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

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Date:   Wed, 25 Mar 2020 12:39:03 +0100
From:   Arnaud POULIQUEN <arnaud.pouliquen@...com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, <linux-kernel@...r.kernel.org>,
        <linux-remoteproc@...r.kernel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Suman Anna <s-anna@...com>,
        Fabien DESSENNE <fabien.dessenne@...com>,
        <linux-stm32@...md-mailman.stormreply.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        xiang xiao <xiaoxiang781216@...il.com>
Subject: Re: [PATCH v7 2/2] tty: add rpmsg driver

Hi,

On 3/24/20 6:44 PM, Randy Dunlap wrote:
> Hi,
> 
> On 3/24/20 10:04 AM, 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.
>>
>> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...com>
>> ---
>>  Documentation/serial/tty_rpmsg.rst |  45 ++++
>>  drivers/tty/Kconfig                |   9 +
>>  drivers/tty/Makefile               |   1 +
>>  drivers/tty/rpmsg_tty.c            | 417 +++++++++++++++++++++++++++++
>>  4 files changed, 472 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..fc1d3fba73c5
>> --- /dev/null
>> +++ b/Documentation/serial/tty_rpmsg.rst
>> @@ -0,0 +1,45 @@
>> +.. SPDX-License-Identifier: GPL-2.0
>> +
>> +=============
>> +The 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.
> 
>                                                                          to make it possible
> 
>> +
>> +The remote processor can instantiate a new tty by requesting:
>> +- a "rpmsg-tty-raw" RPMsg service, for TTY raw data support without flow control
>> +- a "rpmsg-tty-ctrl" RPMSg service, for TTY support with flow control.
>> +
>> +Information related to the RPMsg and associated tty device is available in
>> +/sys/bus/rpmsg/devices/.
>> +
>> +RPMsg TTY without control
>> +---------------------
> 
> extend underline under "control"
> 
>> +
>> +The default end point associated with the "rpmsg-tty-raw" service is directly
>> +used for data exchange. No flow control is available.
>> +
>> +To be compliant with this driver, the remote firmware must create its data end point associated with the "rpmsg-tty-raw" service.
>> +
>> +RPMsg TTY with control
>> +---------------------
> 
> extend underline length.
> 
>> +
>> +The default end point associated with the "rpmsg-tty-ctrl" service is reserved for
>> +the control. A second endpoint must be created for data exchange.
>> +
>> +The control channel is used to transmit to the remote processor the CTS status,
>> +as well as the end point address for data transfer.
>> +
>> +To be compatible with this driver, the remote firmware must create or use its end point associated with "rpmsg-tty-ctrl" service, plus a second endpoint for the data flow.
>> +On Linux rpmsg_tty probes, the data endpoint address and the CTS (set to disable)
>> +is sent to the remote processor.
>> +The remote processor has to respect following rules:
> 
>                                respect the following rules:
> 
>> +- It only transmits data when Linux remote cts is enable, otherwise message
> 
>                                               CTS is enabled,
> 
>> +  could be lost.
>> +- It can pause/resume reception by sending a control message (rely on CTS state).
>> +
>> +Control message structure:
>> +struct rpmsg_tty_ctrl {
>> +	u8 cts;			/* remote reception status */
>> +	u16 d_ept_addr;		/* data endpoint address */
>> +};
> 
> Is that struct packed or padded?

It's packed as payload of the RP message and so define is global size.
This struct is misaligned with the code.

I will take into account your comments in next version.

Thanks for the review,
Arnaud