linux-kernel - Re: [PATCH 0/9] serial: Add Tegra Combined UART driver

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Date:   Mon, 29 Oct 2018 11:04:06 +0200
From:   Pekka Pessi <ppessi@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Jiri Slaby <jslaby@...e.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Jon Hunter <jonathanh@...dia.com>,
        Timo Alho <talho@...dia.com>,
        Mika Liljeberg <mliljeberg@...dia.com>,
        <linux-tegra@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/9] serial: Add Tegra Combined UART driver

Hi Thierry,

What is the use case for the mailbox driver? What kind of entity will be 
there consuming sent messages and sending messages to kernel?

--Pekka

On 10/26/2018 02:16 PM, Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
>
> Hi everyone,
>
> this is a reworked version of Mikko's earlier proposal[0]. I've reworked
> the TCU driver itself so that it relies less on global variables as well
> as added a Kconfig option to allow the console support to be selected. I
> also fixed a couple of issues that manifested themselves as I was moving
> towards the IRQ driven mode (TCU was passing a pointer to a local
> variable which was getting stored in the mailbox's ring buffer and the
> data pointed at was becoming stale by the time the mailbox got around to
> dequeue it).
>
> The biggest bulk of the changes is in the mailbox driver. This series
> addresses all of Jassi's comments from back at the time. One notable
> additional change is that shared mailboxes are now interrupt driven,
> which removes the need for polling mode.
>
> Unfortunately there is still an issue because the TCU uses the mailbox
> in atomic context for both TTY and console modes, so we get a sleeping-
> while-atomic BUG when using mbox_client->tx_block = true in order to
> rate-limit mbox_send_message(). In order to work around this, I added a
> mechanism to mbox_send_message() that will allow blocking from atomic
> context if the mailbox controller implements the new ->flush() callback.
> For Tegra HSP shared mailboxes this is done by spinning on the shared
> mailbox register until the receiver has marked the mailbox as empty. I
> have been running this locally for a couple of days now and it works
> perfectly.
>
> Furthermore this series incorporates Mikko's work in progress on
> splitting up the mailbox controllers and allowing multiple controllers
> to match on the same device tree node during mbox_request_channel().
>
> Last but not least there are no build-time dependencies between the
> mailbox and serial drivers, so I think the easiest way to merge this is
> if Jassi picks up patches 1-5, Greg takes patches 6 & 7 and I pick up
> patches 8-9 into the Tegra tree.
>
> Thanks,
> Thierry
>
> [0]: https://lore.kernel.org/patchwork/project/lkml/list/?series=357641
>
> Mikko Perttunen (5):
>    mailbox: Allow multiple controllers per device
>    dt-bindings: tegra186-hsp: Add shared interrupts
>    dt-bindings: serial: Add bindings for nvidia,tegra194-tcu
>    arm64: tegra: Add nodes for TCU on Tegra194
>    arm64: tegra: Mark TCU as primary serial port on Tegra194 P2888
>
> Thierry Reding (4):
>    mailbox: Support blocking transfers in atomic context
>    mailbox: tegra-hsp: Add support for shared mailboxes
>    mailbox: tegra-hsp: Add suspend/resume support
>    serial: Add Tegra Combined UART driver
>
>   .../bindings/mailbox/nvidia,tegra186-hsp.txt  |   3 +
>   .../bindings/serial/nvidia,tegra194-tcu.txt   |  35 ++
>   .../arm64/boot/dts/nvidia/tegra194-p2888.dtsi |   2 +-
>   arch/arm64/boot/dts/nvidia/tegra194.dtsi      |  38 +-
>   drivers/mailbox/mailbox.c                     |  11 +-
>   drivers/mailbox/tegra-hsp.c                   | 495 +++++++++++++++---
>   drivers/tty/serial/Kconfig                    |  22 +
>   drivers/tty/serial/Makefile                   |   1 +
>   drivers/tty/serial/tegra-tcu.c                | 299 +++++++++++
>   include/linux/mailbox_controller.h            |   4 +
>   include/uapi/linux/serial_core.h              |   3 +
>   11 files changed, 847 insertions(+), 66 deletions(-)
>   create mode 100644 Documentation/devicetree/bindings/serial/nvidia,tegra194-tcu.txt
>   create mode 100644 drivers/tty/serial/tegra-tcu.c
>