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

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Message-ID: <20210114190543.GB255481@xps15>
Date:   Thu, 14 Jan 2021 12:05:43 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH] rpmsg: char: return an error if device already open

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...

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?

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
>