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Date:   Wed, 14 Jun 2023 08:24:35 -0700
From:   Bjorn Andersson <andersson@kernel.org>
To:     Arnaud POULIQUEN <arnaud.pouliquen@foss.st.com>
Cc:     Sarannya S <quic_sarannya@quicinc.com>, quic_bjorande@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: <20230614152435.2quoctx6ouvw4ous@ripper>
References: <1682160127-18103-1-git-send-email-quic_sarannya@quicinc.com>
 <1682160127-18103-2-git-send-email-quic_sarannya@quicinc.com>
 <c44d8942-83e5-01ec-491b-bac1fb27de99@foss.st.com>
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On Mon, Apr 24, 2023 at 02:49:29PM +0200, Arnaud POULIQUEN wrote:
> Hello,
> 
> On 4/22/23 12:42, 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	
> > + * @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;
> 
> Here we return an error if the backend does not implement the ops.
> But the set_flow_control ops is optional.
> Should we return 0 instead with a debug message?
> 

It seems reasonable to allow the software to react to the absence of
flow control support, so a debug message wouldn't help.

But advertising that more explicitly by returning something like
EOPNOTSUPP seems better.

Regards,
Bjorn