Date:   Tue, 5 Oct 2021 18:00:12 +0200
From:   Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
CC:     Ohad Ben-Cohen <ohad@...ery.com>, Jonathan Corbet <corbet@....net>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, Suman Anna <s-anna@...com>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-remoteproc@...r.kernel.org>
Subject: Re: [PATCH v8 2/2] tty: add rpmsg driver

Hello Bjorn,

On 10/4/21 10:12 PM, Bjorn Andersson wrote:
> On Thu 30 Sep 09:05 PDT 2021, Arnaud Pouliquen wrote:
> 
>> This driver exposes a standard TTY interface on top of the rpmsg
>> framework through a rpmsg service.
>>
>> This driver supports multi-instances, offering a /dev/ttyRPMSGx entry
>> per rpmsg endpoint.
>>
> 
> I think this looks pretty good, but it's a while since we discussed
> this, reading your patches again makes me wonder:

That's fair, the last discussion on the topic was over a year ago.

> 
> 1) With all the efforts related to virtualization and adding things such
> as I2C support there, what are the merits of putting a tty driver ontop
> of rpmsg in comparison with directly on virtio - which would be used for
> virtualization as well.

As mentionned in the cover letter, the main advantage of the RPMsg vs virtio is
that RPMsg is able to mix the services on an unique virtio instance. Using
Virtio console implies to add a new virtio instance for each service (or
instance of a service) with associated mailbox channels.
Taking advantage of the RPMsg service mixing is important for platform (such as
stm32MP1) on which coprocessor is limited in term of memory mapping.

I also expect that this service would be available for some other backend than
the virtio one.

> 
> 2) What prevents you from pointing your userspace tool at an rpmsg_char
> endpoint?

You are right rpmsg_char could be an alternative. But advantage of the TTY is:

- It possible to reuse existing applications/tools relying on TTY devices.
An example is a TTY RPMSG device dedicated for coprocessor traces that can be
simply redirect to a log file or mixed to the kernel logs by scripts.
Another exemple is the ability to ease migration from a 2-processors system
solution (with UART-based IPC) to a system solution with an internal
coprocessor. We received such requirement from some ST customers.

- For rpmsg_char you have to share device between TX and RX (only one fopen
allowed per device), while TTY allows you to manage one device in 2 independent
threads/appliaction.

- TTY framework manages intermediate buffer that allow to free Rx RPMsg buffers.

So rpmsg_char and rpmsg_tty don't seem to me to cover exactly the same requierement.

