linux-kernel - Re: [Linux-stm32] [PATCH] rpmsg: char: return an error if device already open

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Message-ID: <6de9ff8f-0be1-387a-df7e-7d77dd859513@st.com>
Date:   Fri, 15 Jan 2021 10:13:35 +0100
From:   Arnaud POULIQUEN <arnaud.pouliquen@...com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Arnaud POULIQUEN - foss <arnaud.pouliquen@...s.st.com>
CC:     Ohad Ben-Cohen <ohad@...ery.com>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-remoteproc@...r.kernel.org" <linux-remoteproc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [Linux-stm32] [PATCH] rpmsg: char: return an error if device
 already open

Hi Mathieu,

On 1/14/21 8:05 PM, Mathieu Poirier wrote:
> On Wed, Jan 06, 2021 at 02:37:14PM +0100, Arnaud Pouliquen wrote:
>> The rpmsg_create_ept function is invoked when the device is opened.
>> As only one endpoint must be created per device. It is not
>> possible to open the same device twice.
>> The fix consists in returning -EBUSY when device is already
>> opened.
>>
>> Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
>> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
>> ---
>>  drivers/rpmsg/rpmsg_char.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
>> index 4bbbacdbf3bb..360a1ab0a9c4 100644
>> --- a/drivers/rpmsg/rpmsg_char.c
>> +++ b/drivers/rpmsg/rpmsg_char.c
>> @@ -127,6 +127,9 @@ static int rpmsg_eptdev_open(struct inode *inode, struct file *filp)
>>  	struct rpmsg_device *rpdev = eptdev->rpdev;
>>  	struct device *dev = &eptdev->dev;
>>  
>> +	if (eptdev->ept)
>> +		return -EBUSY;
>> +
> 
> I rarely had to work so hard to review a 2 line patch...

That means that my commit description was not enough explicit...

> 
> As far as I can tell the actual code is doing the right thing.  If user space is
> trying to open the same eptdev more than once function rpmsg_create_ept() should
> complain and the operation denied, wich is what the current code is doing.  
> 
> There is currently two customers for this API - SMD and GLINK.  The SMD code is
> quite clear that if the channel is already open, the operation will be
> denied [1].  The GLINK code isn't as clear but the fact that it returns NULL on
> error conditions [2] is a good indication that things are working the same way.
> 
> What kind of use case are you looking to address?  Is there any way you can use
> rpdev->ops->create_ept() as it is currently done?

This patch was part of the IOCTL rpmsg series. I sent it separately at Bjorn's
request [1].

I detect the issue using the RPMSG_ADDR_ANY for the source address when tested
it with the rpmsf_virtio bus. In this case at each sys open of the device, a new
endpoint is created because a new source address is allocated.

[1]https://patchwork.kernel.org/project/linux-remoteproc/patch/20201222105726.16906-11-arnaud.pouliquen@foss.st.com/

Thanks,
Arnaud

> 
> Thanks,
> Mathieu
> 
> [1]. https://elixir.bootlin.com/linux/v5.11-rc3/source/drivers/rpmsg/qcom_smd.c#L920
> [2]. https://elixir.bootlin.com/linux/v5.11-rc3/source/drivers/rpmsg/qcom_glink_native.c#L1149
> 
>>  	get_device(dev);
>>  
>>  	ept = rpmsg_create_ept(rpdev, rpmsg_ept_cb, eptdev, eptdev->chinfo);
>> -- 
>> 2.17.1
>>
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