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

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Date:   Thu, 09 Sep 2021 12:44:46 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Jassi Brar" <jassisinghbrar@...il.com>
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 Wed, Sep 8, 2021, at 22:48, Jassi Brar wrote:
> 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.
> 

Ok, I'll use txdone_irq for v2 then and just ignore the HW FIFO.

Thanks,

Sven