Re: [PATCH] usb: gadget: f_fs: Send a create event for endpoint files

[Greg KH <gregkh@linuxfoundation.org>]

<resend as your email headers were corrupt so only got sent to 1 person>

On Tue, Mar 07, 2023 at 03:26:36PM -0800, Jerry Zhang wrote:
> From: Jerry Zhang <jerry@skydio.com>
> 
> If userspace is creating a composite gadget with one or
> more functionfs functions, it generally goes through a process of:
> 
> 1. mount functionfs devices
> 2. start up driver processes
> 3. driver processes open ep0 and write descriptors
> 4. once all descriptors are written, enable gadget
> 
> The user may want the gadget to be enabled as soon as possible.
> However there is no race-free way to know when to enable the gadget
> since the gadget enable process doesn't know the status of the
> other driver processes. Furthermore, other driver processes
> may be fixed binaries, so there may not be an opportunity to
> modify their source to use a cooperative synchronization method.
> 
> This can be resolved by enabling inotify create events for endpoint
> files. The gadget enable process can create inotify watches in each
> driver's directory, and a create event for "ep1" will indicate that
> the driver has finished writing descriptors. Once events for all
> drivers have been received, the gadget is ready to be enabled.
> 
> Signed-off-by: Jerry Zhang <jerry@skydio.com>
> ---
>  drivers/usb/gadget/function/f_fs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index ddfc537c7526..fc038db46e14 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -13,10 +13,11 @@
>  
>  /* #define DEBUG */
>  /* #define VERBOSE_DEBUG */
>  
>  #include <linux/blkdev.h>
> +#include <linux/fsnotify.h>
>  #include <linux/pagemap.h>
>  #include <linux/export.h>
>  #include <linux/fs_parser.h>
>  #include <linux/hid.h>
>  #include <linux/mm.h>
> @@ -1443,10 +1444,11 @@ static struct dentry *ffs_sb_create_file(struct super_block *sb,
>  		dput(dentry);
>  		return NULL;
>  	}
>  
>  	d_add(dentry, inode);
> +	fsnotify_create(dentry->d_parent->d_inode, dentry);
>  	return dentry;
>  }
>  
>  /* Super block */
>  static const struct super_operations ffs_sb_operations = {
> -- 
> 2.37.3
> 

This is a good idea, but where is it now documented that this is the
expected functionality and that userspace can rely on this?

thanks,

greg k-h