linux-kernel - Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same gadget device

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Date:   Wed, 12 Apr 2017 10:30:02 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Felipe Balbi <balbi@...nel.org>
cc:     Greg KH <greg@...ah.com>, Roger Quadros <rogerq@...com>,
        <vivek.gautam@...eaurora.org>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same
 gadget device

On Wed, 12 Apr 2017, Felipe Balbi wrote:

> >> Maybe...  But I can't shake the feeling that Greg KH would strongly 
> >> disagree.  Hasn't he said, many times in the past, that any dynamically 
> >> allocated device structure _must_ have a real release routine?  
> >> usb_udc_nop_release() doesn't qualify.
> >
> > Aw, I wanted to publically yell at someone like the kernel documentation
> > says I am allowed to do so if anyone does such a foolish thing :)
> 
> heh, except that we're not dynamically allocating struct device at all
> :-) Here's what we have for most UDCs (net2280.c included):
> 
> 	struct my_udc {
>         	struct gadget gadget;
>                 [...]
> 	};
> 
> 	probe()
>         {
>         	struct my_udc *u;
> 
> 		u = kzalloc(sizeof(*u), GFP_KERNEL);
>                 [...]
> 		return 0;
> 	}

Allow me to point out that the struct device is embedded inside the
struct gadget (actually struct usb_gadget) embedded inside the struct
my_udc, which _is_ dynamically allocated.  Therefore the struct device
is located in dynamically allocated memory.

> Now, if this kzalloc() would be replaced with devm_kzalloc() wouldn't
> this result on a functionally equivalent execution to the patch I
> proposed above?

It would, and it would be equally wrong.

Alan Stern