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

[Arnaud POULIQUEN]

Hi Bjorn,

On 3/25/20 5:57 PM, 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.
> 
> 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?  
> 
> 
> 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. 
> 
> 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. 
> 
> 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.

Do you think you would find time to move forward with this discussion?
I would like to prepare a v8 to fix issue reported on v7, but as your comments
challenges the driver itself, i would prefer that we first find solutions
that address your concerns.

Thanks,
Arnaud

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