linux-kernel - Re: [PATCH V4 0/4] Add TIOCM Signals support for RPMSG char devices

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Date:   Tue, 9 Oct 2018 18:02:36 +0200
From:   Arnaud Pouliquen <arnaud.pouliquen@...com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
CC:     Arun Kumar Neelakantam <aneela@...eaurora.org>, <ohad@...ery.com>,
        <clew@...eaurora.org>, <sricharan@...eaurora.org>,
        <linux-remoteproc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 0/4] Add TIOCM Signals support for RPMSG char devices

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?

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).

> 
> 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?)...

> 
> 
> 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?

Extra comment: associated documentation update is missing.

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(-)
>>>