linux-kernel - Re: [v3 PATCH] usb: create usb_debug

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Message-ID: <1559201499.8487.40.camel@mhfsdcap03>
Date:   Thu, 30 May 2019 15:31:39 +0800
From:   Chunfeng Yun <chunfeng.yun@...iatek.com>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        <linux-usb@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH] usb: create usb_debug_root for gadget only

Hi Felipe,
On Tue, 2019-05-28 at 11:11 +0300, Felipe Balbi wrote:
> Hi,
> 
> Chunfeng Yun <chunfeng.yun@...iatek.com> writes:
> > diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> > index 7fcb9f782931..88b3ee03a12d 100644
> > --- a/drivers/usb/core/usb.c
> > +++ b/drivers/usb/core/usb.c
> > @@ -1190,7 +1190,7 @@ EXPORT_SYMBOL_GPL(usb_debug_root);
> >  
> >  static void usb_debugfs_init(void)
> >  {
> > -	usb_debug_root = debugfs_create_dir("usb", NULL);
> > +	usb_debug_root = debugfs_create_dir(USB_DEBUG_ROOT_NAME, NULL);
> >  	debugfs_create_file("devices", 0444, usb_debug_root, NULL,
> >  			    &usbfs_devices_fops);
> >  }
> 
> might be a better idea to move this to usb common. Then have a function
> which can be called by both host and gadget to maybe create the
> directory:
> 
> static struct dentry *usb_debug_root;
> 
> struct dentry *usb_debugfs_init(void)
> {
> 	if (!usb_debug_root)
>         	usb_debug_root = debugfs_create_dir("usb", NULL);
> 
> 	return usb_debug_root;
> }
> 
> 
> Then usb core would be updated to something like:
> 
> static void usb_core_debugfs_init(void)
> {
> 	struct dentry *root = usb_debugfs_init();
> 
> 	debugfs_create_file("devices", 0444, root, NULL, &usbfs_devices_fops);
> }
> 
I find a problem when move usb_debugfs_init() and usb_debugfs_cleanup()
into usb common, it's easy to create "usb" directory, but difficult to
cleanup it:

common/common.c

struct dentry *usb_debugfs_init(void)
{
    if (!usb_debug_root)
        usb_debug_root = debugfs_create_dir("usb", NULL);

    return usb_debug_root;
}

void usb_debugfs_cleanup(void)
{
    debugfs_remove_recursive(usb_debug_root);
    usb_debug_root = NULL;
}

core/usb.c

static void usb_core_debugfs_init(void)
{
    struct dentry *root = usb_debugfs_init();

    debugfs_create_file("devices", 0444, root, NULL,
&usbfs_devices_fops);
}

static int __init usb_init(void)
{
    ...
    usb_core_debugfs_init();
    ...
}

static void __exit usb_exit(void)
{
    ...
    usb_debugfs_cleanup();
    // will be error, gadget may use it.
    ...
}

gadget/udc/core.c

static int __init usb_udc_init(void)
{
    ...
    usb_debugfs_init();
    ...
}

static void __exit usb_udc_exit(void)
{
    ...
    usb_debugfs_cleanup();
    // can't cleanup in fact, usb core may use it.
}

How to handle this case? introduce a reference count? do you have any
suggestion?

Thanks a lot