Re: [RFC PATCH 2/5] DT: mailbox: add binding doc for the ARM SMC mailbox

From: Rob Herring <robh@kernel.org>
To: Andre Przywara <andre.przywara@arm.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>,
	Chen-Yu Tsai <wens@csie.org>,
	linux-sunxi@googlegroups.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Jassi Brar <jassisinghbrar@gmail.com>,
	Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org
Subject: Re: [RFC PATCH 2/5] DT: mailbox: add binding doc for the ARM SMC mailbox
Date: Wed, 10 Aug 2016 16:57:02 -0500	[thread overview]
Message-ID: <20160810215702.GA16450@rob-hp-laptop> (raw)
In-Reply-To: <20160809115303.17032-3-andre.przywara@arm.com>

On Tue, Aug 09, 2016 at 12:53:00PM +0100, Andre Przywara wrote:
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  .../devicetree/bindings/mailbox/arm-smc.txt        | 53 ++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/mailbox/arm-smc.txt
> 
> diff --git a/Documentation/devicetree/bindings/mailbox/arm-smc.txt b/Documentation/devicetree/bindings/mailbox/arm-smc.txt
> new file mode 100644
> index 0000000..9919a12
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/mailbox/arm-smc.txt
> @@ -0,0 +1,53 @@
> +ARM SMC Mailbox Driver
> +======================
> +
> +This mailbox driver uses the ARM smc (secure monitor call) instruction to
> +trigger a mailbox-connected activity in firmware running on the very same
> +core as the caller. By nature this operation is synchronous and this
> +driver provides no way for asynchronous messages to be delivered the other
> +way round, from firmware to the OS. However the value of r0/w0 the firmware
> +returns after the smc call is delivered as a received message to the
> +mailbox framework, so a synchronous communication can be established.

Clever

> +
> +One usecase of this mailbox is the SCP interface, which uses shared memory
> +to transfer commands and parameters and mailboxes to trigger a function
> +call. This driver allows SoC without a separate management processor (or
> +when such a processor is not available or used) to use this standardized
> +interface anyway.
> +
> +The driver requires no special hardware, any core which supports the SMC
> +instruction can be used. This requires firmware in monitor mode/EL3 to
> +handle the mailbox message.
> +
> +Mailbox Device Node:
> +====================
> +
> +Required properties:
> +--------------------
> +- compatible:		Shall be "arm,smc-mbox"
> +- #mbox-cells		Shall be 1 - the index of the channel needed.
> +- identifiers		An array of 32-bit values specifying the function

Minimally, needs 'arm' prefix. 'identifiers' is pretty vague. Perhaps 
'arm,smc-func-ids' to say exactly what they are.

> +			IDs used by each mailbox channel. Those function IDs
> +			follow the ARM SMC calling convention standard [1].
> +			There is one identifier per channel and the number
> +			of supported channels is determined by the length
> +			of this array.
> +
> +Example:
> +--------
> +
> +	mailbox: smc_mbox {
> +		#mbox-cells = <1>;
> +		compatible = "arm,smc-mbox";
> +		identifiers = <0x82000001 0x82000002>;
> +	};
> +
> +	scpi {
> +		compatible = "arm,scpi";
> +		mboxes = <&mailbox 0>;
> +		shmem = <&cpu_scp_shmem>;
> +	};
> +
> +
> +[1]
> +http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0028a/index.html
> -- 
> 2.9.0
> 