linux-kernel - Re: [PATCH v2] firmware: arm_scmi: Give SMC transport precedence over mailbox

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Message-ID: <ZwksW2q6-wrY_tKZ@pluto>
Date: Fri, 11 Oct 2024 14:47:07 +0100
From: Cristian Marussi <cristian.marussi@....com>
To: Florian Fainelli <florian.fainelli@...adcom.com>
Cc: linux-arm-kernel@...ts.infreadead.org, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Sudeep Holla <sudeep.holla@....com>,
	Cristian Marussi <cristian.marussi@....com>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:SYSTEM CONTROL & POWER/MANAGEMENT INTERFACE" <arm-scmi@...r.kernel.org>,
	"moderated list:SYSTEM CONTROL & POWER/MANAGEMENT INTERFACE" <linux-arm-kernel@...ts.infradead.org>,
	justin.chen@...adcom.com, opendmb@...il.com,
	kapil.hali@...adcom.com, bcm-kernel-feedback-list@...adcom.com,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2] firmware: arm_scmi: Give SMC transport precedence
 over mailbox

On Mon, Oct 07, 2024 at 04:54:13PM -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, in that order.
> 
> After the commit cited in the Fixes tag and with a kernel configuration
> that enables both the SMC and the Mailbox transports, we would probe
> the mailbox transport, but fail to complete since we would not have a
> mailbox driver available. With each SCMI transport being a platform
> driver with its own set of compatible strings to match, rather than an
> unique platform driver entry point, we no longer match from most
> specific to least specific. There is also no simple way for the mailbox
> driver to return -ENODEV and let another platform driver attempt
> probing. This leads to a platform with no SCMI provider, therefore all
> drivers depending upon SCMI resources are put on deferred probe forever.
> 
> By keeping the SMC transport objects linked first, we can let the
> platform driver match the compatible string and probe successfully with
> no adverse effects on platforms using the mailbox transport.
> 
> Fixes: b53515fa177c ("firmware: arm_scmi: Make MBOX transport a standalone driver")
> Signed-off-by: Florian Fainelli <florian.fainelli@...adcom.com>

I was hoping to find some way to avoid this workaround....
...but I failed :P ... so

LGTM.
Reviewed-by: Cristian Marussi <cristian.marussi@....com>

Thanks,
Cristian