Re: [PATCH v3 03/22] dt-bindings: arm: scmi: add ARM MHU specific mailbox client bindings

[Jassi Brar]

On Tue, Oct 10, 2017 at 4:27 AM, Rob Herring <robh@xxxxxxxxxx> wrote:
> On Mon, Oct 9, 2017 at 9:46 AM, Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
>> On Mon, Oct 9, 2017 at 7:22 PM, Rob Herring <robh@xxxxxxxxxx> wrote:
>>> On Fri, Oct 6, 2017 at 9:26 PM, Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
>>>> On Fri, Oct 6, 2017 at 9:24 PM, Rob Herring <robh@xxxxxxxxxx> wrote:
>>>>> On Fri, Oct 6, 2017 at 6:01 AM, Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
>>>>>> On Fri, Oct 6, 2017 at 4:50 AM, Rob Herring <robh@xxxxxxxxxx> wrote:
>>>>>>> On Thu, Sep 28, 2017 at 02:11:27PM +0100, Sudeep Holla wrote:
>>>>
>>>>>>>
>>>>>>>> +- mbox-data : For each phandle listed in mboxes property, an unsigned 32-bit
>>>>>>>> +           data as expected by the mailbox controller
>>>>>>>
>>>>>>> Shouldn't that be cells as part of mboxes property?
>>>>>>>
>>>>>> A MHU client can send any number of commands (such u32 values) over a channel.
>>>>>> This client (SCMI) sends just one command over a channel, but other
>>>>>> clients may/do send two or more.
>>>
>>> The above definition doesn't support 2 or more as it is 1-1 with channels.
>>>
>> I thought you suggested to make controller driver accept the command
>> as another cell in client's mboxes property.
>> Which we can't do.
>
> Yes, agreed. But I'm wondering since a client may need more than one,
> how would that be expressed?
>
Most (except SCMI) protocols are proprietary and can not be used
generically, so the command codes get hardcoded in the client driver.
SCMI+MHU is going to be rare case when we have a chance to get the code via DT.

>>>>> Okay, then I guess I don't understand why this is in DT.
>>>>>
>>>> Yeah the client has to provide code (u32 value) for the commands it
>>>> sends, and that value is going to be platform specific. For example,
>>>> on Juno the ITS_AN_SCMI_COMMAND may be defined as BIT(7) while on my
>>>> platform it may be 0x4567
>>>>
>>>> For MHU based platforms, it becomes easy if the u32 is passed from DT.
>>>> And that should be ok since that is like a h/w parameter - a value
>>>> chosen/expected by the remote firmware.
>>>
>>> Could it ever be more than 1 cell?
>>>
>> SCMI sends sub-commands via SHMEM, so it is always going to be 1cell for _scmi_.
>> However many firmwares are unlikely to use just one command over a
>> channel - say, the protocol is trivial or the linux and remote have no
>> SHMEM.
>
> I'd hope the normal case is not enumerating commands and sub-commands
> in DT and this is special case of a "generic" protocol with platform
> specific aspects.
>
Yes. It is only for SCMI protocol running over the variations of MHU controller.

Cheers