Re: [PATCH] firmware: arm_scmi: Queue in scmi layer for mailbox implementation

[Sudeep Holla <sudeep.holla@arm.com>]

On Tue, Oct 08, 2024 at 01:10:39PM +0100, Cristian Marussi wrote:
> On Mon, Oct 07, 2024 at 10:58:47AM -0700, Justin Chen wrote:
> > Thanks for the response. I'll try to elaborate.
> > 
> > When comparing SMC and mailbox transport, we noticed mailbox transport
> > timesout much quicker when under load. Originally we thought this was the
> > latency of the mailbox implementation, but after debugging we noticed a
> > weird behavior. We saw SMCI transactions timing out before the mailbox even
> > transmitted the message.
> > 
> > This issue lies in the SCMI layer. drivers/firmware/arm_scmi/driver.c
> > do_xfer() function.
> > 
> > The fundamental issue is send_message() blocks for SMC transport, but
> > doesn't block for mailbox transport. So if send_message() doesn't block we
> > can have multiple messages waiting at scmi_wait_for_message_response().
> > 
> 
> oh...yes...now I can see it...tx_prepare is really never called given
> how the mailbox subsystem de-queues messages once at time...so we end up
> waiting for a reply to some message that is still to be sent...so the
> message inflight is really NOT corrupted because the next remain pending
> until the reply in the shmem is read back , BUT the timeout will drift away
> if you multiple inflights are pending to be sent...
>

Indeed.

> > SMC looks like this
> > CPU #0 SCMI message 0 -> calls send_message() then calls
> > scmi_wait_for_message_response(), timesout after 30ms.
> > CPU #1 SCMI message 1 -> blocks at send_message() waiting for SCMI message 0
> > to complete.
> > 
> > Mailbox looks like this
> > CPU #0 SCMI message 0 -> calls send_message(), mailbox layer queues up
> > message, mailbox layer sees no message is outgoing and sends it. CPU waits
> > at scmi_wait_for_message_response(), timesout after 30ms
> > CPU #1 SCMI message 1 -> calls send_message(), mailbox layer queues up
> > message, mailbox layer sees message pending, hold message in queue. CPU
> > waits at scmi_wait_for_message_response(), timesout after 30ms.
> > 
> > Lets say if transport takes 25ms. The first message would succeed, the
> > second message would timeout after 5ms.
> > 
> > Hopefully this makes sense.
> 
> Yes, of course, thanks, for reporting this, and taking time to
> explain...
> 
> ...in general the patch LGTM...I think your patch is good also because it
> could be easily backported as a fix....can you add a Fixes tag in your
> next version ?
>

Are you seeing this issue a lot ? IOW, do we need this to be backported ?

> Also can you explain in more detail the issue and the solution in the commit
> message....that will help having it merged as a Fix in stables...
>
> ...for the future (definitely NOT in this series) we could probably think to
> get rid of the sleeping mutex in favour of some other non-sleeping form of
> mutual exclusion around the channnel (like in SMC transport) and enable
> (optionally) Atomic transmission support AND also review if the shmem
> layer busy-waiting in txprepare is anymore needed at all...
>

Agreed, if we are locking the channel in SCMI, we can drop the busy-waiting
in tx_prepare and the associated details in the comment as this locking
voids that. It is better have both the changes in the same patch to indicate
the relation between them.

--
Regards,
Sudeep