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Date:   Thu, 15 Jun 2023 07:50:39 -0700
From:   Bjorn Andersson <quic_bjorande@quicinc.com>
To:     Arnaud POULIQUEN <arnaud.pouliquen@foss.st.com>
CC:     Bjorn Andersson <andersson@kernel.org>,
        Sarannya S <quic_sarannya@quicinc.com>, <swboyd@chromium.org>,
        <quic_clew@quicinc.com>, <mathieu.poirier@linaro.org>,
        <linux-kernel@vger.kernel.org>, <linux-arm-msm@vger.kernel.org>,
        <linux-remoteproc@vger.kernel.org>,
        "Deepak Kumar Singh" <quic_deesin@quicinc.com>
Subject: Re: [PATCH V7 1/3] rpmsg: core: Add signal API support
Message-ID: <20230615145039.GA3256591@hu-bjorande-lv.qualcomm.com>
References: <1682160127-18103-1-git-send-email-quic_sarannya@quicinc.com>
 <1682160127-18103-2-git-send-email-quic_sarannya@quicinc.com>
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 <6e51d6d8-cd3a-b0f2-c044-6282749aae89@foss.st.com>
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On Thu, Jun 15, 2023 at 11:01:14AM +0200, Arnaud POULIQUEN wrote:
> 
> 
> On 6/14/23 17:54, Bjorn Andersson wrote:
> > On Sat, Apr 22, 2023 at 04:12:05PM +0530, Sarannya S wrote:
> >> From: Deepak Kumar Singh <quic_deesin@quicinc.com>
> >>
> >> Some transports like Glink support the state notifications between
> >> clients using flow control signals similar to serial protocol signals.
> >> Local glink client drivers can send and receive flow control status
> >> to glink clients running on remote processors.
> >>
> >> Add APIs to support sending and receiving of flow control status by
> >> rpmsg clients.
> >>
> >> Signed-off-by: Deepak Kumar Singh <quic_deesin@quicinc.com>
> >> Signed-off-by: Sarannya S <quic_sarannya@quicinc.com>
> >> ---
> >>  drivers/rpmsg/rpmsg_core.c     | 21 +++++++++++++++++++++
> >>  drivers/rpmsg/rpmsg_internal.h |  2 ++
> >>  include/linux/rpmsg.h          | 15 +++++++++++++++
> >>  3 files changed, 38 insertions(+)
> >>
> >> diff --git a/drivers/rpmsg/rpmsg_core.c b/drivers/rpmsg/rpmsg_core.c
> >> index a2207c0..e8bbe05 100644
> >> --- a/drivers/rpmsg/rpmsg_core.c
> >> +++ b/drivers/rpmsg/rpmsg_core.c
> >> @@ -331,6 +331,25 @@ int rpmsg_trysend_offchannel(struct rpmsg_endpoint *ept, u32 src, u32 dst,
> >>  EXPORT_SYMBOL(rpmsg_trysend_offchannel);
> >>  
> >>  /**
> >> + * rpmsg_set_flow_control() - sets/clears serial flow control signals
> >> + * @ept:	the rpmsg endpoint
> >> + * @enable:	pause/resume incoming data flow	
> > 
> > As shown in the discussion, it's still not clear what true/false means.
> > Also, let's try to clarify that it's a request for the other side to do
> > something:
> > 
> > * rpmsg_set_flow_control() - request remote to pause/resume transmission
> > * ...
> > * @enable: flow restricted
> > * ...
> > 
> > 
> > PS. There's a stray space at the end of the line.
> 
> The notion of flow restricted seems to me also ambiguous. It does
> not specify if the stream is limited in term of bandwidth or stopped.
> 
> What about using XON/XOFF as specified in software flow control[1]
> 
> XOFF	Pause transmission
> XON	Resume transmission
> 
> or simply pause/resume definitions
> 

I agree, that's still ambiguous.

I was concerned about expressing it such that the reader would assume
that calling this means there will be no more data coming, but there
might be things in the queues etc. Expressing it in terms of the state
of transmission is clearer.


/*
 * rpmsg_set_flow_control() - request remote to pause/resume transmission
 ...
 * @enable: Pause transmission
 ...
 */

Does that sound okay and clear to you?

