linux-kernel - Re: [PATCH 3/3] mailbox: sunxi-msgbox: Add a new mailbox driver

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Date:   Wed, 28 Feb 2018 11:51:59 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ", linux-arm-kernel"@lists.infradead.org,
        linux-mediatek@...ts.infradead.org,
        srv_heupstream <linux-arm-kernel@...ts.infradead.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH 3/3] mailbox: sunxi-msgbox: Add a new mailbox driver

Hi,

On 02/28/18 03:16, Jassi Brar wrote:
> On Wed, Feb 28, 2018 at 7:57 AM, Samuel Holland <samuel@...lland.org> wrote:
> ....
> 
>> +/*
>> + * The message box hardware provides 8 unidirectional channels. As the mailbox
>> + * framework expects them to be bidirectional
>>
> That is incorrect. Mailbox framework does not require a channel to be
> TX and RX capable.

Sorry, it would be more accurate to say that the intended mailbox _client_
expects the channels to be bidirectional.

> You should expose each channel as per its physical capability, let the
> client configure it for direction (if its bidirectional) and acquire
> separate channels for RX and TX if it needs to.

Is there any way for the mailbox framework to inform the client that a channel
is uni/bidirectional? Or if the channel supports RX/TX specifically? I couldn't
find any.

It looks like all of the clients assume one case or the other, based on the
hardware they were initially designed to support. For example arm_scpi expects
one channel per client, while ti_sci and st_remoteproc expect two separate RX
and TX channels per client.

Since there's no API for the client to know the hardware properties, modifying
arm_scpi to support unidirectional mailbox channels would break support for all
existing bidirectional mailboxes.

To expose the physical channels individually, there would need to be an API to:
a) Determine if a mailbox channel supports unidirectional or bidirectional
   operation.
b) If a channel is unidirectional, determine which direction(s) it supports.
c) If a unidirectional channel supports both directions, configure which
   direction it should run in.

Since that API doesn't currently exist, I wrote the driver to match what the
intended client expected (as all other mailbox controller drivers currently do).

> Cheers!

Regards,
Samuel