linux-kernel - Re: [PATCH v4 5/5] usb: gadget: udc-core: independent registration of gadgets and gadget drivers

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Date:	Mon, 16 Mar 2015 02:01:58 +0200
From:	Ruslan Bilovol <ruslan.bilovol@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Balbi, Felipe" <balbi@...com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Krzysztof Opasiak <k.opasiak@...sung.com>,
	Peter Chen <peter.chen@...escale.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>
Subject: Re: [PATCH v4 5/5] usb: gadget: udc-core: independent registration of
 gadgets and gadget drivers

HI Alan,

On Fri, Mar 13, 2015 at 4:39 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Thu, 12 Mar 2015, Ruslan Bilovol wrote:
>
>> Change behavior during registration of gadgets and
>> gadget drivers in udc-core. Instead of previous
>> approach when for successful probe of usb gadget driver
>> at least one usb gadget should be already registered
>> use another one where gadget drivers and gadgets
>> can be registered in udc-core independently.
>>
>> Independent registration of gadgets and gadget drivers
>> is useful for built-in into kernel gadget and gadget
>> driver case - because it's possible that gadget is
>> really probed only on late_init stage (due to deferred
>> probe) whereas gadget driver's probe is silently failed
>> on module_init stage due to no any UDC added.
>>
>> Also it is useful for modules case - now there is no
>> difference what module to insert first: gadget module
>> or gadget driver one.
>
>
>> @@ -366,9 +383,16 @@ found:
>>       list_del(&udc->list);
>>       mutex_unlock(&udc_lock);
>>
>> -     if (udc->driver)
>> +     if (udc->driver) {
>> +             struct usb_gadget_driver *driver = udc->driver;
>> +
>>               usb_gadget_remove_driver(udc);
>>
>> +             mutex_lock(&udc_lock);
>> +             list_add(&driver->pending, &gadget_driver_pending_list);
>> +             mutex_unlock(&udc_lock);
>> +     }
>
> I'm not sure this is a good idea.  Gadget drivers probably don't expect
> to be bound again after they are unbound.

This already has been discussed some time ago:
https://lkml.org/lkml/2015/2/9/497
The bottom line was - such gadget drivers are buggy and need to be fixed
since there is no known restrictions in binding gadget drivers to UDC multiple
times

>
> Besides, when would this gadget driver get bound to a UDC?  Not until
> the next UDC is added -- even if there already are some unbound UDCs.

Currently this gadget driver will get bound to a UDC only when next UDC
is added. It seems there is no users of this feature, so I didn't
add full implementation of this (that I had in version #1 if this patch:
https://lkml.org/lkml/2015/1/28/1079 )

>
>
>> @@ -468,6 +491,16 @@ int usb_gadget_unregister_driver(struct usb_gadget_driver *driver)
>>                       break;
>>               }
>>
>> +     if (ret) {
>> +             struct usb_gadget_driver *tmp;
>> +
>> +             list_for_each_entry(tmp, &gadget_driver_pending_list, pending)
>> +                     if (tmp == driver) {
>> +                             list_del(&driver->pending);
>> +                             ret = 0;
>> +                             break;
>> +                     }
>> +     }
>
> You could avoid this loop and simply do list_del(&driver->pending), if
> you made sure driver->pending was initialized.

It would be good to avoid this loop but the question is how to make sure
that driver->pending is not only initialized (prev and next are not NULL),
but also contains valid data?

Best regards,
Ruslan
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