Re: [PATCH 1/2] dt-bindings: firmware: arm,scmi: Document arm,poll-transport property

[Marek Vasut <marek.vasut@mailbox.org>]

On 10/23/25 4:00 PM, Marek Vasut wrote:

Hello again,

>> On Thu, Oct 23, 2025 at 02:35:57PM +0200, Marek Vasut wrote:
>>> Document new property arm,poll-transport, which sets all SCMI 
>>> operation into
>>> poll mode. This is meant to work around uncooperative SCP 
>>> implementations,
>>> which do not generate completion interrupts. This applies primarily 
>>> on mbox
>>> based implementations, but does also cover SMC and VirtIO ones.
>>
>> Hi,
>>
>> ..indeed I was thinking a while ago about exposing the existing force- 
>> polling
>> switch but in my case it was purely a testing-scenario configuration, 
>> so a
>> no-no for the DT, things are different if you have to describe an HW 
>> that has
>> no completion IRQ also on the a2p channel...
> 
> Correct, at least until the SCP on this hardware is updated.
> 
>> ...having said that, though, usually polling-mode is reserved to a few
>> selected commands in a few chosen scenarios (as you may have seen),
>> 'carpet-polling' non-for-testing for all the commands on A2P seems a lot
>> inefficient and heavy...is it really a viable solution ? or these
>> systems use such a low rate of SCMI messages that polling after each and
>> every message is negligible ?
>>
>> ..just to understand the context...
> 
> These systems are early in development and it is likely that the SCP 
> will be updated to generate interrupts properly. Currently, this is not 
> the case, hence the carpet-polling, until this is resolved.

While I was going through the SCMI spec, DEN0056F , page 209 , section 
"4.1 Shared memory based transport" , bullet • Completion interrupts, I 
found it explicitly states:

"
This transport supports polling or interrupt driven modes of 
communication. In interrupt mode, when the callee completes processing a 
message, it raises an interrupt to the caller. Hardware support for 
completion interrupts is optional.
"

-- 
Best regards,
Marek Vasut