Re: [PATCH v2 05/10] mailbox: apple: Add DockChannel FIFO controller

[Yureka Lilian <yureka@cyberchaos.dev>]

On 9/18/26 13:06, Michael Reeves via B4 Relay wrote:
> From: Michael Reeves <michael.reeves077@gmail.com>
>
> DockChannel is a hardware FIFO used by Apple coprocessors for
> low-latency byte-stream communication with the AP.
>
> Add a mailbox controller that preallocates RX storage, tracks IRQ
> enable state in software, and reports TX completion from the TX-empty
> interrupt.
>
> Reject messages larger than the FIFO and return -EBUSY while the
> previous message is still pending. This keeps the provider usable for
> future small-message clients such as serial transports without a TX
> worker.
>
> Handle RX and TX interrupts in a thread so mailbox callbacks and locking
> run outside hard-IRQ context.
>
> Co-developed-by: Hector Martin <marcan@marcan.st>
> Signed-off-by: Hector Martin <marcan@marcan.st>
> Signed-off-by: Michael Reeves <michael.reeves077@gmail.com>
> ---
>   MAINTAINERS                               |   2 +
>   drivers/mailbox/Kconfig                   |  12 +
>   drivers/mailbox/Makefile                  |   2 +
>   drivers/mailbox/apple-dockchannel.c       | 371 ++++++++++++++++++++++++++++++
>   include/linux/mailbox/apple-dockchannel.h |  29 +++
>   5 files changed, 416 insertions(+)
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 7fe2ebb5e..e5be5934c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2677,6 +2677,7 @@ F:	drivers/input/touchscreen/apple_z2.c
>   F:	drivers/iommu/apple-dart.c
>   F:	drivers/iommu/io-pgtable-dart.c
>   F:	drivers/irqchip/irq-apple-aic.c
> +F:	drivers/mailbox/apple-dockchannel.c
>   F:	drivers/mfd/macsmc.c
>   F:	drivers/nvme/host/apple.c
>   F:	drivers/nvmem/apple-efuses.c
> @@ -2695,6 +2696,7 @@ F:	drivers/video/backlight/apple_dwi_bl.c
>   F:	drivers/watchdog/apple_wdt.c
>   F:	include/dt-bindings/interrupt-controller/apple-aic.h
>   F:	include/dt-bindings/pinctrl/apple.h
> +F:	include/linux/mailbox/apple-dockchannel.h
>   F:	include/linux/mfd/macsmc.h
>   F:	include/linux/soc/apple/*
>   F:	include/uapi/drm/asahi_drm.h
> [...]
> diff --git a/drivers/mailbox/apple-dockchannel.c b/drivers/mailbox/apple-dockchannel.c
> new file mode 100644
> index 000000000..d0a66e1b7
> --- /dev/null
> +++ b/drivers/mailbox/apple-dockchannel.c
> @@ -0,0 +1,371 @@
> +// SPDX-License-Identifier: GPL-2.0-only OR MIT
> +/*
> + * Apple DockChannel mailbox controller
> + *
> + * Copyright The Asahi Linux Contributors
> + *
> + * DockChannel is a byte FIFO used by Apple co-processors. This driver exposes a
> + * single FIFO pair as a Linux mailbox channel and moves payload bytes with PIO.
> + * There is no DMA involved, so relaxed MMIO accessors are sufficient for the
> + * FIFO accesses themselves.
> + */
> +
> +#include <linux/device.h>
> +#include <linux/interrupt.h>
> +#include <linux/io.h>
> +#include <linux/mailbox/apple-dockchannel.h>
> +#include <linux/mailbox_controller.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/spinlock.h>
> +#include <linux/unaligned.h>
> +
> +#define APPLE_DOCKCHANNEL_FIFO_SIZE	0x800
> +
> +#define IRQ_MASK			0x0
> +#define IRQ_FLAG			0x4
> +
> +#define IRQ_TX				BIT(2)
> +#define IRQ_RX				BIT(3)

The dockchannel interface used for dockchannel-uart / DebugUSB uses 
different bits here, so these should probably be read from the device 
tree if the mailbox driver is supposed to be somewhat generic.

> [...]