Thanks,
Arnaud

> 
> Thanks,
> Bjorn
> 
>> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
>> ---
>>  Documentation/serial/tty_rpmsg.rst |  15 ++
>>  drivers/tty/Kconfig                |   9 +
>>  drivers/tty/Makefile               |   1 +
>>  drivers/tty/rpmsg_tty.c            | 275 +++++++++++++++++++++++++++++
>>  4 files changed, 300 insertions(+)
>>  create mode 100644 Documentation/serial/tty_rpmsg.rst
>>  create mode 100644 drivers/tty/rpmsg_tty.c
>>
>> diff --git a/Documentation/serial/tty_rpmsg.rst b/Documentation/serial/tty_rpmsg.rst
>> new file mode 100644
>> index 000000000000..b055107866c9
>> --- /dev/null
>> +++ b/Documentation/serial/tty_rpmsg.rst
>> @@ -0,0 +1,15 @@
>> +.. SPDX-License-Identifier: GPL-2.0
>> +
>> +=========
>> +RPMsg TTY
>> +=========
>> +
>> +The rpmsg tty driver implements serial communication on the RPMsg bus to makes possible for
>> +user-space programs to send and receive rpmsg messages as a standard tty protocol.
>> +
>> +The remote processor can instantiate a new tty by requesting a "rpmsg-tty" RPMsg service.
>> +
>> +The "rpmsg-tty" service is directly used for data exchange. No flow control is implemented.
>> +
>> +Information related to the RPMsg and associated tty device is available in
>> +/sys/bus/rpmsg/devices/.
>> diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
>> index 23cc988c68a4..5095513029d7 100644
>> --- a/drivers/tty/Kconfig
>> +++ b/drivers/tty/Kconfig
>> @@ -368,6 +368,15 @@ config VCC
>>  
>>  source "drivers/tty/hvc/Kconfig"
>>  
>> +config RPMSG_TTY
>> +	tristate "RPMSG tty driver"
>> +	depends on RPMSG
>> +	help
>> +	  Say y here to export rpmsg endpoints as tty devices, usually found
>> +	  in /dev/ttyRPMSGx.
>> +	  This makes it possible for user-space programs to send and receive
>> +	  rpmsg messages as a standard tty protocol.
>> +
>>  endif # TTY
>>  
>>  source "drivers/tty/serdev/Kconfig"
>> diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
>> index a2bd75fbaaa4..07aca5184a55 100644
>> --- a/drivers/tty/Makefile
>> +++ b/drivers/tty/Makefile
>> @@ -26,5 +26,6 @@ obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o
>>  obj-$(CONFIG_GOLDFISH_TTY)	+= goldfish.o
>>  obj-$(CONFIG_MIPS_EJTAG_FDC_TTY) += mips_ejtag_fdc.o
>>  obj-$(CONFIG_VCC)		+= vcc.o
>> +obj-$(CONFIG_RPMSG_TTY)		+= rpmsg_tty.o
>>  
>>  obj-y += ipwireless/
>> diff --git a/drivers/tty/rpmsg_tty.c b/drivers/tty/rpmsg_tty.c
>> new file mode 100644
>> index 000000000000..0c99f54c2911
>> --- /dev/null
>> +++ b/drivers/tty/rpmsg_tty.c
>> @@ -0,0 +1,275 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) STMicroelectronics 2021 - All Rights Reserved
>> + */
>> +
>> +#include <linux/module.h>
>> +#include <linux/rpmsg.h>
>> +#include <linux/slab.h>
>> +#include <linux/tty.h>
>> +#include <linux/tty_flip.h>
>> +
>> +#define MAX_TTY_RPMSG	32
>> +
>> +static DEFINE_IDR(tty_idr);	/* tty instance id */
>> +static DEFINE_MUTEX(idr_lock);	/* protects tty_idr */
>> +
>> +static struct tty_driver *rpmsg_tty_driver;
>> +
>> +struct rpmsg_tty_port {
>> +	struct tty_port		port;	 /* TTY port data */
>> +	int			id;	 /* TTY rpmsg index */
>> +	struct rpmsg_device	*rpdev;	 /* rpmsg device */
>> +};
>> +
>> +static int rpmsg_tty_cb(struct rpmsg_device *rpdev, void *data, int len, void *priv, u32 src)
>> +{
>> +	struct rpmsg_tty_port *cport = dev_get_drvdata(&rpdev->dev);
>> +	int copied;
>> +
>> +	if (!len)
>> +		return -EINVAL;
