Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Message-ID: <f3f6ea7d-2051-7a7f-61e0-8a5bba8ca8f2@gmail.com>
Date:   Fri, 22 Apr 2022 10:35:59 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.7.0
Subject: Re: [RESEND PATCH -next] USB: serial: pl2303: implement reset_resume
 member
Content-Language: en-US
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     johan@kernel.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, Hongyu Xie <xiehongyu1@kylinos.cn>,
        stable@vger.kernel.org, "sheng . huang" <sheng.huang@ecastech.com>,
        wangqi@kylinos.cn, xiongxin@kylinos.cn
References: <20220419065408.2461091-1-xy521521@gmail.com>
 <YmGKL05dnA+q/HAM@kroah.com>
From:   Hongyu Xie <xy521521@gmail.com>
In-Reply-To: <YmGKL05dnA+q/HAM@kroah.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


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