linux-kernel - Re: [PATCH v2 5/7] rpmsg: char: Introduce a rpmsg driver for the rpmsg char device

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Date:   Wed, 21 Apr 2021 11:40:53 -0600
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 v2 5/7] rpmsg: char: Introduce a rpmsg driver for the
 rpmsg char device

Good day Arnaud,

On Tue, Apr 13, 2021 at 03:44:56PM +0200, Arnaud Pouliquen wrote:
> A rpmsg char device allows to probe the endpoint device on a remote name
> service announcement.
> 
> With this patch the /dev/rpmsgX interface is created either by a user
> application or by the remote firmware.
> 
> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
> 
> ---
> update from V1:
>  - add missing unregister_rpmsg_driver call on module exit.
> 
> ---
>  drivers/rpmsg/rpmsg_char.c | 59 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 58 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index a64249d83172..4606787b7011 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -26,6 +26,8 @@
>  #include "rpmsg_char.h"
>  #include "rpmsg_internal.h"
>  
> +#define RPMSG_CHAR_DEVNAME "rpmsg-raw"
> +

Why not simply call it rpmsg-char?

>  static dev_t rpmsg_major;
>  
>  static DEFINE_IDA(rpmsg_ept_ida);
> @@ -403,13 +405,67 @@ int rpmsg_chrdev_create_eptdev(struct rpmsg_device *rpdev, struct device *parent
>  }
>  EXPORT_SYMBOL(rpmsg_chrdev_create_eptdev);
>  
> +static int rpmsg_chrdev_probe(struct rpmsg_device *rpdev)
> +{
> +	struct rpmsg_channel_info chinfo;
> +	struct rpmsg_eptdev *eptdev;
> +
> +	if (!rpdev->ept)
> +		return -EINVAL;
> +
> +	memcpy(chinfo.name, RPMSG_CHAR_DEVNAME, sizeof(RPMSG_CHAR_DEVNAME));
> +	chinfo.src = rpdev->src;
> +	chinfo.dst = rpdev->dst;
> +
> +	eptdev = __rpmsg_chrdev_create_eptdev(rpdev, &rpdev->dev, chinfo, NULL);
> +	if (IS_ERR(eptdev))
> +		return PTR_ERR(eptdev);
> +
> +	/* Set the private field of the default endpoint to retrieve context on callback. */
> +	rpdev->ept->priv = eptdev;

This is already done in rpmsg_create_ept() when rpmsg_eptdev_open() is called.

> +
> +	return 0;
> +}
> +
> +static void rpmsg_chrdev_remove(struct rpmsg_device *rpdev)
> +{
> +	int ret;
> +
> +	ret = device_for_each_child(&rpdev->dev, NULL, rpmsg_chrdev_destroy_eptdev);
> +	if (ret)
> +		dev_warn(&rpdev->dev, "failed to destroy endpoints: %d\n", ret);
> +}
> +
> +static struct rpmsg_device_id rpmsg_chrdev_id_table[] = {
> +	{ .name	= RPMSG_CHAR_DEVNAME },
> +	{ },
> +};
> +
> +static struct rpmsg_driver rpmsg_chrdev_driver = {
> +	.probe = rpmsg_chrdev_probe,
> +	.remove = rpmsg_chrdev_remove,
> +	.id_table = rpmsg_chrdev_id_table,
> +	.callback = rpmsg_ept_cb,

Not sure why we need a callback associated to this driver when
rpmsg_eptdev_open() already creates and rpmsg_endpoint.  To me the only thing
having a callback provides is the association between the rpmsg_device and the
rpmsg_endpoint[1] that happens in rpmsg_dev_probe().  The QC folks already do
this association in their platform code[2].  Since this is not done in
__rpmsg_create_ept() a check for rpdev->ept == NULL could be done in
rpmsg_eptdev_open() and do the assignment there. 

[1]. https://elixir.bootlin.com/linux/v5.12-rc6/source/drivers/rpmsg/rpmsg_core.c#L513  
[2]. https://elixir.bootlin.com/linux/v5.12-rc6/source/drivers/rpmsg/qcom_glink_native.c#L1623

> +	.drv = {
> +		.name = "rpmsg_chrdev",
> +	},
> +};
> +
>  static int rpmsg_chrdev_init(void)
>  {
>  	int ret;
>  
>  	ret = alloc_chrdev_region(&rpmsg_major, 0, RPMSG_DEV_MAX, "rpmsg_char");
> -	if (ret < 0)
> +	if (ret < 0) {
>  		pr_err("rpmsg: failed to allocate char dev region\n");
> +		return ret;
> +	}
> +
> +	ret = register_rpmsg_driver(&rpmsg_chrdev_driver);
> +	if (ret < 0) {
> +		pr_err("rpmsg: failed to register rpmsg raw driver\n");
> +		unregister_chrdev_region(rpmsg_major, RPMSG_DEV_MAX);
> +	}
>  
>  	return ret;
>  }
> @@ -417,6 +473,7 @@ postcore_initcall(rpmsg_chrdev_init);
>  
>  static void rpmsg_chrdev_exit(void)
>  {
> +	unregister_rpmsg_driver(&rpmsg_chrdev_driver);
>  	unregister_chrdev_region(rpmsg_major, RPMSG_DEV_MAX);
>  }
>  module_exit(rpmsg_chrdev_exit);
> -- 
> 2.17.1
>