Thanks,
Bjorn

> [1]https://en.wikipedia.org/wiki/Software_flow_control
> 
> Regards,
> Arnaud
> 
> > 
> >> + * @dst:	destination address of the endpoint
> >> + *
> >> + * Return: 0 on success and an appropriate error value on failure.
> >> + */
> >> +int rpmsg_set_flow_control(struct rpmsg_endpoint *ept, bool enable, u32 dst)
> >> +{
> >> +	if (WARN_ON(!ept))
> >> +		return -EINVAL;
> >> +	if (!ept->ops->set_flow_control)
> >> +		return -ENXIO;
> >> +
> >> +	return ept->ops->set_flow_control(ept, enable, dst);
> >> +}
> >> +EXPORT_SYMBOL_GPL(rpmsg_set_flow_control);
> >> +
> >> +/**
> >>   * rpmsg_get_mtu() - get maximum transmission buffer size for sending message.
> >>   * @ept: the rpmsg endpoint
> >>   *
> >> @@ -539,6 +558,8 @@ static int rpmsg_dev_probe(struct device *dev)
> >>  
> >>  		rpdev->ept = ept;
> >>  		rpdev->src = ept->addr;
> >> +
> >> +		ept->flow_cb = rpdrv->flowcontrol;
> >>  	}
> >>  
> >>  	err = rpdrv->probe(rpdev);
> >> diff --git a/drivers/rpmsg/rpmsg_internal.h b/drivers/rpmsg/rpmsg_internal.h
> >> index 39b646d..b6efd3e 100644
> >> --- a/drivers/rpmsg/rpmsg_internal.h
> >> +++ b/drivers/rpmsg/rpmsg_internal.h
> >> @@ -55,6 +55,7 @@ struct rpmsg_device_ops {
> >>   * @trysendto:		see @rpmsg_trysendto(), optional
> >>   * @trysend_offchannel:	see @rpmsg_trysend_offchannel(), optional
> >>   * @poll:		see @rpmsg_poll(), optional
> >> + * @set_flow_control:	see @rpmsg_set_flow_control(), optional
> >>   * @get_mtu:		see @rpmsg_get_mtu(), optional
> >>   *
> >>   * Indirection table for the operations that a rpmsg backend should implement.
> >> @@ -75,6 +76,7 @@ struct rpmsg_endpoint_ops {
> >>  			     void *data, int len);
> >>  	__poll_t (*poll)(struct rpmsg_endpoint *ept, struct file *filp,
> >>  			     poll_table *wait);
> >> +	int (*set_flow_control)(struct rpmsg_endpoint *ept, bool enable, u32 dst);
> >>  	ssize_t (*get_mtu)(struct rpmsg_endpoint *ept);
> >>  };
> >>  
> >> diff --git a/include/linux/rpmsg.h b/include/linux/rpmsg.h
> >> index 523c98b..a0e9d38 100644
> >> --- a/include/linux/rpmsg.h
> >> +++ b/include/linux/rpmsg.h
> >> @@ -64,12 +64,14 @@ struct rpmsg_device {
> >>  };
> >>  
> >>  typedef int (*rpmsg_rx_cb_t)(struct rpmsg_device *, void *, int, void *, u32);
> >> +typedef int (*rpmsg_flowcontrol_cb_t)(struct rpmsg_device *, void *, bool);
> >>  
> >>  /**
> >>   * struct rpmsg_endpoint - binds a local rpmsg address to its user
> >>   * @rpdev: rpmsg channel device
> >>   * @refcount: when this drops to zero, the ept is deallocated
> >>   * @cb: rx callback handler
> >> + * @flow_cb: remote flow control callback handler
> >>   * @cb_lock: must be taken before accessing/changing @cb
> >>   * @addr: local rpmsg address
> >>   * @priv: private data for the driver's use
> >> @@ -92,6 +94,7 @@ struct rpmsg_endpoint {
> >>  	struct rpmsg_device *rpdev;
> >>  	struct kref refcount;
> >>  	rpmsg_rx_cb_t cb;
> >> +	rpmsg_flowcontrol_cb_t flow_cb;
> >>  	struct mutex cb_lock;
> >>  	u32 addr;
> >>  	void *priv;
> >> @@ -106,6 +109,7 @@ struct rpmsg_endpoint {
> >>   * @probe: invoked when a matching rpmsg channel (i.e. device) is found
> >>   * @remove: invoked when the rpmsg channel is removed
> >>   * @callback: invoked when an inbound message is received on the channel
> >> + * @flowcontrol: invoked when remote side flow control status is received
> > 
> > s/status/request/
> > 
> > Regards,
> > Bjorn
> > 
> >>   */
> >>  struct rpmsg_driver {
> >>  	struct device_driver drv;
> >> @@ -113,6 +117,7 @@ struct rpmsg_driver {
> >>  	int (*probe)(struct rpmsg_device *dev);
> >>  	void (*remove)(struct rpmsg_device *dev);
> >>  	int (*callback)(struct rpmsg_device *, void *, int, void *, u32);
> >> +	int (*flowcontrol)(struct rpmsg_device *, void *, bool);
> >>  };
> >>  
> >>  static inline u16 rpmsg16_to_cpu(struct rpmsg_device *rpdev, __rpmsg16 val)
> >> @@ -192,6 +197,8 @@ __poll_t rpmsg_poll(struct rpmsg_endpoint *ept, struct file *filp,
> >>  
> >>  ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept);
> >>  
> >> +int rpmsg_set_flow_control(struct rpmsg_endpoint *ept, bool enable, u32 dst);
> >> +
> >>  #else
> >>  
> >>  static inline int rpmsg_register_device_override(struct rpmsg_device *rpdev,
> >> @@ -316,6 +323,14 @@ static inline ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept)
> >>  	return -ENXIO;
> >>  }
> >>  
> >> +static inline int rpmsg_set_flow_control(struct rpmsg_endpoint *ept, bool enable, u32 dst)
> >> +{
> >> +	/* This shouldn't be possible */
> >> +	WARN_ON(1);
> >> +
> >> +	return -ENXIO;
> >> +}
> >> +
> >>  #endif /* IS_ENABLED(CONFIG_RPMSG) */
> >>  
> >>  /* use a macro to avoid include chaining to get THIS_MODULE */
> >> -- 
> >> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> >> a Linux Foundation Collaborative Project
> >>