linux-kernel - Re: [PATCH RFC v2 0/2] hwspinlock: Introduce raw capability for hwspinlock

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Message-ID: <CAK=WgbatEexJNMipJpE7z5MkMO99dat=L=jH6uKMg+eR3x6CwA@mail.gmail.com>
Date:	Thu, 13 Aug 2015 09:34:13 +0300
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	Lina Iyer <lina.iyer@...aro.org>
Cc:	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Jeffrey Hugo <jhugo@...eaurora.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 0/2] hwspinlock: Introduce raw capability for hwspinlock_device

On Wed, Jul 29, 2015 at 12:51 AM, Lina Iyer <lina.iyer@...aro.org> wrote:
>> Let's not make this more complicated than needed, so please add the
>> hwcaps member to hwspinlock_device instead of to hwspinlock struct. We
>> could always change this later if it proves to be insufficient.
>>
> But this could yield wrong locking scenarios. If banks are allowed RAW
> capability and is not enforced on a per-lock basis, a driver may lock
> using non-raw lock using the _raw API, while another driver may
> 'acquire' the lock (since the value written to the lock would be the
> same as raw api would). That is why you should have the capability on
> hwspinlock and not on hwspinlock_device. Locks that are defined are RAW
> capable should be used as RAW only.
>
> QCOM platform hwlock #7 is unique that different CPUs trying to acquire
> the lock would write different values and hence would be fine. But, the
> same is not true for other locks in the bank.

As far as I understand, there is nothing special about QCOM's hwlock
#7 in terms of hardware. It's exactly the same lock as all the others.

The only difference in hwlock #7 is the way you use it, and that
sounds like a decision the driver should be able to make. It's a
policy, and I'm not sure we should put it in the DT. I'm also not sure
we need this hwlock-specific complexity in the hwspinlock framework.

The driver already makes a decision whether to disable the interrupts
or not and whether to save their state or not. So it can also make a
decision whether to take a sw spinlock at all or not --- if the
hardware allows it. and that if should be encoded in an accessible
vendor specific (not hwlock specific) struct, which is setup by the
underlying vendor specific hwspinlock driver (no DT involved).

Let's go over your aforementioned concerns:
> But this could yield wrong locking scenarios. If banks are allowed RAW
> capability and is not enforced on a per-lock basis, a driver may lock
> using non-raw lock using the _raw API

If this is allowed by the hardware, then this is a valid scenario.
There's no such thing a non-raw lock: a lock is raw if a raw
functionality is required.

> while another driver may
> 'acquire' the lock (since the value written to the lock would be the
> same as raw api would).

Not sure I understand this one. If a lock has already been assigned to
a driver, it cannot be re-assigned to another driver.

Thanks,
Ohad.
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