linux-kernel - Re: [PATCH v5 3/4] dt-bindings: arm: amlogic: amlogic,meson-gx-ao-secure: add secure-monitor property

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Date: Mon, 17 Jun 2024 17:57:53 +0100
From: Conor Dooley <conor@...nel.org>
To: Viacheslav <adeep@...ina.in>
Cc: Rob Herring <robh@...nel.org>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Kevin Hilman <khilman@...libre.com>,
	Jerome Brunet <jbrunet@...libre.com>,
	Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-amlogic@...ts.infradead.org,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v5 3/4] dt-bindings: arm: amlogic:
 amlogic,meson-gx-ao-secure: add secure-monitor property

On Mon, Jun 17, 2024 at 11:21:30AM +0300, Viacheslav wrote:
> Thanks for review.
> 
> 13/06/2024 19.42, Rob Herring wrote:
> > On Tue, Jun 11, 2024 at 07:07:28PM +0100, Conor Dooley wrote:
> > > On Tue, Jun 11, 2024 at 01:25:11PM +0300, Viacheslav wrote:
> > > > Hi!
> > > > 
> > > > 10/06/2024 19.08, Conor Dooley wrote:
> > > > > On Mon, Jun 10, 2024 at 11:39:49AM +0300, Viacheslav Bocharov wrote:
> > > > > > Add secure-monitor property to schema for meson-gx-socinfo-sm driver.
> > > > > 
> > > > > "bindings are for hardware, not drivers". Why purpose does the "secure
> > > > > monitor" serve that the secure firmware needs a reference to it?
> > > > 
> > > > This driver is an extension to the meson-gx-socinfo driver: it supplements
> > > > information obtained from the register with information from the
> > > > SM_GET_CHIP_ID secure monitor call. Due to the specifics of the module
> > > > loading order, we cannot do away with meson-gx-socinfo, as it is used for
> > > > platform identification in some drivers. Therefore, the extended information
> > > > is formatted as a separate driver, which is loaded after the secure-monitor
> > > > driver.
> > > 
> > > Please stop talking about drivers, this is a binding which is about
> > > hardware. Please provide, in your next version, a commit message that
> > > justifies adding this property without talking about driver probing
> > > order etc, and instead focuses on what service the "secure monitor"
> > > provides etc.
> > 
> > To put it another way, how many secure monitors does 1 system have?
> 
> One per system in current device tree.

One per system, or one is currently described per system, but more might
be added later?

> > What do you do if the property is not present? You didn't make it
> > required which is good because that would be an ABI break.
> 
> We need an indication of the ability to use the secure-monitor to obtain
> additional information within the soc driver. It seemed to me that using an
> explicit reference to the secure-monitor is the best choice.
> 
> > 
> > You only need a link in DT if there are different possible providers or
> > some per consumer information to describe (e.g. an interrupt number or
> > clock ID). You don't have the latter and likely there is only 1 possible
> > provider.
> 
> Would replacing the reference to sm with an option, for example,
> use-secure-monitor = <1>; look more appropriate in this case?

Perhaps a silly question, but (provided there's only one per system, why
can't the secure-monitor driver expose a function that you can call to get
a reference to the system-monitor? I did something similar before with
a call to in mpfs_sys_controller_get() mpfs_rng_probe(). Granted,
mpfs-rng is probed from software so it's slightly different to your
case, but the principle is the same and it's not unheard of for code in
drivers/soc to expose interfaces to other drivers like this. You can
just call a function like that, and know whether there's a secure
monitor, without having to retrofit a DT property.

Thanks,
Conor.

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