linux-kernel - Re: [PATCH] remoteproc: core: Move cdev add before device add

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Date:   Thu, 22 Apr 2021 12:04:55 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Siddharth Gupta <sidgup@...eaurora.org>
Cc:     bjorn.andersson@...aro.org, ohad@...ery.com,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        psodagud@...eaurora.org, eberman@...eaurora.org
Subject: Re: [PATCH] remoteproc: core: Move cdev add before device add

Hi Siddharth,

On Tue, Apr 20, 2021 at 12:26:45PM -0700, Siddharth Gupta wrote:
> When cdev_add is called after device_add has been called there is no
> way for the userspace to know about the addition of a cdev as cdev_add
> itself doesn't trigger a uevent notification, or for the kernel to
> know about the change to devt. This results in two problems:
>  - mknod is never called for the cdev and hence no cdev appears on
>    devtmpfs.
>  - sysfs links to the new cdev are not established.
> 
> Based on how cdev_device_add[1] is written, it appears that the correct

Please don't add this kind of reference to the change log as it will become
invalid with time.

> way to use these APIs is to call cdev_add before device_add is called.
> Since the cdev is an optional feature for remoteproc we cannot directly
> use the existing API.

Please explain why the existing API can't be used directly.

> Hence moving rproc_char_device_add() before
> device_add() in rproc_add().
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/char_dev.c#n537
> 
> Signed-off-by: Siddharth Gupta <sidgup@...eaurora.org>
> ---
>  drivers/remoteproc/remoteproc_core.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> index 626a6b90f..562355a 100644
> --- a/drivers/remoteproc/remoteproc_core.c
> +++ b/drivers/remoteproc/remoteproc_core.c
> @@ -2316,6 +2316,11 @@ int rproc_add(struct rproc *rproc)
>  	struct device *dev = &rproc->dev;
>  	int ret;
>  
> +	/* add char device for this remoteproc */
> +	ret = rproc_char_device_add(rproc);
> +	if (ret < 0)
> +		return ret;
> +

I have tested this change and it works.  So how did it work before?

>  	ret = device_add(dev);
>  	if (ret < 0)
>  		return ret;
> @@ -2329,11 +2334,6 @@ int rproc_add(struct rproc *rproc)
>  	/* create debugfs entries */
>  	rproc_create_debug_dir(rproc);
>  
> -	/* add char device for this remoteproc */
> -	ret = rproc_char_device_add(rproc);
> -	if (ret < 0)
> -		return ret;
> -

While reviewing this patch I had another look at rproc_add() and noticed it
doesn't clean up after itself in case of failure.  If any of the conditions
aren't met the function returns but rproc_delete_debug_dir(),
rproc_char_device_remove() and device_del() aren't called.  Please fix that as
part of your next revision.

Thanks,
Mathieu


>  	/* if rproc is marked always-on, request it to boot */
>  	if (rproc->auto_boot) {
>  		ret = rproc_trigger_auto_boot(rproc);
> -- 
> Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>