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

[Arnd Bergmann]

On Wed, Oct 4, 2017 at 4:47 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> On 04/10/17 15:17, Arnd Bergmann wrote:
>> On Wed, Oct 4, 2017 at 3:53 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>>> On 04/10/17 13:35, Arnd Bergmann wrote:
>>>> On Wed, Oct 4, 2017 at 1:07 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:

>> There are probably several ways of doing this better, we should see
>> what the best is we can come up with.
>>
>> I think generally speaking we need a way for a mailbox user to
>> know what it should use as the mailbox data here, so it is
>> able to talk to different incompatible mailbox providers.
>>
>> One idea I had is to use a nested mailbox driver, that turns
>> a doorbell or single-register styled mailbox into a variable-length
>> mailbox by adding a memory region, like
>>
>>     mailbox@1233000 {
>>         compatible = "vendor-hardware-specifc-id";
>>         interrupts = <34>;
>>         reg = <0x1233000 0x100>;
>>         #mbox-cells = <1>;
>>     };
>>
>>     mailbox {
>>            compatible = "shmem-mailbox";
>>            mboxes = <&/mailbox@1233000  25>;
>>            #mbox-cells = <1>;
>>            shmem = <&cpu_scp_lpri &cpu_scp_hpri>;
>>     };
>>
>> This would create one mailbox that only takes a register argument,
>> and another one that can take longer messages based on the first.
>> In your driver, you then refer to the second one and pass the
>> variable-length data into that directly.
>
> 1. IIUC it was intentional not to include shmem as part of mailbox
>    controller binding and was pushed to client drivers as it's generally
>    not part of mailbox IP block. I am not sure if there are any other
>    specific reasons for that, but I may be missing some facts.

Ok, I see.

> 2. I am not sure if we need nested driver/bindings (at-least to begin
>    with). On a platform I don't think both/all modes will be used.
>    I had  proposal for adding doorbell for ARM MHU based on extended
>    bindings, but it was rejected[1]. But I really preferred that over
>    the shim layer I had to add in v3.

Maybe we can come up with a more generic way to do doorbells
on top of mailboxes instead? This sounds like a problem that
would come back with other drivers, so the MHU-specific shim
will not be a permanent solution either.

       Arnd