
>> +modules on the SoC between the CPU and the COP. The hardware spinlock block
>> +also has the capability to interrupt the caller when it has access to the
>> +requested hardware module. To facilitate SoCs like Tegra which do a lot more
>> +than just share data structures between the CPU and the COP, a new callback,
>> +int (*lock_timeout)(struct hwspinlock *lock, unsigned int to), has been added
>> +to struct hwspinlock_ops. The drivers which support functionality similar to
>> +the Tegra SoCs, will use lock_timeout to grant access to the hardware spinlock
>> +block on the SoCs. Such SoC drivers will not support all the apis exposed
>> +by the hwspinlock framework. More information below.
>> +
>>  A common hwspinlock interface makes it possible to have generic, platform-
>>  independent, drivers.
>>  
>> @@ -58,8 +70,11 @@ independent, drivers.
>>     - lock a previously-assigned hwspinlock with a timeout limit (specified in
>>       msecs). If the hwspinlock is already taken, the function will busy loop
>>       waiting for it to be released, but give up when the timeout elapses.
>> -     Upon a successful return from this function, preemption is disabled so
>> -     the caller must not sleep, and is advised to release the hwspinlock as
>> +     If the underlying hardware driver supports lock_timeout in it's ops structure
>> +     then upon a successful return from this function, the caller is allowed
>> +     to sleep since we do not disable premption or interrupts in this scenario.
>> +     If not, upon a successful return from this function, preemption is disabled
>> +     so the caller must not sleep, and is advised to release the hwspinlock as
>>       soon as possible, in order to minimize remote cores polling on the
>>       hardware interconnect.
>>       Returns 0 when successful and an appropriate error code otherwise (most
>> @@ -68,7 +83,10 @@ independent, drivers.
>>  
>>    int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout);
>>     - lock a previously-assigned hwspinlock with a timeout limit (specified in
>> -     msecs). If the hwspinlock is already taken, the function will busy loop
>> +     msecs).
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>> +     If the hwspinlock is already taken, the function will busy loop
>>       waiting for it to be released, but give up when the timeout elapses.
>>       Upon a successful return from this function, preemption and the local
>>       interrupts are disabled, so the caller must not sleep, and is advised to
>> @@ -80,7 +98,10 @@ independent, drivers.
>>    int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to,
>>  							unsigned long *flags);
>>     - lock a previously-assigned hwspinlock with a timeout limit (specified in
>> -     msecs). If the hwspinlock is already taken, the function will busy loop
>> +     msecs).
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>> +     If the hwspinlock is already taken, the function will busy loop
>>       waiting for it to be released, but give up when the timeout elapses.
>>       Upon a successful return from this function, preemption is disabled,
>>       local interrupts are disabled and their previous state is saved at the
>> @@ -93,6 +114,8 @@ independent, drivers.
>>    int hwspin_trylock(struct hwspinlock *hwlock);
>>     - attempt to lock a previously-assigned hwspinlock, but immediately fail if
>>       it is already taken.
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>>       Upon a successful return from this function, preemption is disabled so
>>       caller must not sleep, and is advised to release the hwspinlock as soon as
>>       possible, in order to minimize remote cores polling on the hardware
>> @@ -104,6 +127,8 @@ independent, drivers.
>>    int hwspin_trylock_irq(struct hwspinlock *hwlock);
>>     - attempt to lock a previously-assigned hwspinlock, but immediately fail if
>>       it is already taken.
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>>       Upon a successful return from this function, preemption and the local
>>       interrupts are disabled so caller must not sleep, and is advised to
>>       release the hwspinlock as soon as possible.
>> @@ -114,6 +139,8 @@ independent, drivers.
>>    int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags);
>>     - attempt to lock a previously-assigned hwspinlock, but immediately fail if
>>       it is already taken.
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>>       Upon a successful return from this function, preemption is disabled,
>>       the local interrupts are disabled and their previous state is saved
>>       at the given flags placeholder. The caller must not sleep, and is advised
>> @@ -130,6 +157,8 @@ independent, drivers.
>>  
>>    void hwspin_unlock_irq(struct hwspinlock *hwlock);
>>     - unlock a previously-locked hwspinlock and enable local interrupts.
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>>       The caller should _never_ unlock an hwspinlock which is already unlocked.
>>       Doing so is considered a bug (there is no protection against this).
