Re: [RESEND PATCH -next] USB: serial: pl2303: implement reset_resume member

[Hongyu Xie <xy521521@gmail.com>]

Hi greg,
On 2022/4/22 00:45, Greg KH wrote:
> On Tue, Apr 19, 2022 at 02:54:08PM +0800, Hongyu Xie wrote:
>> From: Hongyu Xie <xiehongyu1@kylinos.cn>
>>
>> pl2303.c doesn't have reset_resume for hibernation.
>> So needs_binding will be set to 1 duiring hibernation.
>> usb_forced_unbind_intf will be called, and the port minor
>> will be released (x in ttyUSBx).
> 
> Please use the full 72 columns that you are allowed in a changelog text.
> 
> 
>> It works fine if you have only one USB-to-serial device.
>> Assume you have 2 USB-to-serial device, nameing A and B.
>> A gets a smaller minor(ttyUSB0), B gets a bigger one.
>> And start to hibernate. When your PC is in hibernation,
>> unplug device A. Then wake up your PC by pressing the
>> power button. After waking up the whole system, device
>> B gets ttyUSB0. This will casuse a problem if you were
>> using those to ports(like opened two minicom process)
>> before hibernation.
>> So member reset_resume is needed in usb_serial_driver
>> pl2303_device.
> 
> If you want persistent device naming, use the symlinks that udev creates
> for your for all your serial devices.  Never rely on the number of a USB
> to serial device.
Let me put it this way. Assume you need to record messages output from 
two machines using 2 USB-to-serial devices(naming A and B, and A is on
USB1-port3, B is on USB1-port4) opened by two minicom process.
The setting for A in minicom would be like:
	"A -    Serial Device      : /dev/ttyUSB0"
The setting for B in minicom would be like:
	"A -    Serial Device      : /dev/ttyUSB1"
Then start to hibernate on your computer. When your PC is in
hibernation, unplug A. After waking up your computer, "/dev/ttyUSB0"
would be released first, then allocated to B. The minicom process used
to record outputs from A is now recording B's outputs. The minicom
process used to record outputs from B is now recording nothing, because
"/dev/ttyUSB1" is not exist anymore. That's the problem I've been
talking about. And I don't think using symlinks will solve this problem.

> 
>> Codes in pl2303_reset_resume are borrowed from pl2303_open.
>>
>> As a matter of fact, all driver under drivers/usb/serial
>> has the same problem except ch341.c.
>>
>> Cc: stable@vger.kernel.org
> 
> How does this meet the stable kernel rule requirements?  It would be a
> new feature if it were accepted, right?
It's not a new feature at all. struct usb_serial_driver already has a
member name reset_resume, there is no implementation in pl2303.c yet.
And ch341.c has one(ch341_reset_resume()), that why I said "all driver
under drivers/usb/serial has the same problem except ch341.c"
> 
> 
> thanks,
> 
> greg k-h