linux-kernel - Re: [PATCH v7 1/4] Documentation: dt: add common bindings for hwspinlock

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Message-ID: <54B94F2F.3050303@ti.com>
Date:	Fri, 16 Jan 2015 11:49:35 -0600
From:	Suman Anna <s-anna@...com>
To:	Mark Rutland <mark.rutland@....com>,
	Rob Herring <robherring2@...il.com>
CC:	Ohad Ben-Cohen <ohad@...ery.com>,
	Kumar Gala <galak@...eaurora.org>,
	Bjorn Andersson <bjorn@...o.se>,
	Josh Cartwright <joshc@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v7 1/4] Documentation: dt: add common bindings for hwspinlock

On 01/16/2015 04:19 AM, Mark Rutland wrote:
> On Thu, Jan 15, 2015 at 02:42:23PM +0000, Rob Herring wrote:
>> On Thu, Jan 15, 2015 at 7:55 AM, Mark Rutland <mark.rutland@....com> wrote:
>>> On Thu, Jan 15, 2015 at 01:52:01PM +0000, Mark Rutland wrote:
>>>> On Wed, Jan 14, 2015 at 08:58:18PM +0000, Suman Anna wrote:
>>>>> This patch adds the generic common bindings used to represent
>>>>> a hwlock device and use/request locks in a device-tree build.
>>>>>
>>>>> All the platform-specific hwlock driver implementations need the
>>>>> number of locks and associated base id for registering the locks
>>>>> present within the device with the driver core. This base id
>>>>> needs to be unique across multiple IP instances of a hwspinlock
>>>>> device, so that each hwlock can be represented uniquely in a
>>>>> system.
>>>>>
>>>>> The number of locks is represented by 'hwlock-num-locks' property,
>>>>> and the base id is represented by the 'hwlock-base-id' property.
>>>>> The args specifier length is dependent on each vendor-specific
>>>>> implementation and is represented through the '#hwlock-cells'
>>>>> property. Client users need to use the property 'hwlocks' for
>>>>> requesting specific lock(s).
>>>>>
>>>>> Note that the document is named hwlock.txt deliberately to keep
>>>>> it a bit more generic.
>>>>>
>>>>> Cc: Rob Herring <robh+dt@...nel.org>
>>>>> Signed-off-by: Suman Anna <s-anna@...com>
>>>>> ---
>>>>> v7: Revised binding info for hwlock-base-id, it is mandatory now
>>>>>
>>>>>  .../devicetree/bindings/hwlock/hwlock.txt          | 55 ++++++++++++++++++++++
>>>>>  1 file changed, 55 insertions(+)
>>>>>  create mode 100644 Documentation/devicetree/bindings/hwlock/hwlock.txt
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/hwlock/hwlock.txt b/Documentation/devicetree/bindings/hwlock/hwlock.txt
>>>>> new file mode 100644
>>>>> index 000000000000..8de7aaf878f9
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/hwlock/hwlock.txt
>>>>> @@ -0,0 +1,55 @@
>>>>> +Generic hwlock bindings
>>>>> +=======================
>>>>> +
>>>>> +Generic bindings that are common to all the hwlock platform specific driver
>>>>> +implementations.
>>>>> +
>>>>> +The validity and need of these common properties may vary from one platform
>>>>> +implementation to another. The platform specific bindings should explicitly
>>>>> +state if an optional property is used. Please also look through the individual
>>>>> +platform specific hwlock binding documentations for identifying the applicable
>>>>> +properties.
>>>>> +
>>>>> +Common properties:
>>>>> +- #hwlock-cells:   Specifies the number of cells needed to represent a
>>>>> +                   specific lock. This property is mandatory for all
>>>>> +                   platform implementations.
>>>>> +- hwlock-num-locks:        Number of locks present in a hwlock device. This
>>>>> +                   property is needed on hwlock devices, where the number
>>>>> +                   of supported locks within a hwlock device cannot be
>>>>> +                   read from a register.
>>>>> +- hwlock-base-id:  An unique base Id for the locks for a particular hwlock
>>>>> +                   device. This property is mandatory for all platform
>>>>> +                   implementations.
>>>>
>>>> This property makes no sense. The ID encoded in the hwlock cells is
>>>> relative to the instance (identified by phandle), not global. So the DT
>>>> has no global ID space.
>>>>
>>>> Why do you think you need this?
>>>
>>> Having looked at the way this proeprty is used, NAK.
>>>
>>> If you need to carve up a Linux-internal ID space, do that dynamically.
>>> There is no need for this property.
>>
>> Better yet, don't create a Linux ID space for this. Everywhere we have
>> one, we want to get rid of it.
> 
> Agreed. A completely opaque token / desc structure would prevent a lot
> of potential abuse and save us from painful breakage.

The regular API to acquire or release a lock on Linux does indeed use
opaque handles, but the id space is needed for communication with other
processors as the handles have no meaning on non-Linux processors. The
id space is the simplest to exchange which lock to use with other
processors compared to the device instance plus the lock number within
that device.

regards
Suman

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