Re: [BUG report]: serial8250 unbind/bind failture

From: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
To: Jun Miao <jun.miao@intel.com>
Cc: "andriy.shevchenko@linux.intel.com" 
	<andriy.shevchenko@linux.intel.com>,
	"linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>
Subject: Re: [BUG report]: serial8250 unbind/bind failture
Date: Mon, 20 Dec 2021 11:42:37 +0100	[thread overview]
Message-ID: <YcBeHUTb8TnQBphS@kroah.com> (raw)
In-Reply-To: <3f82cdd9-c1d9-2dfb-45b0-55d0a419c96a@intel.com>

On Mon, Dec 20, 2021 at 09:08:19PM +0800, Jun Miao wrote:
> 
> On 2021/12/20 17:57, gregkh@linuxfoundation.org wrote:
> > On Mon, Dec 20, 2021 at 09:54:41AM +0000, Miao, Jun wrote:
> > > [Hardware]
> > > Intel Corporation Alder Lake Client Platform/AlderLake-M LP5 RVP, BIOS ADLPFWI1.R00.2265.A01.2107010436 07/01/2021
> > > About x86 old UART, not the PNP device.
> > > 
> > > [how to reproduce]
> > > root@ALD-M:/sys/devices/platform/serial8250/tty/ttyS0/device/driver# echo -n "serial8250" > unbind
> > > root@ALD-M:/sys/devices/platform/serial8250/tty/ttyS0/device/driver# echo -n "serial8250" > bind
> > After you unbound, this device is gone, so how does this path still
> > work?
> > 
> > > [What`s my Confusion]
> > > After the unbind and bind the serial8250_probe cannot register ttyS0.
> > Then do not do that :)
> 
> Hi maintainer,
> 
> Thanks your reply so quickly.
> 
> You mean, this operation is useless and should don`t do that.

Why do you think it is useful?

> But , if this is a PNP device,  it can probe successfully after unbind/bind.

Is that possible?  If so, how?  Has it ever worked before?  Who requires
this to work?  Does this bus and hardware support this type of
functionality for this hardware device?

> > > And there is not dmesg like this: "serial8250: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A".
> > > 
> > > Any one have some advice about 8250 serial ?  It is a common operation if we believe that the old UART which don`t support like this unbind. Or we ignore about this really bug before?
> > What are you trying to do here exactly?  Why are you wanting to unbind
> > the driver from this device?  Why do you then want to bind it again?
> > What problem are you trying to solve by doing this?
> > 
> > thanks,
> > 
> > greg k-h
> 
> There is a testcase to test UART by unbind/bind.   i want to support it on
> old uart  of serial8250 .

Who created this test case and what were they expecting to have happen?
What are they thinking this test case should be doing?  Is this a new
functionality that they need Linux to support?  If so, then new code
probably has to be written :)

bind/unbind is a "best effort" type of thing, not all busses and
hardware types support this at all.

thanks,

greg k-h