>> +	copied = tty_insert_flip_string(&cport->port, data, len);
>> +	if (copied != len)
>> +		dev_dbg(&rpdev->dev, "Trunc buffer: available space is %d\n",
>> +			copied);
>> +	tty_flip_buffer_push(&cport->port);
>> +
>> +	return 0;
>> +}
>> +
>> +static int rpmsg_tty_install(struct tty_driver *driver, struct tty_struct *tty)
>> +{
>> +	struct rpmsg_tty_port *cport = idr_find(&tty_idr, tty->index);
>> +
>> +	if (!cport) {
>> +		dev_err(tty->dev, "Cannot get cport\n");
>> +		return -ENODEV;
>> +	}
>> +
>> +	tty->driver_data = cport;
>> +
>> +	return tty_port_install(&cport->port, driver, tty);
>> +}
>> +
>> +static int rpmsg_tty_open(struct tty_struct *tty, struct file *filp)
>> +{
>> +	return tty_port_open(tty->port, tty, filp);
>> +}
>> +
>> +static void rpmsg_tty_close(struct tty_struct *tty, struct file *filp)
>> +{
>> +	return tty_port_close(tty->port, tty, filp);
>> +}
>> +
>> +static int rpmsg_tty_write(struct tty_struct *tty, const u8 *buf, int len)
>> +{
>> +	struct rpmsg_tty_port *cport = tty->driver_data;
>> +	struct rpmsg_device *rpdev;
>> +	int msg_max_size, msg_size;
>> +	int ret;
>> +
>> +	rpdev = cport->rpdev;
>> +
>> +	dev_dbg(&rpdev->dev, "Send msg from tty->index = %d, len = %d\n", tty->index, len);
>> +
>> +	msg_max_size = rpmsg_get_mtu(rpdev->ept);
>> +	if (msg_max_size < 0)
>> +		return msg_max_size;
>> +
>> +	msg_size = min(len, msg_max_size);
>> +
>> +	/*
>> +	 * Use rpmsg_trysend instead of rpmsg_send to send the message so the caller is not
>> +	 * hung until a rpmsg buffer is available. In such case rpmsg_trysend returns -ENOMEM.
>> +	 */
>> +	ret = rpmsg_trysend(rpdev->ept, (void *)buf, msg_size);
>> +	if (ret) {
>> +		dev_dbg(&rpdev->dev, "rpmsg_send failed: %d\n", ret);
>> +		return ret;
>> +	}
>> +
>> +	return msg_size;
>> +}
>> +
>> +static unsigned int rpmsg_tty_write_room(struct tty_struct *tty)
>> +{
>> +	struct rpmsg_tty_port *cport = tty->driver_data;
>> +	int size;
>> +
>> +	size = rpmsg_get_mtu(cport->rpdev->ept);
>> +	if (size < 0)
>> +		return 0;
>> +
>> +	return size;
>> +}
>> +
>> +static const struct tty_operations rpmsg_tty_ops = {
>> +	.install	= rpmsg_tty_install,
>> +	.open		= rpmsg_tty_open,
>> +	.close		= rpmsg_tty_close,
>> +	.write		= rpmsg_tty_write,
>> +	.write_room	= rpmsg_tty_write_room,
>> +};
>> +
>> +static struct rpmsg_tty_port *rpmsg_tty_alloc_cport(void)
>> +{
>> +	struct rpmsg_tty_port *cport;
>> +	int err;
>> +
>> +	cport = kzalloc(sizeof(*cport), GFP_KERNEL);
>> +	if (!cport)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	mutex_lock(&idr_lock);
>> +	cport->id = idr_alloc(&tty_idr, cport, 0, MAX_TTY_RPMSG, GFP_KERNEL);
>> +	mutex_unlock(&idr_lock);
>> +
>> +	if (cport->id < 0) {
>> +		err = cport->id;
>> +		kfree(cport);
>> +		return ERR_PTR(err);
>> +	}
>> +
>> +	return cport;
>> +}
>> +
>> +static void rpmsg_tty_release_cport(struct rpmsg_tty_port *cport)
>> +{
>> +	mutex_lock(&idr_lock);
>> +	idr_remove(&tty_idr, cport->id);
>> +	mutex_unlock(&idr_lock);
>> +
>> +	kfree(cport);
>> +}
>> +
>> +static const struct tty_port_operations rpmsg_tty_port_ops = { };
>> +
>> +static int rpmsg_tty_probe(struct rpmsg_device *rpdev)
>> +{
>> +	struct rpmsg_tty_port *cport;
>> +	struct device *dev = &rpdev->dev;
>> +	struct device *tty_dev;
>> +	int ret;
>> +
>> +	cport = rpmsg_tty_alloc_cport();
>> +	if (IS_ERR(cport)) {
>> +		dev_err(dev, "Failed to alloc tty port\n");
>> +		return PTR_ERR(cport);
>> +	}
>> +