>>       Upon a successful return from this function, preemption and local
>> @@ -138,6 +167,8 @@ independent, drivers.
>>    void
>>    hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags);
>>     - unlock a previously-locked hwspinlock.
>> +     Not supported if lock_timeout is exported from the ops struct by the
>> +     underlying driver.
>>       The caller should _never_ unlock an hwspinlock which is already unlocked.
>>       Doing so is considered a bug (there is no protection against this).
>>       Upon a successful return from this function, preemption is reenabled,
>> diff --git a/drivers/hwspinlock/hwspinlock_core.c b/drivers/hwspinlock/hwspinlock_core.c
>> index 61c9cf1..520c2c9 100644
>> --- a/drivers/hwspinlock/hwspinlock_core.c
>> +++ b/drivers/hwspinlock/hwspinlock_core.c
>> @@ -91,6 +91,10 @@ int __hwspin_trylock(struct hwspinlock *hwlock, int mode, unsigned long *flags)
>>  
>>  	BUG_ON(!hwlock);
>>  	BUG_ON(!flags && mode == HWLOCK_IRQSTATE);
>> +	BUG_ON(hwlock->bank->ops->trylock && hwlock->bank->ops->lock_timeout);
>> +
>> +	if (!hwlock->bank->ops->trylock)
>> +		return -EINVAL;
>>  
>>  	/*
>>  	 * This spin_lock{_irq, _irqsave} serves three purposes:
>> @@ -182,6 +186,9 @@ int __hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int to,
>>  
>>  	expire = msecs_to_jiffies(to) + jiffies;
>>  
>> +	if (hwlock->bank->ops->lock_timeout)
>> +		return hwlock->bank->ops->lock_timeout(hwlock, expire);
>> +
>>  	for (;;) {
>>  		/* Try to take the hwspinlock */
>>  		ret = __hwspin_trylock(hwlock, mode, flags);
>> @@ -245,7 +252,10 @@ void __hwspin_unlock(struct hwspinlock *hwlock, int mode, unsigned long *flags)
>>  	 */
>>  	mb();
>>  
>> -	hwlock->bank->ops->unlock(hwlock);
>> +	if (hwlock->bank->ops->lock_timeout)
>> +		return hwlock->bank->ops->unlock(hwlock);
>> +	else
>> +		hwlock->bank->ops->unlock(hwlock);
>>  
>>  	/* Undo the spin_trylock{_irq, _irqsave} called while locking */
>>  	if (mode == HWLOCK_IRQSTATE)
>> @@ -328,8 +338,8 @@ int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev,
>>  	struct hwspinlock *hwlock;
>>  	int ret = 0, i;
>>  
>> -	if (!bank || !ops || !dev || !num_locks || !ops->trylock ||
>> -							!ops->unlock) {
>> +	if (!bank || !ops || !dev || !num_locks ||
>> +	    (!ops->trylock && !ops->lock_timeout) || !ops->unlock) {
>>  		pr_err("invalid parameters\n");
>>  		return -EINVAL;
>>  	}
>> diff --git a/drivers/hwspinlock/hwspinlock_internal.h b/drivers/hwspinlock/hwspinlock_internal.h
>> index d26f78b..b0ba642 100644
>> --- a/drivers/hwspinlock/hwspinlock_internal.h
>> +++ b/drivers/hwspinlock/hwspinlock_internal.h
>> @@ -26,6 +26,8 @@ struct hwspinlock_device;
>>  /**
>>   * struct hwspinlock_ops - platform-specific hwspinlock handlers
>>   *
>> + * @lock_timeout: attempt to take the lock and wait with a timeout.
>> + * 		  returns 0 on failure and true on success. may_sleep.
>>   * @trylock: make a single attempt to take the lock. returns 0 on
>>   *	     failure and true on success. may _not_ sleep.
>>   * @unlock:  release the lock. always succeed. may _not_ sleep.
>> @@ -35,6 +37,7 @@ struct hwspinlock_device;
>>   */
>>  struct hwspinlock_ops {
>>  	int (*trylock)(struct hwspinlock *lock);
>> +	int (*lock_timeout)(struct hwspinlock *lock, unsigned int to);
>>  	void (*unlock)(struct hwspinlock *lock);
>>  	void (*relax)(struct hwspinlock *lock);
>>  };

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