linux-kernel - Re: [RFC PATCH 1/2] dt-bindings: arm: Add secure-clocks binding description

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Date:   Fri, 24 Apr 2020 17:05:49 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Etienne Carriere <etienne.carriere@...aro.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>, devicetree@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] dt-bindings: arm: Add secure-clocks binding description

On Thu, Apr 23, 2020 at 1:49 AM Etienne Carriere
<etienne.carriere@...aro.org> wrote:
>
> Describe how clocks property can leverage secure- property prefix
> for clocks handled exclusively or shared by secure and non-secure
> worlds.
>
> Signed-off-by: Etienne Carriere <etienne.carriere@...aro.org>
> ---
>  .../devicetree/bindings/arm/secure.txt        | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/arm/secure.txt b/Documentation/devicetree/bindings/arm/secure.txt
> index f27bbff2c780..9bc94921f2a6 100644
> --- a/Documentation/devicetree/bindings/arm/secure.txt
> +++ b/Documentation/devicetree/bindings/arm/secure.txt
> @@ -53,6 +53,25 @@ Valid Secure world properties
>     status = "disabled";                             /* disabled in both */
>     status = "disabled"; secure-status = "disabled"; /* disabled in both */
>
> +- secure-clocks : specifies the Phandle list secure world shall use
> +  for the related clocks whereas property "clocks" specifies the
> +  clock Phandle list non-secure shall use for the that clocks.
> +  This configuration can apply for example when a hardware clock is
> +  shared by the 2 worlds and the hardware implements a specific interface
> +  for each world, i.e.:
> +
> +  clocks = <&clk DMA_NS>;      /* NS relies on clock handle DMA_NS */
> +  secure-clocks = <&clk DMA_S>;        /* S relies on clock handle DMA_S */

The device has to know what it's clocks are for and should know if
some clocks are secure only.

> +  Another example where use of "clocks" and "secure-clocks" can apply
> +  is when hardware implements a clock that secure and non-secure must
> +  share, as a shared GPIO bank clock, and secure world relies on clock
> +  device driver whereas non-secure world relies on a software service
> +  exposed by secure world as SCMI clock device. I.e.:
> +
> +  clocks = <&scmi_clk 2>;      /* NS relies on SCMI resources */
> +  secure-clocks= <&clk 5>;     /* S accesses the SoC reset interfaces */

If you have this case, I don't think this is the solution. I don't
think it scales well and you probably need separate DTs. It's
something to solve in the system DT project.

Rob