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Anyway here I only share my opinion. It is not my role to give the direction.

> 
>> And What about using the "software flow control" instead? [1]
>>
>> [1] https://en.wikipedia.org/wiki/Software_flow_control
>>
>>>         return 0;
>>> @@ -137,6 +152,7 @@ static int rpmsg_eptdev_open(struct inode *inode,
>>> struct file *filp)
>>>           return -EINVAL;
>>>       }
>>>   +    ept->sig_cb = rpmsg_sigs_cb;
>>>       eptdev->ept = ept;
>>>       filp->private_data = eptdev;
>>>   @@ -155,6 +171,7 @@ static int rpmsg_eptdev_release(struct inode
>>> *inode, struct file *filp)
>>>           eptdev->ept = NULL;
>>>       }
>>>       mutex_unlock(&eptdev->ept_lock);
>>> +    eptdev->sig_pending = false;
>>>         /* Discard all SKBs */
>>>       skb_queue_purge(&eptdev->queue);
>>> @@ -265,6 +282,9 @@ static __poll_t rpmsg_eptdev_poll(struct file
>>> *filp, poll_table *wait)
>>>       if (!skb_queue_empty(&eptdev->queue))
>>>           mask |= EPOLLIN | EPOLLRDNORM;
>>>   +    if (eptdev->sig_pending)
>>> +        mask |= EPOLLPRI;
>>> +
>>>       mask |= rpmsg_poll(eptdev->ept, filp, wait);
>>>         return mask;
>>> @@ -274,11 +294,30 @@ static long rpmsg_eptdev_ioctl(struct file *fp,
>>> unsigned int cmd,
>>>                      unsigned long arg)
>>>   {
>>>       struct rpmsg_eptdev *eptdev = fp->private_data;
>>> +    bool set;
>>> +    u32 val;
>>> +    int ret;
>>>   -    if (cmd != RPMSG_DESTROY_EPT_IOCTL)
>>> -        return -EINVAL;
>>> +    switch (cmd) {
>>> +    case TIOCMGET:
>>> +        eptdev->sig_pending = false;
>>> +        ret = put_user(eptdev->rsigs, (int __user *)arg);
>>> +        break;
>>> +    case TIOCMSET:
>>> +        ret = get_user(val, (int __user *)arg);
>>> +        if (ret)
>>> +            break;
>>> +        set = (val & TIOCM_DTR) ? true : false;
>>> +        ret = rpmsg_set_flow_control(eptdev->ept, set);
>>> +        break;
>> Could this directly be handled by the driver on open close?
>> If application wants to suspend the link it could just close de
>> /dev/rpmsgX.
> All clients may not require setting flow control.

Agree, but this could be conditioned by rpdrv->signals, right?
And this could avoid to expose controls 
- in open/close the rpmsg_set_flow_control would be called,
- in rpmsg_eptdev_write_iter an error would be returned ( or 0)
  if remote side has suspended the transmission.

But perhaps you need more that a ON/OFF flow control?

Regards,
Arnaud

>>   Regards,
>> Arnaud
>>
>>> +    case RPMSG_DESTROY_EPT_IOCTL:
>>> +        ret = rpmsg_eptdev_destroy(&eptdev->dev, NULL);
>>> +        break;
>>> +    default:
>>> +        ret = -EINVAL;
>>> +    }
>>>   -    return rpmsg_eptdev_destroy(&eptdev->dev, NULL);
>>> +    return ret;
>>>   }
>>>     static const struct file_operations rpmsg_eptdev_fops = {



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