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

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Date:   Wed, 4 Oct 2017 12:07:50 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        ALKML <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Roy Franz <roy.franz@...ium.com>,
        Harb Abdulhamid <harba@...eaurora.org>,
        Nishanth Menon <nm@...com>, Loc Ho <lho@....com>,
        Alexey Klimov <alexey.klimov@....com>,
        Ryan Harkin <Ryan.Harkin@....com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3 02/22] dt-bindings: arm: add support for ARM System
 Control and Management Interface(SCMI) protocol

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@....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.

> 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.

>> +- 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.

>> +Each protocol supported shall have a sub-node with corresponding compatible
>> +as described in the following sections. If the platform supports dedicated
>> +communication channel for a particular protocol, the 3 properties namely:
>> +mboxes, mbox-names and shmem shall be present in the sub-node corresponding
>> +to that protocol.
>> +
>> +Clock/Performance bindings for the clocks/OPPs based on SCMI Message Protocol
>> +------------------------------------------------------------
>> +
>> +This binding uses the common clock binding[1].
>> +
>> +Required properties:
>> +- #clock-cells : Should be 1. Contains the Clock ID value used by SCMI commands.
>> +
> 
> 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
-- 
Regards,
Sudeep