On Tue, May 8, 2012 at 10:32 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 8 May 2012, Greg Kroah-Hartman wrote:
>
>> On Mon, May 07, 2012 at 10:39:32PM -0400, Alan Stern wrote:
>> > On Mon, 7 May 2012, Greg Kroah-Hartman wrote:
>> >
>> > > > It looks like you're replacing a reset_resume method with a regular
>> > > > resume method. Is that really the right thing to do?
>> > >
>> > > As only 2 usb-serial drivers have ever implemented reset_resume, and
>> > > they both did the same exact thing that their resume functions did, I
>> > > think it is safe as resume will be called if reset_resume is not set,
>> > > right?
>> >
>> > Actually no. If the device is going through a reset-resume and a
>> > driver's reset_resume method pointer isn't set, the driver will be
>> > unbound and then later rebound. It's not a matter of just calling the
>> > resume method instead.
>> >
>> > Although maybe for serial devices it doesn't make much difference.
>>
>> Maybe for the sierra device it might matter. So, should we just have a
>> reset_resume callback in the usbserial driver structure? If that is
>> set, then we can set the usb_driver field as well, which should keep
>> things working properly I think.
>
> I don't know how much difference it makes in the end. Even if we add a
> reset_resume callback to usb-serial, if the lower serial drivers don't
> know how to reinitialize the hardware then the operation probably won't
> work. Settings that should be retained (like the current baud rate)
> won't be.
>
> Overall I suspect it's best for now just not to have reset-resume
> support in usb-serial. Of the two drivers that claimed to implement
> it, only ch341 actually did something at all reasonable (it called
> ch341_configure()). The sierra driver did not try to reinitialize
> anything.
In fact, reset_resume of ch341 is introduced to fix one problem, see below
commit:
commit 1ded7ea47b8829a06068c3bb5e3ebe471076617a
Author: Ming Lei <tom.leiming@xxxxxxxxx>
Date: Fri Feb 20 21:23:09 2009 +0800
USB: ch341 serial: fix port number changed after resume
I remembered that the problem addressed is a generic one, and it may
exist on other devices, but looks no general solution can be done in
usb-serial core.
Thanks,
--
Ming Lei
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