linux-kernel - Re: [PATCH 0/3] Apple Mailbox Controller support

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Date:   Wed, 8 Sep 2021 15:48:08 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Sven Peter <sven@...npeter.dev>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Hector Martin <marcan@...can.st>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Stan Skowronek <stan@...ellium.com>,
        Devicetree List <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Apple Mailbox Controller support

On Tue, Sep 7, 2021 at 9:55 AM Sven Peter <sven@...npeter.dev> wrote:
>
> Hi,
>
> This series adds support for the mailbox HW found in the Apple M1. These SoCs
> have various co-processors controlling different peripherals (NVMe, display
> controller, SMC (required for WiFi), Thunderbolt, and probably more
> we don't know about yet). All these co-processors communicate with the main CPU
> using these mailboxes. These mailboxes transmit 64+32 bit messages, are
> backed by a hardware FIFO and have four interrupts (FIFO empty and FIFO not
> empty for the transmit and receive FIFO each).
>
> The hardware itself allows to send 64+32 bit message using two hardware
> registers. A write to or read from the second register transmits or receives a
> message. Usually, the first 64 bit register is used for the message itself and
> 8 bits of the second register are used as an endpoint. I originally considered
> to have the endpoint exposed as a mailbox-channel, but finally decided against
> it: The hardware itself only provides a single channel to the co-processor and
> the endpoint bits are only an implementation detail of the firmware. There's
> even one co-processor (SEP) which uses 8 bits of the first register as its
> endpoint number instead.
> There was a similar discussion about the BCM2835 / Raspberry Pi mailboxes
> which came to the same conclusion [1].
>
> These mailboxes also have a hardware FIFO which make implementing them with the
> current mailbox a bit tricky: There is no "transmission done" interrupt because
> most transmissions are "done" immediately. There is only a "transmission fifo
> empty" level interrupt. I have instead implemented this by adding a fast-path to
> the core mailbox code as a new txready_fifo mode.
> The other possibilities (which would not require any changes to the core mailbox
> code) are to either use the polling mode or to enable the "tx fifo empty"
> interrupt in send_message and then call txready from the irq handler before
> disabling it again. I'd like to avoid those though since so far I've never seen
> the TX FIFO run full which allows to almost always avoid the context switch when
> sending a message. I can easily switch to one of these modes if you prefer to
> keep the core code untouched though.
>
Yes, please keep the api unchanged.
Let us please not dig our own tunnels when the existing ways serve the purpose.

Thanks.