RE: [PATCH 1/1] usb: gadget: core: wait gadget device .release finishing at usb_del_gadget_udc

[Jun Li <jun.li@nxp.com>]

> -----Original Message-----
> From: Peter Chen <peter.chen@nxp.com>
> Sent: Saturday, August 1, 2020 2:54 PM
> To: Alan Stern <stern@rowland.harvard.edu>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>; balbi@kernel.org;
> linux-usb@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>
> Subject: RE: [PATCH 1/1] usb: gadget: core: wait gadget device .release finishing
> at usb_del_gadget_udc
> 
> 
> >
> > On 20-07-31 10:12:48, Alan Stern wrote:
> > > On Fri, Jul 31, 2020 at 02:06:20PM +0000, Peter Chen wrote:
> > > > On 20-07-31 14:25:20, Greg Kroah-Hartman wrote:
> > > > > On Fri, Jul 31, 2020 at 12:11:32PM +0000, Peter Chen wrote:
> > > > >
> > > > > Grab a reference from somewhere else and do not give it up for a
> > > > > long time.
> > > > >
> > > >
> > > > So wait_for_completion_timeout is suitable? The similar use case
> > > > is when we open the file at the USB Drive at Windows, and we click
> > > > "Eject", it will say "The device is currently in use", and refuse our "Eject"
> > > > operation.
> > > >
> > > > When we try to remove the gadget, if the gadget is in use, we
> > > > could refuse the remove operation, reasonable?
> > >
> > > No, the real solution is to fix the UDC drivers.  They need to
> > > allocate the gadget structure dynamically instead of reusing it.
> > > And they should have a real release routine that deallocates the gadget structure.
> > >
> > > Alan Stern
> >
> > So, the basic routine should like below. I thought the usb_gadget
> > should be deallocated before the UDC driver remove itself (UDC device
> > is the parent of usb_gadget device), I may not need to wrong about it,
> > it is just a memory region, it could release later.
> >
> > xxx_udc_release(struct device *gadget_dev) {
> > 	struct usb_gadget *gadget = container_of(gadget_dev, struct
> > 			usb_gadget, dev);
> > 	kfree(gadget);
> > }
> >
> >
> > xxx_udc_probe(pdev)
> > {
> > 	udc_priv_data = kzalloc(sizeof(*udc_priv_data), GFP_KERNEL);
> > 	gadget = kzalloc(sizeof(struct usb_gadget), GFP_KERNEL);
> > 	udc_priv_data->gadget = gadget;
> > 	...
> > 	usb_add_gadget_udc_release(&pdev->dev, gadget, xxx_udc_release);
> >
> > }
> >
> > At xxx_udc_remove(pdev)
> > {
> > 	usb_del_gadget_udc(udc_priv_data->gadget);
> > 	/* need to never reference udc_priv_data->gadget any more */
> > 	udc_priv_data other deinit;
> > 	kfree(udc_priv_data);
> > }
> >
> > Since all structures xxx_udc_release uses are common one, it could
> > replace usb_udc_nop_release at udc/core.c.
> >
> 
> Since gadget structure is allocated at UDC drivers, I think it should be freed at
> the same place. Current common release function at udc/core.c may not a good idea
> per our discussion.

Could all those allocation and free in UDC core? Have them in one central place
will ease/force UDC drivers to correctly handle this.

Li Jun
> 
> Peter