Re: [PATCH v7 2/2] tty: add rpmsg driver

[Bjorn Andersson]

On Wed 25 Mar 09:57 PDT 2020, Arnaud POULIQUEN wrote:

> Hi Bjorn,
> 
> On 3/24/20 9:52 PM, Bjorn Andersson wrote:
> > On Tue 24 Mar 10:04 PDT 2020, Arnaud Pouliquen wrote:
> > [..]
> >> diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
> >> index 020b1cd9294f..c2465e7ebc2a 100644
> >> --- a/drivers/tty/Makefile
> >> +++ b/drivers/tty/Makefile
> >> @@ -34,5 +34,6 @@ 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
> > [..]
> >> +static struct rpmsg_device_id rpmsg_driver_tty_id_table[] = {
> >> +	{ .name	= TTY_CH_NAME_RAW },
> >> +	{ .name	= TTY_CH_NAME_WITH_CTS},
> > 
> > I still don't like the idea that the tty devices are tied to channels by
> > fixed names.
> 
> This point has been discussed with Xiang, he has the same kind of requirement. 
> My proposal here is to do this in two steps. First a fixed name, then
> in a second step we can extend the naming using the implementation proposed
> by Mathieu Poirier:
> 
> [1]https://lkml.org/lkml/2020/2/12/1083
> 
> Is this patch could answer to your requirement?
> 
> if requested i can I can integrate the Mathieu's patch in this patchset.
>  
> > 
> > This makes the driver unusable for communicating with any firmware out
> > there that provides tty-like data over a channel with a different name -
> > such as modems with channels providing an AT command interface (they are
> > not named "rpmsg-tty-raw").
> 
> I'm not fixed on the naming, any proposal is welcome.
> If we use the patch [1], could be renamed 
> "rpmsg-tty". then for AT command could be something like "rpmsg-tty-at"
> 
> But here seems we are speaking about service over TTY and not over RPMsg.
> 
> > 
> > I also fail to see how you would distinguish ttys when the firmware
> > provides more than a single tty - e.g. say you have a modem-like device
> > that provides an AT command channel and a NMEA stream.
> 
> Today it is a limitation. In fact this limitation is the same for all RPMsg
> devices with multi instance.
> The patch [1] will allow to retrieve the instance by identifying
> the service device name in /sys/class/tty/ttyRPMSG<X>/device/name
> 
> > 
> > 
> > These are the reasons why drivers/rpmsg/rpmsg_char registers a "control
> > device", from which you can spawn new char devices. As I've said before,
> > I really think the same approach should be taken for ttys - perhaps by
> > just extending the rpmsg_char to allow it to create tty devices in
> > addition to the "packet based" char device?
> > 
> I'm not very familiar with the rpmsg_char so please correct me if i'm wrong:
> 
> The rpmsg_char exposes to userland an interface to manage rpmsg channels
> (relying on a char device). This interface offers the  possibility to create
> new channels/endpoints and send/received related messages. 
>  
> Thus, the application declares the RPMsg channels which is bound if they matches
> with the remote processor channel (similar behavior than a kernel rpmsg driver).
> There is no constrain on the service once same service is advertised by remote
> firmware.
> 
> In addition, a limitation of the rpmsg_char device is that it needs to be
> associated with an existing device, as example the implementation in qcom_smd
> driver.
> 

Correct, the rpmsg_char control device must be associated with a
transport instance, e.g. a virtio rpmsg instance sitting on a
remoteproc. This is necessary in order to be able to tie the dynamically
created rpmsg_char endpoints (i.e. the thing that is similar to your tty
devices) to a particular transport/remoteproc..

The reason why qcom_smd needs to be involved is because of the problem
that I want the control device to appear without depending on particular
channels being exposed by the firmware.

> If i try to figure out how to implement TTY using the rpmsg_char:
> I should create a rpmsg_char dev in the rpmsg tty driver. Then application
> will create channels related to its service. But in this case
> how to ensure that channels created are related to the TTY service?  
> 

My proposal/wish is that 1) rpmsg_char is implemented for virtio/rpmsg,
so that the control device is registered as virtio rpmsg is initiated
and 2) that rpmsg_char is extended to allow creating tty devices in
addition to the existing interface (if the existing read/write interface
isn't enough).

> 
> I would also expect to manage RPMsg TTY such as a generic TTY: without
> extra interface and auto mounted as an USB TTY. this means that the
> /dev/ttyRMPSGx are created automatically at remote firmware startup
> (without any application action). For instance a generic application 
> (e.g. minicom) could control an internal remote processor such as
> an external processor through a TTY link. 
> 

And that's not possible using the two-stage approach rpmsg_char takes,
instead I use udev rules to invoke the ioctl on the control device.

The benefit is that the design of the firmware is not tied to the design
of the Linux system.

> Then we have also similar RPMsg driver for I2C and SPI virtual link. So extend
> the rpmsg_char to support TTY seems not a good solution for long terms. 
> 

What do you mean with this? Are you saying that running tty over rpmsg
over SPI is a bad idea?

> For these reasons i would prefer to have a specific driver. And found a solution
> to allow user to differentiate the TTY instances.
> 
> Anyway I am very interesting in having more details of an implementation relying
> on rpmsg_char if you still thinking that is the good approach here.
> 

I do think it's a good idea to decouple the system design on the Linux
side from the naming of channels provided by the firmware.

Regards,
Bjorn

> Thanks for your comments, 
> Arnaud
> 
> > Regards,
> > Bjorn
> >