>> +	tty_port_init(&cport->port);
>> +	cport->port.ops = &rpmsg_tty_port_ops;
>> +
>> +	tty_dev = tty_port_register_device(&cport->port, rpmsg_tty_driver,
>> +					   cport->id, dev);
>> +	if (IS_ERR(tty_dev)) {
>> +		dev_err(dev, "Failed to register tty port\n");
>> +		ret = PTR_ERR(tty_dev);
>> +		goto  err_destroy;
>> +	}
>> +
>> +	cport->rpdev = rpdev;
>> +
>> +	dev_set_drvdata(dev, cport);
>> +
>> +	dev_dbg(dev, "New channel: 0x%x -> 0x%x : ttyRPMSG%d\n",
>> +		rpdev->src, rpdev->dst, cport->id);
>> +
>> +	return 0;
>> +
>> +err_destroy:
>> +	tty_port_destroy(&cport->port);
>> +	rpmsg_tty_release_cport(cport);
>> +
>> +	return ret;
>> +}
>> +
>> +static void rpmsg_tty_remove(struct rpmsg_device *rpdev)
>> +{
>> +	struct rpmsg_tty_port *cport = dev_get_drvdata(&rpdev->dev);
>> +
>> +	dev_dbg(&rpdev->dev, "Removing rpmsg tty device %d\n", cport->id);
>> +
>> +	/* User hang up to release the tty */
>> +	if (tty_port_initialized(&cport->port))
>> +		tty_port_tty_hangup(&cport->port, false);
>> +
>> +	tty_unregister_device(rpmsg_tty_driver, cport->id);
>> +
>> +	tty_port_destroy(&cport->port);
>> +	rpmsg_tty_release_cport(cport);
>> +}
>> +
>> +static struct rpmsg_device_id rpmsg_driver_tty_id_table[] = {
>> +	{ .name	= "rpmsg-tty" },
>> +	{ },
>> +};
>> +MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_tty_id_table);
>> +
>> +static struct rpmsg_driver rpmsg_tty_rpmsg_drv = {
>> +	.drv.name	= KBUILD_MODNAME,
>> +	.id_table	= rpmsg_driver_tty_id_table,
>> +	.probe		= rpmsg_tty_probe,
>> +	.callback	= rpmsg_tty_cb,
>> +	.remove		= rpmsg_tty_remove,
>> +};
>> +
>> +static int __init rpmsg_tty_init(void)
>> +{
>> +	int err;
>> +
>> +	rpmsg_tty_driver = tty_alloc_driver(MAX_TTY_RPMSG, TTY_DRIVER_REAL_RAW |
>> +					    TTY_DRIVER_DYNAMIC_DEV);
>> +	if (IS_ERR(rpmsg_tty_driver))
>> +		return PTR_ERR(rpmsg_tty_driver);
>> +
>> +	rpmsg_tty_driver->driver_name = "rpmsg_tty";
>> +	rpmsg_tty_driver->name = "ttyRPMSG";
>> +	rpmsg_tty_driver->major = 0;
>> +	rpmsg_tty_driver->type = TTY_DRIVER_TYPE_CONSOLE;
>> +
>> +	/* Disable unused mode by default */
>> +	rpmsg_tty_driver->init_termios = tty_std_termios;
>> +	rpmsg_tty_driver->init_termios.c_lflag &= ~(ECHO | ICANON);
>> +	rpmsg_tty_driver->init_termios.c_oflag &= ~(OPOST | ONLCR);
>> +
>> +	tty_set_operations(rpmsg_tty_driver, &rpmsg_tty_ops);
>> +
>> +	err = tty_register_driver(rpmsg_tty_driver);
>> +	if (err < 0) {
>> +		pr_err("Couldn't install rpmsg tty driver: err %d\n", err);
>> +		goto error_put;
>> +	}
>> +
>> +	err = register_rpmsg_driver(&rpmsg_tty_rpmsg_drv);
>> +	if (err < 0) {
>> +		pr_err("Couldn't register rpmsg tty driver: err %d\n", err);
>> +		goto error_unregister;
>> +	}
>> +
>> +	return 0;
>> +
>> +error_unregister:
>> +	tty_unregister_driver(rpmsg_tty_driver);
>> +
>> +error_put:
>> +	tty_driver_kref_put(rpmsg_tty_driver);
>> +
>> +	return err;
>> +}
>> +
>> +static void __exit rpmsg_tty_exit(void)
>> +{
>> +	unregister_rpmsg_driver(&rpmsg_tty_rpmsg_drv);
>> +	tty_unregister_driver(rpmsg_tty_driver);
>> +	tty_driver_kref_put(rpmsg_tty_driver);
>> +	idr_destroy(&tty_idr);
>> +}
>> +
>> +module_init(rpmsg_tty_init);
>> +module_exit(rpmsg_tty_exit);
>> +
>> +MODULE_AUTHOR("Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>");
>> +MODULE_DESCRIPTION("remote processor messaging tty driver");
>> +MODULE_LICENSE("GPL v2");
>> -- 
>> 2.17.1
>>

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