Date:   Thu, 7 Oct 2021 12:20:14 +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 10/5/21 6:24 PM, Bjorn Andersson wrote:
> On Tue 05 Oct 09:00 PDT 2021, Arnaud POULIQUEN wrote:
> 
>> 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.
>>
> 
> The limitations related to mapping additional virtio pipes does seem
> like a reasonable one. I just hope we don't end up duplicating much of
> the virtualization effort, with the recently concluded I2C client
> support coming to mind.

Difficult to have a prediction on this.
This is something that should be coming with rational.The I2C management would
be a good example. If we consider a I2C bus managed by the coprocessor. A main
processor can have to access to a device on this bus.
Is limited number of access to the device justify to reserve a dedicated memory
area and a mailbox channel just for few device configurations on runtime?

Anyway supported more virtio services in remoteproc and OpenAMP library seems
something like a good middle term objective. The work I started around
remoteproc virtio, I guess, is going in this direction.

> 
>> I also expect that this service would be available for some other backend than
>> the virtio one.
>>
> 
> Last time we discussed this I believe I mentioned the AT command
> channels found in Qualcomm modems. Since then that got pushed into
> drivers/net/wwan, for the few other cases rpmsg_char works fine.
> 
>>>
>>> 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.
>>
> 
> I presume the transport itself provided by rpmsg_char would work just
> fine for this, but that these applications expects something from the
> tty framework, with its known ioctls etc?

The rts mechanism i implemented in previous version is also something
interesting to add on for the flow control.

> 
>> - 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.
>>
> 
> At least in the Qualcomm case, where the channels have a state and
> clients opening and closing the rpmsg_char will affect that state it
> simplifies things a lot not to support a multi-client model. So I think
> there's merit to this difference.
> 
>> - TTY framework manages intermediate buffer that allow to free Rx RPMsg buffers.
>>
> 
> rpmsg_char does the same thing, by putting incoming messages on
> epddev->queue.

That's right, I forgot that, thanks for pointing it out.

> 
>> So rpmsg_char and rpmsg_tty don't seem to me to cover exactly the same requierement.
>>
> 
> No, you're right there's some differences here.

So can I consider that there is no blocking point on your side to move forward
with this Rpmsg service?

Thanks,
Arnaud
> 
> Regards,
> Bjorn
> 
>> 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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