Re: [PATCH] firmware: arm_scmi: Give SMC transport precedence over mailbox

[Florian Fainelli]

Hi Cristian,

On October 7, 2024 4:52:33 AM PDT, Cristian Marussi 
<cristian.marussi@xxxxxxx> wrote:
> On Sat, Oct 05, 2024 at 09:33:17PM -0700, Florian Fainelli wrote:
> > Broadcom STB platforms have for historical reasons included both
> > "arm,scmi-smc" and "arm,scmi" in their SCMI Device Tree node compatible
> > string.
>
> Hi Florian,
>
> did not know this..

It stems from us starting with a mailbox driver that did the SMC call, 
and later transitioning to the "smc" transport proper. Our boot loader 
provides the Device Tree blob to the kernel and we maintain 
backward/forward compatibility as much as possible.

> > After the commit cited in the Fixes tag and with a kernel
> > configuration that enables both the SCMI and the Mailbox transports, we
> > would probe the mailbox transport, but fail to complete since we would
> > not have a mailbox driver available.
> Not sure to have understood this...
>
> ...you mean you DO have the SMC/Mailbox SCMI transport drivers compiled
> into the Kconfig AND you have BOTH the SMC AND Mailbox compatibles in
> DT, BUT your platform does NOT physically have a mbox/shmem transport
> and as a consequence, when MBOX probes (at first), you see an error from
> the core like:
>
>    "arm-scmi: unable to communicate with SCMI"
>
> since it gets no reply from the SCMI server (being not connnected via
> mbox) and it bails out .... am I right ?

In an unmodified kernel where both the "mailbox" and "smc" transports 
are enabled, we get the "mailbox" driver to probe first since it matched 
the "arm,scmi" part of the compatible string and it is linked first into 
the kernel. Down the road though we will fail the initialization with:

[    1.135363] arm-scmi arm-scmi.1.auto: Using scmi_mailbox_transport
[    1.141901] arm-scmi arm-scmi.1.auto: SCMI max-rx-timeout: 30ms
[    1.148113] arm-scmi arm-scmi.1.auto: failed to setup channel for 
protocol:0x10
[    1.155828] arm-scmi arm-scmi.1.auto: error -EINVAL: failed to setup 
channels
[    1.163379] arm-scmi arm-scmi.1.auto: probe with driver arm-scmi 
failed with error -22

Because the platform device is now bound, and there is no mechanism to 
return -ENODEV, we won't try another transport driver that would attempt 
to match the other compatibility strings. That makes sense because in 
general you specify the Device Tree precisely, and you also have a 
tailored kernel configuration. Right now this is only an issue using 
arm's multi_v7_defconfig and arm64's defconfig both of which that we 
intend to keep on using for CI purposes.


> If this is the case, without this patch, after this error and the mbox probe
> failing, the SMC transport, instead, DO probe successfully at the end, right ?

With my patch we probe the "smc" transport first and foremost and we 
successfully initialize it, therefore we do not even try the "mailbox" 
transport at all, which is intended.

> IOW, what is the impact without this patch, an error and a delay in the
> probe sequence till it gets to the SMC transport probe 9as second
> attempt) or worse ? (trying to understand here...)

There is no recovery without the patch, we are not giving up the 
arm_scmi platform device because there is no mechanism to return -ENODEV 
and allow any of the subsequent transport drivers enabled to attempt to 
take over the platform device and probe it again.

Thanks!
--
Florian