linux-kernel - Re: [PATCH RFC v2 2/2] hwspinlock: qcom: Lock #7 is special lock, uses dynamic proc

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Date:	Wed, 10 Jun 2015 14:13:19 -0600
From:	Lina Iyer <lina.iyer@...aro.org>
To:	Bjorn Andersson <bjorn@...o.se>
Cc:	Ohad Ben-Cohen <ohad@...ery.com>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeffrey Hugo <jhugo@...eaurora.org>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Andy Gross <agross@...eaurora.org>
Subject: Re: [PATCH RFC v2 2/2] hwspinlock: qcom: Lock #7 is special lock,
 uses dynamic proc_id

On Wed, Jun 10 2015 at 11:33 -0600, Bjorn Andersson wrote:
>On Tue, Jun 9, 2015 at 9:23 AM, Lina Iyer <lina.iyer@...aro.org> wrote:
>> Hwspinlocks are widely used between processors in an SoC, and also
>> between elevation levels within in the same processor.  QCOM SoC's use
>> hwspinlock to serialize entry into a low power mode when the context
>> switches from Linux to secure monitor.
>>
>> Lock #7 has been assigned for this purpose. In order to differentiate
>> between one cpu core holding a lock while another cpu is contending for
>> the same lock, the proc id written into the lock is (128 + cpu id). This
>> makes it unique value among the cpu cores and therefore when a core
>> locks the hwspinlock, other cores would wait for the lock to be released
>> since they would have a different proc id.  This value is specific for
>> the lock #7 only.
>>
>> Declare lock #7 as raw capable, so the hwspinlock framework would not
>> enfore acquiring a s/w spinlock before acquiring the hwspinlock.
>>
>
>Hi Lina,
>
>Very sorry for slacking off and missing v1 of this.
>
No worries. Thanks for reviewing.

>I'm puzzled to the concept of using the hwspinlock framework for
>lock-only locks. The patch your proposed is rather clean and as long
>as there's no lock-debugging added to the framework it would work...
>
>
>Blindly declaring lock #7 as special on all Qualcomm hwspinlocks I do
>however not like at all. There's nothing in either the SFPB nor TCSR
>mutex hardware that dictates this fact, it's a system configuration
>fact. As such this "requirement" should be described in the device
>tree.
>
Its not a mutable entity, but sure.

>The puzzling part of the value to be written is strongly cpuidle
>implementation defined makes me wonder if it belong in this driver at
>all.
>
>At least this should be configured/flagged by some devicetree
>property. "qcom,lock-by-cpu-id-locks = <7, ...>"?
>
Okay.

>
>The other alternative to these patches would be to just consume the
>syscon in cpuidle and opencode the locking there. It isolates the
>cpuidle specifics of this to the original place and it isn't using
>only one side of the hwspinlock framework...
>
Well, ultimately a hwspinlock is just a writel, so that is a
possibility, if we want. But it is a hwspinlock, therefore the use of
the framework seems appropriate, even amidst the unique behavior of the
lock.

Thanks,
Lina


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