Re: [PATCH v3 02/22] dt-bindings: arm: add support for ARM System Control and Management Interface(SCMI) protocol

[Sudeep Holla <sudeep.holla@arm.com>]

On 04/10/17 13:35, Arnd Bergmann wrote:
> On Wed, Oct 4, 2017 at 1:07 PM, Sudeep Holla <sudeep.holla@arm.com> wrote:
>> Hi Arnd,
>>
>> Thanks for taking a look at this.
>>
>> On 04/10/17 11:50, Arnd Bergmann wrote:
>>> On Thu, Sep 28, 2017 at 3:11 PM, Sudeep Holla <sudeep.holla@arm.com> wrote:
>>>> +
>>>> +The SCMI is intended to allow agents such as OSPM to manage various functions
>>>> +that are provided by the hardware platform it is running on, including power
>>>> +and performance functions.
>>>> +
>>>> +This binding is intended to define the interface the firmware implementing
>>>> +the SCMI as described in ARM document number ARM DUI 0922B ("ARM System Control
>>>> +and Management Interface Platform Design Document")[0] provide for OSPM in
>>>> +the device tree.
>>>> +
>>>> +Required properties:
>>>> +
>>>> +The scmi node with the following properties shall be under the /firmware/ node.
>>>> +
>>>> +- compatible : shall be "arm,scmi"
>>>> +- mboxes: List of phandle and mailbox channel specifiers. It should contain
>>>> +         exactly one or two mailboxes, one for transmitting messages("tx")
>>>> +         and another optional for receiving the notifications("rx") if
>>>> +         supported.
>>>> +- mbox-names: shall be "tx" or "rx"
>>>
>>> The example below does not have the mbox-names property. If you require
>>> exactly two mailboxes, why do you need the names anyway?
>>>
>>
>> Good question. I can drop it, but would like to keep in case we need to
>> extend it in future. We can always use then to identify.
> 
> I don't think it's necessary, as long you always need to have the first two,
> but it doesn't hurt either.
>
> Just make the description match the example.
> 

Sure.

>>> However, your example does have a #addresss-cells/#size-cells
>>> property that are not documented here. Please add them here as either
>>> optional or required, and describe what the permitted values are and
>>> how the address is interpreted.
>>>
>>
>> Ah right, I didn't notice that. I will add it. It was added to provide
>> the protocol number in "reg" property.
> ...
>>> How does the OS identify the fact that a subnode uses the clock binding?
>>> Do you need to look for the #clock-cells property, or is this based on the
>>> unit address?
>>>
>>
>> Yes it depends on #clock-cells property. That's the main reason for
>> adding #clock-cells
> 
> I'm still unclear on this. Do you mean we look for a subnode with
> reg=<0x14> and then assume it's a clock node and require the
>  #clock-cells to be there, 

Yes that's how it's used. Presence of subnode with reg=0x14 indicates
clock protocol and #clock-cells to indicate that it's clock provider
expecting 1 parameter from consumer which is the clock identifier.

or do we look through the sub-nodes to
> find one with the #clock-cells property and then look up the 'reg'
> property to find out which protocol number to use?
> 

Not this way. Do you see any issues ?

>>>> +- shmem : List of phandle pointing to the shared memory(SHM) area as per
>>>> +         generic mailbox client binding.
>>>> +
>>>> +See Documentation/devicetree/bindings/mailbox/mailbox.txt for more details
>>>> +about the generic mailbox controller and client driver bindings.
>>>> +
>>>> +The mailbox is the only permitted method of calling the SCMI firmware.
>>>> +Mailbox doorbell is used as a mechanism to alert the presence of a
>>>> +messages and/or notification.
>>>
>>> This looks odd: why not make the message itself part of the mailbox
>>> protocol here, and leave the shmem as a implementation detail of the
>>> mailbox driver?
>>>
>>
>> I am not sure if I follow you here. But generally shmem can be memory
>> carved out of anything in the system and it's dependent on the protocol
>> and the remote firmware rather than the mailbox hardware itself.
> 
> I think the problem is the way we use the mailbox API in Linux, which
> is completely abstract at the moment: it could be a pure doorbell, a
> single-register for a data, some structured memory, or a
> variable-length message. The assumption today is that the mailbox
> user and the mailbox driver agree on the interpretation of that
> void pointer.
> 

Unfortunately true.

> This breaks down here, as you require the message to be a
> variable-length message in a fixed physical location, but assume that
> the mailbox serves only as a doorbell.
> 

Yes.

> The solution might be to extend the mailbox API slightly, to
> have explicit support for variable-length messages, and implement
> support for that in either mailbox drivers or as an abstraction
> on top of doorbell-type mailboxes.
> 
I got the concept. But are you also suggesting that in bindings it shmem
should be associated with mailbox controller rather than client ?

-- 
Regards,
Sudeep