linux-kernel - Re: [PATCH] firmware: arm_scmi: add timeout in do_xfer_with

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Message-Id: <D8X9JJGPGDNL.1OTKIJODRFKNN@gmail.com>
Date: Thu, 03 Apr 2025 22:50:17 +0300
From: "Matthew Bystrin" <dev.mbstr@...il.com>
To: "Cristian Marussi" <cristian.marussi@....com>, "Sudeep Holla"
 <sudeep.holla@....com>
Cc: <arm-scmi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-arm-kernel@...ts.infradead.org>, "Philipp Zabel"
 <p.zabel@...gutronix.de>, "Peng Fan" <peng.fan@....com>
Subject: Re: [PATCH] firmware: arm_scmi: add timeout in
 do_xfer_with_response()

Hi Sudeep, Cristian,

Thanks for having a look on the patch.

Cristian Marussi, Apr 02, 2025 at 19:05:
> > Please post this patch along with the vendor specific protocols mentioned
> > above and with the reasoning as why 2s is not sufficient.
>
> Ack on this, it would be good to understand why a huge 2 secs is not
> enough...and also...

I've been working on firmware update using SCMI vendor/platform-specific
extension on FPGA prototype, so not posted it initially. I'm open to share the
details if needed, but need some extra time for preparations. For now I'm
posting a brief description of the extension. It has 2 commands:

- Obtain firmware version number.
- Update firmware. Firmware image is placed into shared physically contiguous
  memory, Agent sends to platform micro controller (PuC) physical address and
  size of the update image to start update procedure. After update is completed
  (successfully or not) PuC sends delayed response.

	Agent ----     start update         ---> Platform uC
	Agent <--- update procedure started ---- Platform uC
	...
	Agent <--- (async) update completed ---- Platform uC

I've faced timeout problem with the async completion response. And update can't
be done faster than 10s due to SPI flash write speed limit.

Why not to use notifications?

First of all, semantics. IIUC notifications can be sent by PuC in any time. This
is not suitable for updates, because procedure is initiated by an agent, not by
a platform.

Secondly, code implementing  notification waiting duplicates delayed response
code. I had implemented it as a proof-of-concept before I prepared this patch.

> > Also instead of churning up existing users/usage, we can explore to had
> > one with this timeout as alternative if you present and convince the
> > validity of your use-case and the associated timing requirement.
> > 
>
> ...with the proposed patch (and any kind of alternative API proposed
> by Sudeep) the delayed response timeout becomes a parameter of the method
> do_xfer_with_response() and so, as a consequence, this timoeut becomes
> effectively configurable per-transaction, while usually a timeout is
> commonly configurable per-channel,

Totally agree, usually it is. And that's why I didn't change do_xfer() call.
Here is the thing I want to pay attention to.

Let's focus on delayed responses. I think delayed response timeout should not be
defined by transport but rather should be defined by _function_ PuC providing.
And of course platform and transport could influence on the timeout value.

> so valid as a whole for any protocol
> on that channel across the whole platform, AND optionally describable as
> different from the default standard value via DT props (like max-rx-timeout).
>
> Is this what we want ? (a per-transaction configurable timeout ?)
>
> If not, it could be an option to make instead this a per-channel optional
> new DT described property so that you can configure globally a different
> delayed timeout.

Taking into account my previous comment, I don't think that having a per-channel
timeout for delayed response would solve the problem in the right way. What
about having a per-protocol timeout at least?

> If yes, how this new parameter is meant to be used/configured/chosen ?
> on a per-protocol/command basis, unrelated to the specific platform we run on ?

As delayed timeout is IMO should be defined by PuC services (in other words by
command), new parameter can be set directly in the driver. If we talking about
per-protocol solution, using DT is also a good approach.

> Thanks,
> Cristian

Sudeep, hope I also answered your comments from the last email as well.

Thanks,
Matthew