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Date:   Tue, 9 Oct 2018 11:27:12 -0700
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Arnaud Pouliquen <arnaud.pouliquen@st.com>
Cc:     Arun Kumar Neelakantam <aneela@codeaurora.org>, ohad@wizery.com,
        clew@codeaurora.org, sricharan@codeaurora.org,
        linux-remoteproc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V4 0/4] Add TIOCM Signals support for RPMSG char devices
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On Tue 09 Oct 09:02 PDT 2018, Arnaud Pouliquen wrote:

> hello Bjorn,
> 
> On 10/08/2018 06:23 PM, Bjorn Andersson wrote:
> > On Mon 08 Oct 06:08 PDT 2018, Arnaud Pouliquen wrote:
> > 
> >> Hi Arun, Bjorn,
> >>
> >> On 10/08/2018 08:38 AM, Arun Kumar Neelakantam wrote:
> >>> Glink transport support signals to exchange state notification between
> >>> local and remote side clients. Adding support to send/receive the signal
> >>> command and notify the clients through callback and POLL notification.
> >>
> >> Please correct me if i'm wrong...My concern here is that this patchset
> >> implements a rpmsg service in the rpmsg core. I would separate this from
> >> the rpmsg core, as this is not part of the rpmsg protocol but seems
> >> linked to the serial protocol itself.
> >> Could it be implemented in rpmsg_char, using a dedicated channel...?
> >>
> > 
> > rpmsg_char does expose a rpmsg channel (be it virtio, smd or glink) to
> > user space. This patch series add support for invoking TIOCMGET and
> > TIOCMSET on these channels.
> 
> I'm not familiar with this concept, that could explain that i don't
> understand this patchset...
> 
> TIOCMGET and TIOCMSET is related to the serial/console flow control, to
> control remote modem/processor, right?
> 

Correct, using the known tty ioctls for flow control we allow userspace
to communicate flow control information to the rpmsg_char driver.

> it seems be implemented only to support the rpmsg_char ioctl interface.
> When i have a look to the glink code, signal is treated by a message
> sent to remote processor. Therefore it seems that it could be treated as
> a service on top of rpmsg (so treat it in rpmsg_char instead of extend
> the rpmsg protocol to treat it in glink driver).
> 

The glink message is a control message, so it's not possible to pass
this inside a channel. As such it's not possible to solve this entirely
in rpmsg_char.

Looking at SMD, this is a set of bits in a per-channel control
structure. So there it's clearer that it's some side-band control
information.


Further more, the rpmsg_char driver just exposes a channel to user
space, it does not care about the data inside. As such it's not possible
to generically extend it to support this with in-band messages.

> > 
> > In addition to adding the client side of this to rpmsg_char it provides
> > this support for glink, but the same mechanism exists in smd - while
> > this is not supported (today) by the virtio rpmsg.
> This is my main concern, i would like to be sure that this service is
> not related to specific needs introduced by the rmpsg char implementation:
> In this case this should not be part of the rpmsg core but perhaps some
> ops directly provided to the rpmsg_char on registration
> (rpmsg_chrdev_register_device?)...
> 

Arun's imminent need is for a user space client that needs to flow
control the incoming data stream. But the possibility of controlling the
incoming flow of data is useful in a number of situations.

> > 
> > 
> > I'm uncertain of how we could implement this mechanism for virtio rpmsg,
> > given that it as a transport doesn't really have a concept of
> > channels/flows - but it's really useful to have!
> > 
> > PS. rpmsg_set_signals() can be called from any rpmsg device to perform
> > flow control of the communication channel.
> 
> For my point of view this patch-set extends the rpmsg protocol to add
> channel flow control.
> Does it make sense to have a flow control in rpmsg protocol?
> if yes, should it be linked to a channel or to the remote processor itself?
> 

Having per-channel flow control particularly useful in scenarios where
one has multiple types of data flowing over a shared underlying FIFO -
such as virtio rpmsg. As this allows these different applications to
limit the data rate without having to use application specific side band
controls.

> Extra comment: associated documentation update is missing.
> 

Good catch, we should have a section in Documentation/rpmsg.txt
describing this mechanism.

Regards,
Bjorn

> Regards,
> Arnaud
> 
> > 
> > Regards,
> > Bjorn
> > 
> >> Regards
> >> Arnaud
> >>
> >>>
> >>> Changes since v1:
> >>> - Split the patches as per functional areas like core, char, glink
> >>> - Add set, clear mask for TIOCMSET
> >>> - Merge the char signal callback and POLLPRI patches
> >>>
> >>> Changes since v2:
> >>> - Modify the rpmsg_get_signals function prototype
> >>>
> >>> Changes since v3:
> >>> - Correct the TICOMGET case handling as per new rpmsg_get_signals prototype
> >>> - Update the rpmsg_get_signals function header
> >>>
> >>> Arun Kumar Neelakantam (4):
> >>>   rpmsg: core: Add signal API support
> >>>   rpmsg: glink: Add support to handle signals command
> >>>   rpmsg: char: Add TIOCMGET/TIOCMSET ioctl support
> >>>   rpmsg: char: Add signal callback and POLLPRI support
> >>>
> >>>  drivers/rpmsg/qcom_glink_native.c | 126 ++++++++++++++++++++++++++++++++++++++
> >>>  drivers/rpmsg/rpmsg_char.c        |  74 +++++++++++++++++++++-
> >>>  drivers/rpmsg/rpmsg_core.c        |  41 +++++++++++++
> >>>  drivers/rpmsg/rpmsg_internal.h    |   5 ++
> >>>  include/linux/rpmsg.h             |  26 ++++++++
> >>>  5 files changed, 269 insertions(+), 3 deletions(-)
> >>>