linux-kernel - Re: [PATCH] Lock 7 is cpuidle specific, use non-generic value for locking

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Date:	Thu, 12 Mar 2015 16:16:00 -0600
From:	Lina Iyer <lina.iyer@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	bjorn.andersson@...ymobile.com, linux-arm-msm@...r.kernel.org,
	jhugo@...eaurora.org, agross@...eaurora.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Lock 7 is cpuidle specific, use non-generic value for
 locking

On Thu, Mar 12 2015 at 15:12 -0600, Stephen Boyd wrote:
>On 03/12/15 13:48, Lina Iyer wrote:
>> On Thu, Mar 12 2015 at 14:35 -0600, Stephen Boyd wrote:
>>> On 03/12/15 12:38, Lina Iyer wrote:
>>>> ---
>>>
>>> sign off?
>>>
>> :) I was just hacking it to make it easier to understand. Sure.
>>
>>>>  drivers/hwspinlock/qcom_hwspinlock.c | 15 +++++++++++----
>>>>  1 file changed, 11 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/hwspinlock/qcom_hwspinlock.c
>>>> b/drivers/hwspinlock/qcom_hwspinlock.c
>>>> index 93b62e0..7642524 100644
>>>> --- a/drivers/hwspinlock/qcom_hwspinlock.c
>>>> +++ b/drivers/hwspinlock/qcom_hwspinlock.c
>>>> @@ -25,16 +25,23 @@
>>>>
>>>>  #include "hwspinlock_internal.h"
>>>>
>>>> -#define QCOM_MUTEX_APPS_PROC_ID    1
>>>> -#define QCOM_MUTEX_NUM_LOCKS    32
>>>> +#define QCOM_MUTEX_APPS_PROC_ID        1
>>>> +#define QCOM_MUTEX_CPUIDLE_OFFSET    128
>>>> +#define QCOM_CPUIDLE_LOCK        7
>>>> +#define QCOM_MUTEX_NUM_LOCKS        32
>>>>
>>>>  static int qcom_hwspinlock_trylock(struct hwspinlock *lock)
>>>>  {
>>>>      struct regmap_field *field = lock->priv;
>>>>      u32 lock_owner;
>>>>      int ret;
>>>> +    u32 proc_id;
>>>>
>>>> -    ret = regmap_field_write(field, QCOM_MUTEX_APPS_PROC_ID);
>>>> +    proc_id = hwspin_lock_get_id(lock) == QCOM_CPUIDLE_LOCK ?
>>>> +            QCOM_MUTEX_CPUIDLE_OFFSET + smp_processor_id():
>>>
>>> So we assume that the caller will always be the CPU that is locking the
>>> lock? Also, do we assume that the remote side knows our CPU scheme here?
>>> smp_processor_id() returns the logical CPU and not the physical CPU
>>> number so hopefully the remote side doesn't care about logical CPU
>>> numbers being written to the lock value.
>>
>> The remote side (SCM) doesnt care the value written. We use 128+cpu to
>> be unique in Linux(128 is to make sure it doesnt clash with predefined
>> values used across by other processors.
>>
>>
>
>It looks like the remote side unlocks it too? It doesn't seem like this
>will work with the framework very well. The framework has a kernel
>spinlock attached to the hwspinlock so when we lock the hwspinlock we
>also lock the kernel spinlock and we only release the kernel spinlock
>when the kernel unlocks the hwspinlock. In this case it seems like
>cpuidle wants to have it's own kernel spinlock and just use the trylock
>loop part of __hwspin_lock_timeout() without taking any kernel side
>locks. Plus it wants to write a specific value to the lock.
>
Right.
Just noticed that part of the hwspinlock. Yes SCM unlocks the
hwspinlock. So I cannot hold any lock in Linux. May need changes in the
hwspinlock framework. Seems like an additional flag in hwspinlock to not
lock any in the trylock path work work. Hmm....



>-- 
>Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>a Linux Foundation Collaborative Project
>
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