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Subject: Re: [PATCH V10 2/2] mailbox: introduce ARM SMC based mailbox
To:     Peng Fan <peng.fan@nxp.com>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "jassisinghbrar@gmail.com" <jassisinghbrar@gmail.com>,
        "sudeep.holla@arm.com" <sudeep.holla@arm.com>,
        "andre.przywara@arm.com" <andre.przywara@arm.com>
Cc:     "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        dl-linux-imx <linux-imx@nxp.com>
References: <1569824287-4263-1-git-send-email-peng.fan@nxp.com>
 <1569824287-4263-3-git-send-email-peng.fan@nxp.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
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Message-ID: <2c8fa412-33c2-57c7-20b7-37b3b70ce524@gmail.com>
Date:   Fri, 8 Nov 2019 09:32:43 -0800
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Peng,

On 9/29/19 11:20 PM, Peng Fan wrote:
> From: Peng Fan <peng.fan@nxp.com>
> 
> This mailbox driver implements a mailbox which signals transmitted data
> via an ARM smc (secure monitor call) instruction. The mailbox receiver
> is implemented in firmware and can synchronously return data when it
> returns execution to the non-secure world again.
> An asynchronous receive path is not implemented.
> This allows the usage of a mailbox to trigger firmware actions on SoCs
> which either don't have a separate management processor or on which such
> a core is not available. A user of this mailbox could be the SCP
> interface.

Sorry for not spotting this, or rather asking this earlier, but I do
have one question below.

[snip]

> +static int arm_smc_send_data(struct mbox_chan *link, void *data)
> +{
> +	struct arm_smc_chan_data *chan_data = link->con_priv;
> +	struct arm_smccc_mbox_cmd *cmd = data;
> +	unsigned long ret;
> +
> +	if (ARM_SMCCC_IS_64(chan_data->function_id)) {
> +		ret = chan_data->invoke_smc_mbox_fn(chan_data->function_id,
> +						    cmd->args_smccc64[0],
> +						    cmd->args_smccc64[1],
> +						    cmd->args_smccc64[2],
> +						    cmd->args_smccc64[3],
> +						    cmd->args_smccc64[4],
> +						    cmd->args_smccc64[5]);
> +	} else {
> +		ret = chan_data->invoke_smc_mbox_fn(chan_data->function_id,
> +						    cmd->args_smccc32[0],
> +						    cmd->args_smccc32[1],
> +						    cmd->args_smccc32[2],
> +						    cmd->args_smccc32[3],
> +						    cmd->args_smccc32[4],
> +						    cmd->args_smccc32[5]);
> +	}

Why did not we use unsigned long for the args_smccc[] array to be bit
width independent, this is what the PSCI infrastructure does and it
looks a lot nicer IMHO. More question below.

[snip]

> +
> +#ifndef _LINUX_ARM_SMCCC_MBOX_H_
> +#define _LINUX_ARM_SMCCC_MBOX_H_
> +
> +#include <linux/types.h>
> +
> +/**
> + * struct arm_smccc_mbox_cmd - ARM SMCCC message structure
> + * @args_smccc32/64:	actual usage of registers is up to the protocol
> + *			(within the SMCCC limits)
> + */
> +struct arm_smccc_mbox_cmd {
> +	union {
> +		u32 args_smccc32[6];
> +		u64 args_smccc64[6];
> +	};
> +};

Why is this being moved to a separate header file and not within the
driver's main file? It is not like we offer the ability for a driver to
embed this ARM SMC mailbox driver as a library, and customize the values
of the SMC arguments (maybe we should do that, as a later patch) except
for the function_id. If you have a "public" header, there is usually a
service or some configuration that your driver would offer, which is not
the case here.
-- 
Florian