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SubjectRe: [PATCH v8 2/2] tty: add rpmsg driver
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.

> 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?

> - 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.

> 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.

Regards,
Bjorn

> Thanks,
> Arnaud
>
> >
> > Thanks,
> > Bjorn
> >
> >> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.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@foss.st.com>");
> >> +MODULE_DESCRIPTION("remote processor messaging tty driver");
> >> +MODULE_LICENSE("GPL v2");
> >> --
> >> 2.17.1
> >>

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