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Subject: Re: [PATCH v7 1/2] rpmsg: core: add API to get MTU
To:     Jiri Slaby <jslaby@suse.cz>, Ohad Ben-Cohen <ohad@wizery.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>, <linux-remoteproc@vger.kernel.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
CC:     Suman Anna <s-anna@ti.com>,
        Fabien DESSENNE <fabien.dessenne@st.com>,
        <linux-stm32@st-md-mailman.stormreply.com>,
        xiang xiao <xiaoxiang781216@gmail.com>
References: <20200324170407.16470-1-arnaud.pouliquen@st.com>
 <20200324170407.16470-2-arnaud.pouliquen@st.com>
 <356fe539-6ec4-6000-1f68-23404fc5d373@suse.com>
 <9f302a19-e684-ffff-823a-02af7f90403c@suse.cz>
From:   Arnaud POULIQUEN <arnaud.pouliquen@st.com>
Message-ID: <1edf98cf-dfb6-0e66-a58d-abba5d2a65c4@st.com>
Date:   Wed, 1 Apr 2020 13:34:19 +0200
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On 4/1/20 8:29 AM, Jiri Slaby wrote:
> On 01. 04. 20, 8:28, Jiri Slaby wrote:
>> On 24. 03. 20, 18:04, Arnaud Pouliquen wrote:
>>> Return the rpmsg buffer MTU for sending message, so rpmsg users
>>> can split a long message in several sub rpmsg buffers.
>>>
>>> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
>>> Acked-by: Suman Anna <s-anna@ti.com>
>>> ---
>>>  drivers/rpmsg/rpmsg_core.c       | 21 +++++++++++++++++++++
>>>  drivers/rpmsg/rpmsg_internal.h   |  2 ++
>>>  drivers/rpmsg/virtio_rpmsg_bus.c | 10 ++++++++++
>>>  include/linux/rpmsg.h            | 10 ++++++++++
>>>  4 files changed, 43 insertions(+)
>>>
>>> diff --git a/drivers/rpmsg/rpmsg_core.c b/drivers/rpmsg/rpmsg_core.c
>>> index e330ec4dfc33..a6ef54c4779a 100644
>>> --- a/drivers/rpmsg/rpmsg_core.c
>>> +++ b/drivers/rpmsg/rpmsg_core.c
>>> @@ -283,6 +283,27 @@ int rpmsg_trysend_offchannel(struct rpmsg_endpoint *ept, u32 src, u32 dst,
>>>  }
>>>  EXPORT_SYMBOL(rpmsg_trysend_offchannel);
>>>  
>>> +/**
>>> + * rpmsg_get_mtu() - get maximum transmission buffer size for sending message.
>>> + * @ept: the rpmsg endpoint
>>> + *
>>> + * This function returns maximum buffer size available for a single message.
>>> + *
>>> + * Return: the maximum transmission size on success and an appropriate error
>>> + * value on failure.
>>> + */
>>> +
>>> +ssize_t rpmsg_get_mtu(struct rpmsg_endpoint *ept)
>>> +{
>>> +	if (WARN_ON(!ept))
>>> +		return -EINVAL;
>>> +	if (!ept->ops->get_mtu)
>>> +		return -ENOTSUPP;
>>
>> Hmm, I don't think all callers of tty_write_room() handle negative values...
>>
>> But some drivers also return negative values. There is some work to be
>> done, adding to TODO.
>>
>> For the time being, I suggest returning 0 instead.
> 
> Or better, convert the negative to 0 in rpmsg_tty_write_room for now.
> 
>> thanks,-- 

Right, seems that a negative return of tty_write_room seems not always handled
so i will force to 0 in rpmsg_tty in case of error returned. I will also
add a comment in code that explain the context.
  
Thanks,
Arnaud